Free-living Phagotrophic Protists. Introducing the role of remineralization and placing protists in a food web context

Transcription

1 Free-living Phagotrophic Protists Introducing the role of remineralization and placing protists in a food web context

2 Tuesday: Feeding selectively Model Protist: Ingestion (methods) and Metabolism Today: Last few slides: Metabolism -- growth efficiency and temperature effects Continue with: 1) Excretion, Remineralization in the Model Grazer 2) How do protists link to higher trophic levels? Size considerations... 3) Metazoans that directly impact the microbial food web

3 Metabolism: Excretion/Remineralization New Handout...

4 Protist Excretion Grazing of flagellate on diatom No bacteria With bacteria diatom abundance flagellate abundance particulate nitrogen NH 4 & urea Goldman & Caron 1985

5 Factors affecting Excretion Rates Ciliate feeding on bacteria Flagellate feeding on bacteria Ferrier-Pages & Rassoulzadegan 1994

6 Why waste that food? Concept: Organisms tend to retain the nutrients that are limiting to growth and excrete the nutrients that are available in excess. Underlying assumption: Organisms try to maintain a constant stoichiometry of elements, such as C, N, P, in their own cells through conserving or excreting/egesting food.

7 Prey (food) Quality Matters Paraphysomonas (flagellate) fed phytoplankton grown in chemostats GGE (G/I) Caron et al n.b., GGE s probably 10-15% high, because of re-ingestion

8 Elemental Ratios Organism C:N N:C Bacteria Phagotrophic Protist Algae (healthy)* Algae (N-limited) *healthy ratio from Redfield ratio N:C (predator) < N:C (prey), then excess N excreted e.g., protozoa feeding on bacteria excrete excess N, but if feeding on algal cell would conserve N.

9 Relative Importance of Microbial Loop Organisms with regard to Remineralization Who are the important nutrient cyclers? An example: DOM BACT ALGAE Parameter BACT ZOOFL CILIATE Ingestion(C) Respiration (C) AE GGE C:N(prey) C:N(pred)

10 DOM Uptake by Bacteria Bacteria C:N = 5, DOM C:N = 6.6 (from algae) Bacteria are generally not efficient recyclers of nutrients (Nitrogen). High GGE, C:N(bact) << C:N(DOM)

11 Impact of Bacterial GGE? Cycling of DOC during diatom blooms: two examples Bacterial GGE Kirchman et al HMW: mainly polysacharides (high C:N) LMW: mainly amino acids (low C:N) Amon & Benner 1994

12 Food (DOM) quality matters Bacteria grown on substrates, ranging in C:N from At C:N > 6 (~algal C:N), GGE = 40-50%, Nitrogen regeneration: 0-20% Bacteria are significant respirers of C, but if DOM poor in N, they have little role in N remineralization

13 Protists feeding on bacteria Protist C:N = 5.6, Bacteria C:N = 5 Bacterivores expected to excrete and egest a significant portion of consumed food. Protist C:N > Bacteria C:N

14 Protist feeding on Algae Protist C:N = 5.6, Algae C:N = 6.6 Protist herbivores would be expected to excrete and egest less, because their elemental ratio is closer to that of their prey.

15 Protist Egestion: Contribution to DOM pool Strom et al Protist Grazer: in all experiments, more DOC produced in presence of grazing % of algal carbon released as DOC (30% carbohydrates) Copepod Grazer: in 2/4 experiments, more DOC produced in presence of grazing

16 Summary Role of organisms as nutrient remineralizers 1) low GGE increases with 2) low C:N(prey) relative to C:N(pred) 3) small size = high specific rates - because larger organisms (metazoans) also have to fuel metabolic products into reproduction - also, sinking velocity of fecal material decreases with small size

17 Phagotrophic Protists as Links to Higher Trophic Levels Bigger organisms tend to feed on smaller ones... 10:1 predator:prey size ratios Filter feeders vs. Direct Interception feeders Metazoan predators that are important microbial loop consumers vs. those that aren t Implications for food web efficiency Classic vs. Microbial Food Web High vs. Low Energy Ecosystems

18 Prey:Predator Size Ratios All organisms feed selectively, the optimal range of prey being determined by: Sensory mechanisms & thresholds for detecting prey Physical constraints on contact (encounter) frequency Minimum size that can be effectively captured/handled Maximum size that can be effectively captured/handled

19 Feeding Mechanisms: Searching for Food Increasingly complex sensory & capture mechanisms are required to offset the fact that organism mass increases at a faster rate than surface area Flagellates Ciliates Crustacea Fish Direct Contact Filter feeding Structures Chemo & Mechanical Receptors Visual mm very large Body Size (length) Radiolarians Strategy: increase surface are at expense of mass & organizational complexity Salps Baleen whales behavior: exploit patches Karen figure after Selph, a drawing OCN 626, by M. Fall Landry,pictures 2009 from J. Drazen (fish), D. Keith (salp), K. Sime (whale), (copepod), R. Patterson (flagellates)

20 Food particle size as a function of predator size line = 1:10 food:predator size filled circles: filter feeders open circles: direct interception feeders Fenchel 1986

21 Protist optimum Prey Selection: Is it 10:1? Hansen et al. 1994

22 Dinoflagellate s place in food webs 1) predators of autotrophs 2) prey of mesozooplankton 3) predators of juvenile/naupliar mesozooplankton 4) predators and prey of other HTD

23 Metazoans: Raptors above/on the 1:10 line ambush predators/raptors raptorial feeders

24 Metazoan predators: Crustaceans -- hard-bodied (chitin exoskeleton) with specialized, segmented feeding, swimming, and sensory appendages. Responsive to mechanical, chemical and light cues. Daily vertical migration in many forms. Copepods: most numerous multicellular animals. Many species benthic or parasitic. Most are <3 mm, the largest free-living form ~16 mm. Suspension feeding via water currents generated by mouth parts, particles strained by appendages with fine setae. Raptorial feeding by forms with larger, less setose maxillae and maxillipeds for grasping and manipulating prey. Most pelagic forms omnivorous. Euphausiids: stalked compound eyes, shrimp-like body. Omnivorous, some suspension feed on larger phytoplankton, highly motile, some exhibit schooling (e.g., krill in Antarctic waters). Amphipods: sessile compound eyes, legs usually modified for grasping. Most benthic, most open ocean forms live on, or in association with, gelatinous zooplankton. Most are predators. The crustaceans also include shrimp, crabs, lobster and many other forms that are planktonic only as larvae (meroplankton, as opposed to holoplankton - plankton for entire life). Photo credits: copepod: euphausiid: Uwe Kils, amphipod: shrimp larvae: geochange.er.usgs.gov/sw/impacts/biology crab larvae: ibss.iuf.net/people/skryabin/merop.html lobster larvae: Russell Bradford, CSIRO

25 Crustacean feeding appendages filter feeder: crushing mandible, fine hairs on appendages e.g., Calanus sp. Filter feeder Cutting edge of mandible Predator predator: slicing mandible, no hairs on appendages e.g., Euchaeta sp.

26 Why they don t feed on small organisms Herbivorous copepod, Acartia clausi ~15 µm ~7 µm ~5 µm Florian Hantzshe Nival & Nival 1976

27 Metazoan Predators: Chaetognaths -- arrow worms with elongated body separated into head, trunk, and tail. Exclusively marine. Head with paired eyes and prehensile, chitinous jaws with hooks. Ambush predator, responds to mechanical cues, swallows prey (copepods, fish larvae, other chaetognaths) whole. Hermaphroditic, testes in tail, ovaries in trunk. chaetognath: R. Hopcroft, Univ. Alaska - Fairbanks chaetognath head: chaetognath with copepod: Jean-Marie Cavanihac, Karen Selph, OCN 626, Fall uk.org

28 Metazoans: Filter feeders below 1:10 line filter feeders microbial loop organisms

29 Gelatinous Zooplankton: grouped by morphological traits, not genetic relatedness, bodies with high water contents, secrete mucus nets so that animal is effectively bigger and can capture more food Medusae: carnivores, food captured with tentacles armed with paralyzing nematocysts. Siphonophores, including the Portugese Man-of-War, are particularly significant in oceanic waters. Ctenophores: comb jellies, characterized by bands of joined cilia around body surface. Tentaculate forms have 2 tentacles with special sticky cells to entangle prey; Lobate types capture prey contacting their large lobed feeding surfaces. Salps: Barrel-shaped animal with muscle bands that contract to force water into a buccal opening and out of an atrial opening. Ciliary-mucus filter feeders - particles in the water current entering the buccal opening are captured on a net of mucus strands. Chiefly oceanic -- sometimes occur in dense swarms. Photo credits: ctenophores: & Marsh Youngbluth, life.bio.sunysb.edu/marinebio/plankton medusae: & salps: &

30 jellyzone.com Appendicularians (Hemichordata) a.k.a. larvaceans -- mature forms retain appearance of tadpole chordate larvae, head with tail. Body enclosed in a feeding house. Undulations of tail cause water to enter house through coarse filter where fine particles are concentrated. The house is abandoned periodically and a new house is built. Old houses are important component of marine snow. Because of house mesh size, potentially important as grazers Karen of Selph, picoplankton OCN 626, Fall 2009

31 Short-circuits vs. Baby-steps effect on food web Filter feeders that Short-circuit the food chain: Appendicularians (1-10 mm) Bacteria & Sm. flagellates Salps (~10 cm) Sm. flagellates Neocalanus spp., SubArctic Pacific (5-10 mm) 2 µm+ Anchovetta (10+ mm) Diatoms Baleen Whales (10s of meters) Krill Raptorial feeders that reduce food web efficiency: Paraphysomonas (flagellate) Diatoms (1/2 size) Didinium (ciliate) Paramecium (ciliate) (equal size) Oblea (dinoflagellate) Lg. Diatoms (e.g., Ditylum) Pelagic Foraminifera Copepods Lg. Predatory Copepods Copepods Sharks Other large prey

32 Classic View of simple, linear food chain: diatoms copepods fish

33 Trophic Cascades: this model of ecosystems hypothesizes that, by their presence or absence, higher trophic levels will determine whether or not blooms will occur at the base of the system (as opposed to just resource limitation/sufficiency) Location: Station ALOHA Manipulation: Bacterial net growth in a size-truncated food web Implied Grazer Chain: 2-5 µm HF 5-20 µm HF Bacteria Calbet & Landry 1999 Western Arabian Sea plankton: Reckermann & Veldhuis 1997

34 OLIGOTROPHIC (Low Latitude) 0.06 Ocean Ecosystems Divided 6 EUTROPHIC (High Latitude) 20 Fish (GGE = 0.2) 0.3 FISH MacroZP (GGE = 0.2) MesoZP (GGE = 0.2) 6.3 Ciliate (GGE = 0.3) 21 Flagellate (GGE = 0.3) 20 COPEPODS 100 PHYTO= Bacteria (GGE = 0.4) Phyto = 100 FISH 100 PHYTO=100 (UPWELLING)

35 Low Energy Stable Systems Low energy Lack of nutrient re-supply Low nutrients (oligotrophic) Small Phytoplankton (high surface:volume ratio) Long food chains (small consumers at base) Relatively stable system

36 High energy (storm activity, eddy action, upwelling, etc.) High Energy Unstable Systems High nutrients (eutrophic) Large Phytoplankton (small, too!) Composite Spring Picture Mean Chlorophyll (µg/l) at the surface Short food chain (dynamic) (superimposed on stable long food chain) Unstable (dynamic) system

37 Macro+ Meso Autotrophic Food Web Structure Heterotrophic Fish Copepods eutrophic Micro diatoms & dinoflagellates Ciliates Nano oligotrophic Pico a-flagellates cyanobacteria Mixotrophs flag level 3 zooflagellates level 2 flag level 1 H-Bacteria viruses DOM Nutrients