Chlorophyta. Generally, vegetative or reproductive flagellate cells are isokont with smooth flagella.

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1 The Chlorophyta are primarily freshwater; only about 10% of the algae are marine, whereas 90% are freshwater. Some orders are predominantly marine (Caulerpales, Dasycladales, Siphonocladales), whereas others are predominantly freshwater (Ulotrichales, Coleochaetales) or exclusively freshwater (Oedogoniales, Zygnematales). The freshwater species have a cosmopolitan distribution, with few species endemic in a certain area. In the marine environment, the green algae in the warmer tropical and semitropical waters tend to be similar everywhere in the world. This is not true of the Chlorophyta in the colder marine waters; the waters of the Northern and Southern hemispheres have markedly different species. Chlorophyta Generally, vegetative or reproductive flagellate cells are isokont with smooth flagella. Microtubular hairs do not occur on the flagella, although fibrillar hairs and Golgi-produced scales, are present (though rare) in some genera. Their species include unicellular flagellates like Tetraselmis, immobile like Chlorocytis, multicellular colonies like Dictyosphaerium, simple or branched filaments like Cladophora, pseudoparenchymatous thalli like Codium and parenchymatous laminae like Ulva. Most are microscopic, but some species like Caulerpa can reach over 10 meters long. Pyramimonas obovata. (a) Semidiagrammatic drawing of a cell showing the organelles, two of the four flagella, and the layers of scales on the body and the flagella. (b) Whole cell. (c) Inner flagella scale. (d) Outer flagellar scale. (e) Flagellar hair. (f) Inner body scale. (g) Intermediate body scale, top and side view. (h) Outer body scale. (c) Chloroplast; (f) flagellum; (m) mitochondrion; (n) nucleus; (p) pyrenoid; (r) scale reservoir; (s) starch; (sc) scales. Pyramimonas obovata Caulerpa Accumulation of carotenoids occurs under conditions of nitrogen deficiency, high irradiance or high salinity. This is particularly true in Dunaliella where β-carotene accumulates between thylakoids in the chloroplast, and Haematococcus, where astaxanthin accumulates in lipid globules outside the chloroplast. Hematochrome is a general term for these carotenoids. Accumulation of hematochromes color the cells orange or red, with hematochrome accumulating up to 8 12% of the cellular contents in Dunaliella. Animals can not synthesize carotenoids and they acquire the pigments through the food chain from primary producers. Hematochromes are responsible for the coloring in fish, crustaceans and birds (such as the pink in flamingos). Cell walls usually have cellulose as the main structural polysaccharide, although xylans or mannans often replace cellulose in the Caulerpales. The primitive algae in the Prasinophyceae have extracellular scales, or a wall derived from interlacing scales, composed of acidic polysaccharides. Algae in the Volvocales have walls composed of glycoproteins. Chloroplast pigments are similar to those of higher plants; chlorophyll a and b are present. The main carotenoid is lutein. The siphonaceous genera, as well as the unicells Tetraselmis and Mesostigma, are the only green algae to have siphonoxanthin and its ester siphonein. The chemical structures of carotenoids of the Chlorophyta. ١

2 Chloroplasts are surrounded only by the double-membrane chloroplast envelope, with no chloroplast endoplasmic reticulum (Fig. 5.2). The thylakoids are grouped into bands of 3 to 5 thylakoids. In some of the siphonaceous genera (e.g., Caulerpa), amyloplasts containing starch grains and a few thylakoids occur in the chloroplasts. Chlorophyta are distinguished from other algae because their main storage product is starch, which accumulates in the chloroplast and not in the cytoplasm. Starch formation is often associated with a pyrenoid (Fig. 5.2). The starch is similar to that of higher plants and is composed of amylose and amylopectin. The photosynthetic pathways are similar to those of higher plants, many of these pathways first being worked out in green algae such as Chlorella. Dunaliella salina ( red color ) Haematococcus Fig. 5.2 Semidiagrammatic drawing of a cell in a Volvox vegetative colony. The colony wall (CW) is distinct from the cell wall (W). (C) Chloroplast; (E) eyespot; (F) flagellum; (G) Golgi; (M) mitochondrion; (N) nucleus; (P) pyrenoid; (S) starch. Phototaxis and eyespots There are two types of phototactic movement in the Chlorophyta: movement by flagella and Movement by the secretion of mucilage. Most of the flagellated cells that show phototactic movement have an eyespot. In the Chlorophyta, the eyespot or stigma is always in the chloroplast, usually in the anterior portion near the flagella bases (Fig. 5.2). The eyespot consists of one to a number of layers of lipid droplets, usually in the stroma. The eyespot is usually colored orangered from the carotenoids in the lipid droplets. Contractile vacuoles are common in the vegetative cells of unicellular and colonial flagellate chlorophytes. Usually in biflagellate genera there are two contractile vacuoles at the base of the flagella. When there are two contractile vacuoles, they contract alternately. Contractile vacuoles are sometimes called pulsating vacuoles because of their alternate filling and emptying action. The phototactic response varies with light intensity; organisms with positive phototaxis at moderate light intensities exhibited negative phototaxis at very high light intensities. At a given light intensity, temperature has an effect on phototaxis. Haematococcus zoospores at a given light intensity will be negatively phototactic at 4 C, positively phototactic at C, and very strongly phototactic at 35 C. photoreceptor system The portion of the plasma membrane overlying the eyespot and has large number of protein particles represent a part of the photoreceptor system. The photoreceptor in Chlamydomonas consists of a chromophore (colored substance) linked to a protein (opsin or apoprotein) that is embedded in the plasma membrane. The chromophore is 11-cis-retinal (the aldehyde of vitamin A). In Chlamydomonas reinhardtii, the protein is chlamyopsin, while in Volvox carteri the protein is volvoxopsin Combined, the chromophore 11-cis-retinal and the protein produce a rhodopsin, a general class of compounds that absorb light maximally around wavelengths of 500 nm. Green algae can also show geotactic responses to gravity Chlamydomonas exhibits negative geotaxis by swimming against gravity. (a) The structure of 11-cis-retinal, part of the photoreceptor molecule in Chlamydomonas. ٢

3 Sexual reproduction Sexual reproduction in the Chlorophyceae may be isogamous, anisogamous, or oogamous. Usually gametes are specialized cells and not vegetative cells, although in the one celled Volvocales the latter can occur. If the species is isogamous or anisogamous, the gametes are usually not formed in specialized cells although in the oogamous species, gametes are normally formed in specialized gametangia. Whereas most Chlorophyta form motile flagellated gametes (zoogametes), in the Zygnematales aplanogametes or amoeboid gametes are formed. gametogenesis is induced by environmental changes, or the presence of two sexually different strains. fertilization of sexually different gametes occur under a complex hormonal control. production of a chemotactic substance by the egg that attracts the spermatozoids. at random. Asexual reproduction There are a number of types of asexual reproduction, the simplest being fragmentation of colonies into two or more parts, each part becoming a new colony. Zoosporogenes is commonly occurs, usually induced by a change in the environment of the alga. zoospores are normally produced in vegetative cells, and only in a few cases are they formed in specialized sporangia. Zoospores are usually formed in the younger parts of filaments, and the number of zoospores is generally a power of two in uninucleate genera. Aplanospores are non-flagellated and have a wall distinct from the parent cell wall (e.g., Trebouxia). Aplanospores are considered to be abortive zoospores and have the ability to form a new plant on germination. Autospores are aplanospores that have the same shape as the parent cell, and are common in the Chlorellales (e.g., Chlorella). Autospores are usually formed in a multiple of two in the parent cell. Coenobia are colonies with a definite number of cells arranged in a specific manner (e.g., Volvox). Genera with colonies arranged in coenobia form daughter colonies with a certain number of cells. In maturation of the daughter coenobia, there is enlargement but no division of vegetative cells in the coenobia. Microtubular roots Microtubular roots can have one of two basic configurations: (1) There can be a microtubular root consisting of a large broad band of microtubules (MBA: microtubular band) with a smaller second microtubular root (M) (Charophyceae), or zoospore of Coleochaete Position of flagella in cells the flagella are attached in a lateral position in the cell. at the anterior end. Flagellar roots Flagellar basal bodies are anchored in the protoplast by microtubular roots and/or rhizoplasts (fibrose roots). Charophyceae: scaly cell with one large root, one smaller root, and flagella extending at an angle from the point of insertion. Ultrastructure of flagellar apparatus in green algae. (b) Chaetosphaeridium (Coleochaetophyceae) zoospore, side view of unilateral type of flagellar apparatus. BB: basal body. MBA: microtubular band. MLS: multilayered structure. F: flagella. TSC: transversely striate fiber connecting the basal bodies The characteristics of Microtubular roots configuration vary depending on whether the position of flagella is apical or lateral. If flagella are inserted laterally, the microtubular roots are formed by the wide microtubular band (MBA) and the smaller microtubular root (M) If there are two flagella located at the apex, the microtubules that anchor the flagella are the cruciately arranged microtubular roots. scales (2) there can be four groups of cruciately arranged microtubular roots running from the basal bodies (Ulvophyceae and Chlorophyceae). The cruciately arranged microtubular roots have what is called an X-2-X-2 arrangement. This notation refers to the fact that two of the microtubular roots are usually composed of two microtubules, whereas the two other roots can have different numbers of microtubules in different organisms. Thus Chlamydomonas moewusii has a arrangement, whereas motile cells of Ulothrix sp. have a arrangement. microtubular root Chlorophyceae: cruciate roots and the cell covered with a theca. (F) Fibrous roots; (M) microtubular root; (T) theca. Ultrastructure of flagellar apparatus in the green algae Batophora (Ulvopphyceae), anterior and lateral view of root system. CP: caping plate connecting the basal bodies. BB: basal body. FR: fibrose roots or rhizoplasts. 2R and 5R: two and five stranded mictotubular roots. ٣

4 Multilayered structure A multilayered structure (MLS) consists of a more or less rectangular body attached to the anterior end of the single broad band of microtubules in the Charophyceae. The MLS lies directly beneath the basal bodies (BB) of the flagella (F). The MLS consists of four layers. Two microtubules layers and two electrondense layers in between. MLS may be present in the Prasinophyceae and Charophyceae, but is absent in the Chlorophyceae and Ulvophyceae. Rhizoplasts (fibrous roots) A rhizoplast (FR: fibrose roots ) is a fiber system with a striate appearance when observed by electron microscopy, composed of contractile proteins. A rhizoplast runs from the basal bodies posteriorly toward the nucleus. The method of contraction of the rhizoplast may be similar to that of muscle. Rhizoplasts may be present in the Prasinophyceae, Chlorophyceae, and Ulvophyceae, but are absent in the Charophyceae. FR FR Right: Drawing of a green algal cell containing a multilayered structure. (F) Flagellum; (Mt) microtubule; (MLS) multilayered structure; (R) microtubular root. Six to ten cellulose synthetase molecules are grouped into a single subunit. The subunits are, in turn, aggregated into terminal complexes. In the Chlorophyta, there are two different types of terminal complexes: 1 In the Charophyceae, terminal complexes have subunits aggregated into rosettes. 2 In the Chlorophyceae and Ulvophyceae, terminal complexes consist of linear rows of subunits. Ultrastructure of flagellar apparatus in the green algae Batophora (Ulvopphyceae), anterior and lateral view of root system. CP: caping plate connecting the basal bodies. BB: basal body. FR: fibrose roots or rhizoplasts. 2R and 5R: two and five stranded mictotubular roots. Occurrence of scales or a wall on the motile cells Proteins of cellulose Synthetase in the plasma membrane have aggregated into terminal complexes along two phylogenetic lines in the Chlorophyta. Rosettes of cellulose synthetase proteins have evolved in the Charophyceae, while aggregations of linear rows have evolved in the Chlorophyceae and Ulvophyceae. Persistent interzonal spindle and phragmoplast (Charophyceae) ٤ In the Charophyceae, the microtubular spindle persists even after the daughter nuclei have separated in telophase. The daughter nuclei are separated by the length of the persistent spindle while a new cross wall is formed by a phragmoplast. Wall formation by a phragmoplast initially involves the production of vesicles by the dictyosomes (Golgi apparatus). The Golgi vesicles contain the components of the new cross walls. The persistent spindle microtubules may function in guiding the vesicles to the area of the new cross wall which will separate the two daughter cells. The vesicles fuse, releasing their contents which form the new cross wall. Plasmodesmata are formed in the cross wall between the daughter cells where the persistent spindle microtubules traverse the cross wall. In some of the primitive Charophyceae, the new cross wall is formed by a phragmoplast in association with an infurrowing of the plasma membrane. In the more advanced Charophyceae (those in the Coleochaetales and Charales), the cross wall is formed only by a phragmoplast. Motile cells covered with scales may occur in the Prasinophyceae, Charophyceae, and Ulvophyceae. The presence of scales on the motile cells is probably the primitive condition. Then, the scales became interweaved along their edges so a coherent cell covering was formed, as in the genus Tetraselmis (Fig. 5.12). The end result of this was the formation of theca that covers the motile cells in the Chlorophyceae. This theca has a crystalline substructure and is composed of hydroxyproline-rich glycoproteins associated with various polysaccharides. The theca in motile cells of the Chlorophyceae is thus not to be confused with the cell walls of non-motile stages of the more advanced Chlorophyta, which have cellulose as the main skeletal molecule. Cellulose is produced by the enzyme cellulose synthetase that occurs as proteins embedded in the plasma membrane of the cell. Drawing of a cell of Tetraselmis. (C) Chloroplast; (E) eyespot; (F) flagellum; (FS) flagellar scales; (N) nucleus; (P) pyrenoid; (R) rhizoplast (only one of two is shown); (S) starch; (T) theca. Cell division Three of the classes in the Chlorophyta are characterized by the type of spindle in the class and the way the new cross wall is formed. Two types of interzonal spindles occur in telophase cells of the Chlorophyta: the persistent type and the collapsing type. The new cell wall can be formed between the daughter cells by means of a phragmoplast, a cleavage furrow, or a phycoplast.

5 Collapsing interzonal spindle and a phragmoplast (Chlorophyceae) In the Chlorophyceae, the mitotic spindle collapses after nuclear division. This results in the two daughter nuclei coming close together in telophase because there is no longer a persistent interzonal spindle to hold the nuclei apart. The collapsed spindles, produce a new set of microtubules that are arranged parallel to the plane of cell division and perpendicular to the former position of the spindle microtubules, the phycoplast, whose function is to keep the nuclei separate. Dictyosome vesicles fuse between the phycoplast microtubules, forming the new cross wall. The Chlorophyceae have motile cells with a cell wall (theca) and thus differ from the Ulvophyceae and Charophyceae. Persistent interzonal spindle and a cleavage furrow (Ulvophyceae) In the Ulvophyceae, the interzonal spindle persists during telophase, holding the daughter nuclei apart while the new cross wall is formed by an infurrowing of the plasma membrane. As the plasma membrane furrows inward, dictyosome vesicles fuse with the plasma membrane, behind the infurrowing, producing the new cross wall. Glycolate degradation Glycolate, the major substrate of photorespiration, is derived from phosphoglycolate, which is formed by the oxygenation of ribulose-1,5- diphosphate: The glycolate is metabolized in microbodies called peroxisomes. In the peroxisomes, glycolate is oxidized to glyoxylate. The H 2 O 2 produced in the reaction is cleaved by the enzyme catalase to H 2 O and O 2 : The persistence or not of the nuclear membrane during mitosis is also a major character in the systematics of green algae. In most cases, the nuclear membrane remains during cell division (closed mitosis), but in other cases, like in some charophytes, it disappears (open mitosis). In Trebouxiophyceae, centrioles are in the metaphase plane instead of at the poles (Fig. d). The above oxidation of glycolate can be catalyzed by the enzyme glycolate dehydrogenase or the enzyme glycolate oxidase. In the Charophyceae, the reaction is catalyzed by glycolate oxidase, whereas in the Chlorophyceae and the Ulvophyceae, the reaction is catalyzed by glycolate dehydrogenase. Glycolate oxidase probably represents the primitive condition since Cyanophora paradoxa in the Glaucophyta catalyzes the reaction with this enzyme (c) Closed mitosis. (d) Closed mitosis with centrioles located in the plane of metaphase. (e) Open division. Superoxide dismutase Superoxide dismutases (SOD) are a group of enzymes that catalyze the reaction: Urea degradation In the Charophyceae, Ulvophyceae, and higher plants, urea is broken down by the enzyme urease: Superoxide dismutases are important because they take highly reactive, potentially damaging oxygen radicals (O - 2 ) and other toxic oxygen species (OH, singlet oxygen) and convert them to less toxic moieties. There are three forms of superoxide dismutases, named for the metal in their reaction centers: iron SOD (FeSOD), occurs in the chloroplast manganese SOD (MnSOD), occurs in the mitochondrion copper-zinc SOD (Cu/Zn SOD), occurs in the cytosol. FeSOD and MnSOD occur in all classes of green algae and most land plants. Cu/Zn SOD, however, occurs only in the Charophyceae and land plants In the Chlorophyceae, urea is broken down by the enzyme urea amidolyase ٥

6 Prasinophyceae (Micromonadophyceae) Prasinophyceans contain a small group of unicellular scaly or naked green algae with interzonal spindles that are persistent during cytokinesis. Scaled species are covered by one or several layers of organic scales. Most are flagellate, provided with one to eight flagella that often emerge from a depression. The flagellar root system can be cross-shaped or show a multilayer structure (MLS). Species of this group live both in marine and freshwater. A common genus is Pyramimonas, a unicellular heart-shaped (cordate) cells, with four flagella arising from an anterior flat-bottomed depression and the body of the cell covered by three layers of different types of organic scales. Classification The four important classes in the Chlorophyta are the Prasinophyceae (Micromonadophyceae), Charophyceae, Ulvophyceae, and the Chlorophyceae. Their classification scheme was based largely on ultrastructural characteristics. Later investigations utilizing molecular genetics have verified their work. Figure. Diagram of Pyramimonas lunata, note the three bands of scales. OBS: outermost body scales. IBS: intermediate body scales. UBS: undermost body scales. CH: chloroplast. CV: cylindrical vesicle. F: flagella. FR: flagellar microtubular root. M: mitochondria. N: nucleus. P: pyrenoid. S: starch grain. SR: scale reservoir. ST: stigma Chlorophyceae Chlorophyceans show great morphological diversity and live almost exclusively in inland waters, predominantly freshwater. in telophase, the interzonal spindle collapses; phycoplast produces the new cross wall in cell division; motile cells with two or four apical flagella attached at anterior end of cell, whose microtubular root system is crossed (cruciate) and sometimes with rhizoplast, no multilayered structure (MLS);. Mitosis is closed. The cell wall is composed of cellulose and an external layer of crystalline glycoproteins. theca outside of the cells is common in motile cells; eyespots common; glycolate breakdown by glycolate dehydrogenase; urea broken down by urea amidolyase; zygote undergoes a dormant period; meiosis occurs when the zygote germinates. Tetraselmis is commonly found in marine waters. The cells are oval-shaped and surrounded by a theca that is formed by the coalescence of many small stellate scales that are produced in the Golgi apparatus. Four flagella are inserted in an apical pit in the cell. The flagella are covered with hairs and scales, and the flagella emerge from an opening in the theca. There is a cup-shaped chloroplast with a basal pyrenoid. A nucleus occurs in the center of the cell, and Golgi bodies are found anterior to the nucleus. Tetraselmis has a very high lipid level; their amino acids stimulate feeding in marine organisms. Tetraselmis sp. is known to produce lipids that can be transformed into biodiesel. Drawing of a cell of Tetraselmis. (C) Chloroplast; (E) eyespot; (F) flagellum; (FS) flagellar scales; (N) nucleus; (P) pyrenoid; (R) rhizoplast (only one of two is shown); (S) starch; (T) theca. Volvocales In the Volvocales the vegetative cells are flagellated and motile. The algae can be unicellular or multicellular. If they are multicellular, then the number of cells in the vegetative colony is a multiple of two. Almost all of these organisms are freshwater, being abundant in waters high in nitrogenous compounds. There are two important families in the order: Family 1 Chlamydomonadaceae: unicellular algae. In this family are all the unicellular Volvocales. All of the genera are uninucleate, and usually the chloroplast is cup-shaped. Examples are Chlamydomonas, Dunaliella (Dunaliella acidophila, D. salina), Hematococcus and Phacotus. Family 2 Volvocaceae: colonial algae formed into coenobia, colonies with a definite number of cells arranged in a specific manner. Example include Volvox (500 to cells), Gonium (4 to 32 cells), Pandorina (4 to 32 cells), Volvulina (18 cells in four ranks of four), Stephanoon (8 to 16 cells), Eudorina (16 to 128 cells), Platydorina. Classification The Chlorophyceae are divided into the following important orders: Order 1 Volvocales: vegetative cells flagellated and motile. There are two important families in the order: Family 1 Chlamydomonadaceae: unicellular algae. Family 2 Volvocaceae: colonial algae formed into coenobia, colonies with a definite number of cells arranged in a specific manner. Order 2 Tetrasporales: non-filamentous colonies with immobile vegetative cells capable of cell division; pseudocilia may be present. Two families will be considered here: Family 1 Tetrasporaceae: cells with pseudocilia. Family 2 Palmellaceae: cells without pseudocilia. Order 3 Prasiolales: marine, freshwater or terrestrial algae composed of unicells, short filaments or small sheet-like thalli. Order 4 Chlorellales: unicells or nonfilamentous colonial algae; vegetative cells non-motile. Order 5 Trebouxiales: green algae involved in a lichen symbiosis. Order 6 Sphaeropleales: unbranched filaments with new walls formed inside the old filament walls, resulting in H-shaped wall pieces. Order 7 Chlorosarcinales: daughter cells retained within parent cell wall; no plasmodesmata present. Order 8 Chaetophorales: branched or unbranched filaments; plasmodesmata present. Order 9 Oedogoniales: uninucleate filamentous freshwater algae with a unique type of cell division; motile spores and gametes with a whorl of flagella at one pole. Volvox Gonium Pandorina Eudorina ٦

7 Prasiolales: marine, freshwater or terrestrial algae composed of unicells, short filaments or small sheet-like thalli. The members of the order are characterized by a stellate chloroplast with a central pyrenoid. Prasiola stipitata is example. Tetrasporales: non-filamentous colonies with immobile vegetative cells capable of cell division; pseudocilia may be present. Pseudocilia long hairlike immobile, non-functional flagella but are evidently related to them because the pseudocilia have a normal basal body but an abnormal 9+0 configuration of microtubules near the base of the pseudocilia. The number of microtubules lessens and becomes more irregular as the end of the pseudocilium is approached. Two families will be considered here: Family 1 Tetrasporaceae: The elongated gelatinous thalli of the Tetrasporaceae have vegetative cells in groups of two to four, with each cell having two pseudocilia. Example is Tetraspora gelatinosa Family 2 Palmellaceae: cells without pseudocilia. Members of the Palmellaceae have their cells united in small gelatinous colonies that are generally amorphous but may be of definite shape. Palmella and Botryococcus braunii are examples. The life cycle of Prasiola stipitata. Tetraspora gelatinosa Palmella Trebouxiales: green algae involved in a lichen symbiosis. This order represents the lichen algae group. Note that not all lichenized algae are in this group: other phycobionts (algal partner in a lichen symbiosis, the fungus is the mycobiont) include cyanobacteria, Trente pholia (Ulvophyceae), and Heterococcus (Xanthophyceae). Trebouxia is the most common green alga as a phycobiont in a lichen symbiosis. Trebouxia sp.: (a) vegetative cell; (b) zoospores being released; (c) aplanospores being released. The Trebouxia-containing lichen Xanthoria parietina; (d) whole thallus; (e) section of thallus showing spherical Trebouxia cells. Chlorellales: In this order are the algae that have a non-motile vegetative thallus where the thallus comprises only a single cell or a coenobium composed of a definite number of cells arranged in a specific manner. Asexual reproduction occurs by zoospores or aplanospores that are commonly autospores. These autospores are probably no more than daughter cells of the parent thallus. Sexual reproduction can be isogamous, anisogamous, or oogamous. The order is exclusively freshwater. Chlorococcum, Hydrodictyon reticulatum, Chlorella, Prototheca and Scenedesmus are examples. Prototheca causes protothecosis in animals and humans. This disease is more common than is supposed, but because the small round colorless cells resemble yeasts, the disease, at least in animals, is often incorrectly diagnosed. Prototheca cells are fairly common in the soil, and it is from here that most infections occur. Massive infections have been reported in humans but usually only as secondary invaders after the primary invader has seriously lowered the body s defenses. More common in humans is the subcutaneous type of infection that starts initially as a small lesion and spreads slowly through the lymph glands, covering large areas of the body and preventing the sufferer from performing normal work duties Lesions of protothecosis on the foot and lower leg of a man from Sierra Leone. Chlorosarcinales: The algae in this order have a type of cell division (desmoschisis) that results in the formation of a number of cells, each with its own cell wall, within the cell wall of the parent cell. The genera in the Chlorosarcinales lack the plasmodesmata. Sphaeropleales: unbranched filaments with new walls formed inside the old filament walls, resulting in H-shaped wall pieces. Chlorosarcina sp. (a) Packet of cells dividing by desmoschisis to produce daughter colonies, each enclosed within the cell wall of the parent cell. (b) Colony-forming zoospores. (c) Zoospore. Microspora crassior. (a) Filament. (b) Diagrammatic representation of wall deposition. Initially the interphase cell is enclosed by two H-shaped segments and a central cylinder. At cytokinesis, a cross wall is deposited in the middle of the cylinder. During cell expansion, the two H-shaped segments move apart while a new cylinder is secreted inside the H-shaped segments. ٧

8 Oedogoniales: uninucleate filamentous freshwater algae with a unique type of cell division; motile spores and gametes with a whorl of flagella at one pole (stephanokonts). Sexual reproduction is oogamous, and asexual reproduction can be by zoospores or akinetes. (a) Bulbochaete gigantea, filament with zygote. (b),(c) Oedocladium hazenii, germling with rhizoid (b) and portion of a filament with empty androsporangia, immature dwarf males, and an unfertilized egg (c). Chaetophorales: branched or unbranched filaments. The Chaetophorales, along with the Oedogoniales, mark the most complex organization attained within the Chlorophyceae. The Chaetophorales have plasmodesmata, a characteristic of true multicellularity. The Chaetophorales have branched filaments with uninucleate cells that contain a single parietal chloroplast. Many of the plants exhibit heterotrichy; that is, there are two different types of filaments, one constituting the prostrate system and the other the erect system. Stigeoclonium is a common freshwater alga found attached in flowing water. It has a very wide tolerance to organic pollution and can be indicative of heavily polluted water although it will also grow in unpolluted water. (a) Fritschiella tuberosa showing prostrate system (ps), protonema (pr), projecting secondary branches (sec), and a rhizoid (r). Also shown is the tip of a branch. (b) Stigeoclonium farctum, old and young plants with erect and prostrate portions. (c) Chaetophora incrassata. (d) Draparnaldia glomerata. Ulvophyceans are grouped in at least six orders according to their morphology, number of nuclei per cell, cellular and reproductive features and molecular sequence analysis. Ulvophyceae Ulvophyceans are cosmopolitan and predominantly marine. Most of their over 1,200 species are macroscopic benthic algae which have a life cycle with alternation of generations. Their mobile cells have apical flagella attached at anterior end of cell; motile cells have near-radial symmetry externally; flagella roots consist of four cruciately arranged microtubular roots and sometimes a rhizoplast; no multilayered structure (MLS). Scales may be present on motile cells During cell division, they have persistent interzonal spindle in telophase; cleavage furrow produces the new cross wall in cell division; eyespots common; glycolate broken down by glycolate dehydrogenase; urea broken down by urease; no dormant zygotes; alternation of generations common. The order Ulotrichales comprises unbranched filaments like Ulothrix or Urospora, two marine and freshwater genera, and branched filaments like Gomontia or monostromatic (single cell layered) Monostroma. Cells are uninucleate and contain a parietal, band-like chloroplast with one or several pyrenoids. The filament is attached to substrate by a special basal cell called the holdfast. All cells are able to divide except the holdfast. In sexual reproduction, biflagellate isogametes produced by a cell of the filament are released through a pore. Gametes fuse in pairs to form quadriflagellate zygotes, which develop a thick wall originating a resting cell. After meiosis, new thalli are produced. Asexual reproduction can occur by fragmentation of filaments or by the production of quadriflagellate zoospores. Classification The following orders are placed in the Ulvophyceae. Order 1 Ulotrichales: uninculeate filamentous algae with a parietal chloroplast. Order 2 Ulvales: uninucleate cells with a parietal chloroplast; thallus is a hollow cylinder or a sheet, one or two cells thick. Order 3 Cladophorales: multinucleate filamentous algae with a parietal perforate or reticulate chloroplast. Order 4 Dasycladales: thallus has radial symmetry composed of an erect axis bearing branches; thallus uninucleate but multinucleate just before reproduction; gametes formed in operculate cysts. Order 5 Caulerpales: coenocytic algae lacking cellulose in the walls; siphonoxanthin and siphonein usually present. Order 6 Siphonocladales: algae with segregative cell division; siphonoxanthin present. Trentepohliales is a terrestrial order, growing on humid soil, rocks, buildings, tree bark, leaves, stems, and fruit. Urospora penicilliformis Gomontia Monostroma ٨

9 Life cycle of Ulva presents isomorphic alternation of generations. Every gametophyte cell can produce several gametes. Female gametes are bigger than male gametes. After fertilization, a quadriflagellate zygote is produced. This zygote fixes to the substrate to form a cylindrical thallus that becomes laminar at maturity (Fig. 31). Diploid sporophytes produce quadriflagellate zoospores by meiosis. The order Ulvales comprises parenchymatic thalli, which can be tubular as in Blidingia, laminar with a single cell layer as in Ulvaria, or two layers thick as in Ulva. These genera have diffuse growth, and all cells except the basal ones have reproductive capacity. Asexual reproduction can occur by parthenogenetic gametes. Figure 31. Life cycle in Ulva arasaki Blidingia Ulvaria Ulva The life cycles are haplo-diplontic with the isomorphic alternation of generations as in Cladophora or pseudoheteromorphic as in Spongomorpha in which a unicellular zygote (Codiolum) lives for several months after germination. Fertilization is by isogamy or anisogamy with biflagellate gametes. Quadriflagellate zoospores arise after meiotic division. Cladophoraceae live in marine, brackish and inland waters. Cladophorales includes about 400 species with filamentous and pseudoparenchimatous thalli. The body is constituted by septate filaments formed by large cylindrical cells. Each cellular segment contains multiple nuclei and a reticulate chloroplast with many pyrenoids. The family Cladophoraceae comprises single filaments as in Chaetomorpha or branched filaments as in Cladophora (Fig. 30B). Spongomorpha (a) Chaetomorpha aerea. (b) Cladophora microcladioides. At maturity, the nucleus contained in the axis is thought to divide by meiosis, and the resulting haploid nuclei then undergo mitosis and move towards the cap rays. A septum is formed in each cap ray, and it then becomes a gametophore and later a cyst (Fig. 33). Mature cysts release isogamous biflagellate gametes by abscision of an operculum. The zygote generates vegetative thalli. Dasycladales It comprises species with thalli formed by a nonseptate primary axis with branches in whorls, exhibiting radial symmetry. Acetabularia has uninucleate diploid thalli that become coenocytic and haploid at maturity. The thallus is formed by a cylindrical axis attached to substrate by a holdfast of rhizoidal branches. At the apex, whorls of adherent branches form the so-called cap rays. ٩

10 The order Caulerpales (Bryopsidales) includes about 500 species of multinucleate, siphonous ulvophyceans of varied morphology which were previously treated as orders and are now treated as families. Thalli lack crosswalls except at sites of reproductive cell formation. Chloroplasts are discoid, numerous and may or not have pyrenoids. Some genera, like Udotea and Caulerpa, contain two types of plastids: pigmented plastids and unpigmented plastids or leucoplasts in which starch accumulates. Some taxa contain polysaccharides like xylans or mannans in their cell walls instead of cellulose. The genus Cymopolia is multinucleate. The nuclei have a heterogeneous appearance in the different parts of the thallus. Dasycladales occurs in shallow waters of tropical to temperate seas. They are often calcified, for which there are numerous known fossil genera. Udotea (a) Caulerpa prolifera. (b) Caulerpa floridana. The large genus Caulerpa is a uniaxial siphonous form with a rhizoidal horizontal axis from which erect photosynthetic axes emerge. Caulerpa is characterized by the occurrence of cylindrical wall ingrowths or trabeculae traversing the central lumen of siphon and providing mechanical support. Chloroplasts and leucoplast are present. Sexual reproduction by anisogametes is known, and zygote development produces a new diploid thallus. Caulerpa grows in sandy beds from tropical to template sea waters. Members of Bryopsidales, like Halimeda, show flattened articulated thalli, whose walls are calcified in the form of araganonite crystals. If the thallus is injured, rupture can result in the loss of large amounts of cytoplasm, but repair of the wound occurs rapidly, by contraction of protoplasm and plug formation. This plug formation is related to the terpenoid caulerpenyne which also has the function of defense against herbivores. The majority of Bryopsidales are seaweeds which occur in warm tropical to temperate waters and live on the littoral or in deeper waters. Caulerpa verticillata Caulerpa sertularioides Halimeda This genus is characterized by a heteromorphic life cycle involving a branching siphonous sporophyte and a vesicular coenocytic gametophyte, known as the Halicystis phase. Gametophytes produce small pale gametes and larger dark green gametes in different individuals. Sporophytes produce multinucleate zoospores provided with a crown of subapical flagella. In Derbesia and Pedobesia, the thallus is constituted by a uniaxial system with pinnate branches (Fig. 30D). The chloroplasts are pigmented and have a pyrenoid that accumulates starch. Derbesia marina The life cycle of Derbesia. Derbesia Pedobesia solieri, coenocytic thallus and sporangium ١٠

11 Other pseuparenchymatous thalli are the fan-shaped of Udotea and the terete stalk of Penicillus (Fig. 32). Codium, a widespread marine genus, is common in temperate waters. Its thalli exhibit varied forms from hollow spheres to prostrate crusts and branched cylindrical axes, which can be up to 1 m in length. They are multiaxial, composed of interwoven siphons arranged in a colorless medullary region and a pigmented cortical one. Chloroplasts are discoid without pyrenoids. The tips of coenocytic siphons are dilated into enlarged utricles that form a compact palisade-like surface layer. Gametangia are produced laterally on the utricles. Sexual reproduction is by biflagellate anisogametes Udotea Penicillus Gametangia In Siphonocladus, segregative cell division takes place from unicellular young thalli constituted by a coenocytic vesicle. In the process of segregative cell division, cell content is divided to form spherical vesicles that secrete an enveloping membrane. The vesicles increase in size until they contact each other, finally producing lateral protrusions which grow to form a thallus with a branched appearance. The mature thallus consists of an erect axis with lateral branches The order Siphonocladales includes multicellular algae that presents a characteristic type of cell division known as segregative cell division. These organisms are wholly marine, and are usually tropical. The cells are multinucleate, with reticulate chloroplasts. Their chloroplasts contain a particular xanthophyll, called siphonaxanthin. Sexual reproduction is by isogamous in most cases. Siphonocladus, Anadyomene and Valonia are the most common genera of this order. Segregative cell division in Siphonocladus tropicus: (a) germling; (b) cytoplasm in spherical masses; (c) expansion of cytoplasmic masses; (d) lateral branches forming; (e) mature thallus. Siphonocladus Valonia aegagropila (note segregative cell division) The main characteristics that distinguish the order Trentepohliales from other green chlorophycean algae are as follows: Differentiated reproductive cells; The presence of β-carotene and haematochrome (i.e., astaxanthin), which color the algal thallus yellow, orange, or red; The absence of a pyrenoid in the chloroplasts; Unique flagellar apparatus; Transverse cell walls with plasmodesmata. The terrestrial order Trentepohliales consists of a single family, the Trentepohliaceae, with six genera. These algae are not aquatic, but rather subaerial, growing on humid soil, rocks, buildings, tree bark, leaves, stems, and fruit. Some species are endophytic or parasitic, whereas others grow in close association with fungi, forming lichens. The cells are uni- or multinucleate, with several parietal chloroplasts that can be discoid or band-shaped, sometimes appearing reticulate. Most trentepohlialean genera develop a filamentous structure that forms either uniseriated, branched, erect tufts or laterally coherent, prostrate discs. Others are highly reduced and produce only a short vegetative filament a few cells in length. Reproduction occurs by asexual quadriflagellate zoospores or sexual biflagellate gametes. Discoid thallus in Phycopeltis sp red branched filaments of Trentepohlia sp. ١١

12 Charophyceae It is believed that this is the line of algal evolution that led to the development of land plants (embryophytes). The algae in the class are predominantly freshwater algae. Walls of the Charophyceae are composed of cellulosic microfibrils, and most are heavily calcitied, hence the common name 'stoneworts'. The motile cells of the Charophyceae are asymmetrical and have two laterally or subapically inserted flagella. The microtubular root system contains a multilayered structure (MLS) that is associated with a broad microtubular root and a second, smaller, microtubular root. Rhizoplasts (fibrous roots) are not present. scales common outside of motile cells; The mitotic spindle is persistent during cytokinesis, and cell division occurs by means of a phragmoplast producing new cross walls. No eyespots occur. Sexual reproduction results in the formation of a dormant zygote. Meiosis occurs when the zygote germinates. Glycolate is broken down by glycolate oxidase, whereas urea is broken down by urease. Economic Importance The economic importance of the Trentepohliales can be both positive and negative. Negative factors are Damage to buildings (stone and concrete). Trentepohlialean algae are accounted among the factors responsible for the progressive mechanical degradation of buildings, or biodeterioration and biocides for the control have been evaluated. Damage to economically important plants. Cephaleuros infections can cause death (necrosis) of the cells just beneath the algal thallus and perhaps injure the host plants. Positive factors The use of the trentepohlialean biodiversity as bioindicator of environmental conditions. Pollutants in the air have an effect on the algal biodiversity. Source of carotenoids. Commercial production of carotenoids is currently based on Dunaliella, a green alga of saline habitats, but recent research established Trentepohlia as a potential source for carotenoids since it appears to accumulate a larger quantity. Klebsormidiales The algae in this order are terrestrial or freshwater algae with unbranched filaments that do not have holdfasts. Each cell contains a single parietal chloroplast. There are no plasmodesmata between cells. The zoospores are naked and are released through a pore in the wall. Sexual reproduction is isogamous by biflagellate gametes. Algae definitely placed in this order on the basis of their cytology include Stichococcus (Fig. a), Klebsormidium (Fig. b), and Raphidonema (Fig. c). Classification Within the Charophyceae are four important orders: Order 1 Klebsormidiales: unbranched filaments without holdfasts; plasmodesmata absent; zoospores naked and released through a pore in the wall. Order 2 Zygnematales: sexual reproduction by conjugation; unicells or unbranched filaments without holdfasts; plasmodesmata absent in filamentous forms; flagellated cells not produced. Order 3 Coleochaetales: oogamous sexual reproduction; motile cells have a covering of scales, sheathed setae present; unicells, branched filaments or discoid thalli. Order 4 Charales: oogamous sexual reproduction; sterile cells surround antheridia and oogonia; male gametes covered with scales; zoospores not produced; complex plant body with apical growth and differentiation into nodes and internodes; plasmodesmata present. There are basically three types of chloroplasts in the order: (1) spirally twisted bands extending the length of the cell as in Spirogyra and Spirotaenia; Spirogyra Spirotaenia (2) an axial plate extending the length of the cell as in Mougeotia and Mesotaenium. In these algae, there is a marked chloroplast orientation in response to light intensity. Zygnematales The Zygnematales are a closely related group of freshwater algae that are unicells or unbranched filaments without holdfasts plasmodesmata absent in filamentous forms flagellated cells not produced. Zygnematales are unique among the Chlorophyta in having sexual reproduction by isogamous conjugation in which the gametes are nonflagellated. The union of the two gametes can be through a conjugation tube formed by the parent cells, or the gametes can move from their parent cells into the medium and fuse. The zygote or zygospore forms a cell wall and goes through a resting period before germinating meiotically. The life cycle is therefore primarily haploid, with the zygote representing the diploid generation. Zygnematalean algae, particularly Mougeotia, dominate freshwaters affected by acid precipitation. Mougeotia Mesotaenium Cylindrocystis (3) two stellate chloroplasts next to each other as in Cylindrocystis ١٢

13 Family 2 Mesotaeniaceae: basically unicellular nonfilamentous; and the cells taper to their ends; cell walls without pores; no new semicell formed in cell division. The nucleus is central in the cell, and there are three types of chloroplast structure, which are similar to those in the Zygnemataceae. Mesotaenium, Cylindrocystis, and Spirotaenia are in the family. These organisms are indicative of water low in magnesium and calcium and therefore usually unpolluted water. Within the Zygnematales there are three families, Zygnemataceae, Mesotaeniaceae and Desmidaceae (some phycologists group the last two families into one family). Family 1 Zygnemataceae: cylindrical cells united permanently into unbranched filaments; cell wall without pores. Spirogyra, Mougeotia and Zygogonium are members of this family. Members of the Zygnemataceae are among the most common filamentous freshwater algae, favoring small stagnant bodies of water, but with a few found attached in the littoral zone of lakes (Spirogyra adnata) and in flowing water. They are especially abundant in the spring months, generally occurring as bright green free-floating masses, with some type of attaching organ present in young stages of several genera. Spirogyra Mougeotia (a) Mesotaenium de greyi, high- and low-intensity orientations of the chloroplast. (b) Spirotaenia condensata. (c) Cylindrocystis brebissonii. Zygogonium Coleochaetales The algae in the Coleochaetales are characterized by the presence of sheathed setae and by oogamous sexual reproduction. They have asymmetrical motile cells covered with scales, microtubular roots consisting of a large and small band, a persistent interzonal spindle, and a phragmoplast at cytokinesis; glycolate is degraded by glycolate oxidase. Some of the species in the Coleochaetales have evolved a protective sheath around the oogonium. Family 3 Desmidaceae: basically nonfilamentous; cell walls with pores; they have two distinct halves or semicells separated by a median constriction; new semicell formed in cell division. The cells can be solitary, joined end to end in filamentous colonies or united in amorphous colonies. Some of these plants have a relatively complex and attractive shape. The desmids are usually indicators of relatively unpolluted water, low in calcium and magnesium, with a slightly acidic ph. Normally a large number of species are present in such waters, without a single species comprising most of the population. (a) Closterium moniliforme. (b) Euastrum affine. (c) Micrasterias radiata. (d) Xanthidium antilopaeum. (e) Staurastrum curvatum. (f) Spondylosium moniliforme. (g) Pleurotaenium nodosum. Reproduction is oogamous, with sterile cells surrounding the antheridia and oogonia. The male gametes (the sperm), bear two somewhat unequal flagella, subapically inserted and covered by scales. The sperm body is divided into three regions (head, middle and tail regions) and is elongate. No asexual zoospores are formed. Cell division occurs by a phragmoplast, resulting in the formation of a cross wall with plasmodesmata. Only one flagellar root occurs, which is generally associated with a multilayered structure (MLS). Cell walls are composed of cellulose and are frequently heavily calcified, this feature leads to the deposition marl (CaCO3 and MgCO3 deposits) in freshwater habitats where Charales abound (such as the bottom of lakes) and hence the common name of the Charales, stoneworts. Charales The algae in the Charales are primarily freshwater, with only a few species occurring in brackish water. They are most common at the bottom of clear lakes, usually forming extensive growths. The green algae in the Charales have a complex plant body with apical growth and differentiation of the body into nodes and internodes. Each node bears a whorl of branches composed of a number of cells that cease to grow after they have reached a certain length. The internode consists of a single long cell in Nitella. In some genera, such as Chara, the internodal cell is ensheathed (corticated) by a layer of vertically elongated cells of a much smaller diameter originating at the nodes. The axis is attached to the substratum by uniseriately branched rhizoids. ١٣

14 The sexual organs are the globule (male) and nucule (female). Globules and nucules are borne on nodes, usually on the same plant. In Chara, the nucule is above the globule. Commercial exploitation of microscopic green algae Commercial exploitation of microscopic green algae comprises relatively few chlorophycean genera among which there are Chlorella, Dunaliella and Haematococcus. Dunaliella spp. from hypersaline environments have been extensively studied, cultivated and commercialized as a source of natural -carotene. Haematococcus Agardh. Haematococcus is a freshwater unicellular alga with ovoid cells actively motile by two smooth apical flagella. The cell content is usually green with a small part of the cell orange-red pigmented owing to the accumulation of the ketocarotenoid astaxanthin in globules outside the chloroplast. As the conditions become unfavourable, green and motile cells round off, then produce thicker walls, loose the flagella and finally cease to swim. In these cysts the protoplast fills the whole cell and a great increase in cell dimension occurs together with a change of pigmentation from green to orange-red, due to the increase of astaxanthin deposition. When the conditions become favourable for growth, the cysts germinate releasing a large number of new motile cells. The astaxanthin represents a high valuable product not only as a colouring agent for fishes and crustaceans in aquaculture but, since it is a potent natural antioxidant, it may also find important applications in medicine. Chlorella Beijerinck. Chlorella is a cosmopolitan genus with small, unicellular, ovoidal nonmotile cells; it does not produce zoospores. Cells have a thin cell wall, and cup-shaped chloroplast. Pyrenoid may be present. The accumulation of starch occurs within the chloroplast. Chlorella reproduces by forming daughter cells or autospores (4 8 16) of the same shape as the parent cell. It grows in autotrophic, heterotrophic and mixotrophic conditions. Besides autotrophic strains, heterotrophic strains are also cultivated. Chlorella is the most important species in the microalgal industry; it is cultivated and sold essentially as health food. Botryococcus braunii was proposed and cultivated as a renewable source of liquid fuel, owing to the high hydrocarbon content. Cells are uninucleate and typically contain numerous ellipsoidal chloroplasts arranged in twisted, longitudinal, parallel rows. In the center of the cells is a single large vacuole. Cytoplasmic streaming is particularly evident in the large internodal cells. The cytoplasm is divided into an inner endoplasm next to the vacuole, and an outer ectoplasm containing the chloroplasts. Streaming endoplasm follows the orientation of the spirally arranged stationary chloroplasts in the ectoplasm at a rate of 60 µm s -1 at 20 C. Streaming is due to the interaction of actin microfilaments with cytoplasmic microtubules Schematic view of part of an internodal cell of Nitella. The chloroplasts are stationary in the ectoderm, whereas the endoderm moves by cytoplasmic streaming. ١٤