Water-column stratification governs the community structure of subtropical marine picophytoplanktonemi4_

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1 Environmental Microbiology Reports (2011) doi: /j x Water-column stratification governs the community structure of subtropical marine picophytoplanktonemi4_ Heather A. Bouman, 1 * Osvaldo Ulloa, 1 Ray Barlow, 2 William K. W. Li, 3 Trevor Platt, 3,4 Katrin Zwirglmaier, 5 David J. Scanlan, 5 and Shubha Sathyendranath 4 1 Departamento de Oceanografía and Centro de Investigación Oceanográfica COPAS, Universidad de Concepción, Casilla 160-C, Concepción, Chile. 2 Marine and Coastal Management, Private Bag X2, Rogge Bay 8012 Cape Town, South Africa. 3 Bedford Institute of Oceanography, Dartmouth, Nova Scotia B2Y 4A2, Canada. 4 Plymouth Marine Laboratory, Prospect Place, The Hoe, Plymouth PL1 3DH, UK. 5 Department of Biological Sciences, University of Warwick, Coventry CV4 7AL, UK. Summary The increase in the areal extent of the subtropical gyres over the past decade has been attributed to a global tendency towards increased water-column stratification. Here, we examine how vertical stratification governs the community structure of the picophytoplankton that dominate these vast marine ecosystems. We analysed phytoplankton community composition in the three Southern Subtropical basins of the Pacific, Indian and Atlantic Oceans using a variety of methods and show that the distributions of picocyanobacteria and photosynthetic picoeukaryotes (PPEs) are strongly correlated with depth and strength of vertical mixing: the changes in community structure occur at various taxonomic levels. In well-mixed waters, PPEs, in particular haptophytes, dominate, whereas in strongly stratified waters, picocyanobacteria of the genus Prochlorococcus are prevalent, regardless of whether the relative contributions to total biomass are assessed in terms of pigment or of carbon. This ecological diochotomy within the picophytoplankton supports the hypothesis that genomic streamlining provides a selective advantage for Prochlorococcus in highly stable, oligotrophic systems, but may restrict their ability to dominate in regions subject to dynamic mixing. Received 20 June, 2010; accepted 26 October, *For correspondence. heather.bouman@earth.ox.ac.uk; Tel ; Fax Present address: Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK. Introduction Owing to their large size, the subtropical gyres contribute a significant fraction to the total primary production of the global ocean (Karl, 1999). In the past, they were considered to be static, oceanic deserts. But over the last few decades this paradigm has been challenged and it is now believed that physical forcing events occurring over a variety of time scales (days to decades) can cause marked changes in the taxonomic structure and biogeochemical function of subtropical gyre communities (Karl, 1999; Letelier et al., 2000; Karl et al., 2001; Corno et al., 2007). Analysis of surface chlorophyll data detected by earthorbiting satellites over the past decade reveals that the oligotrophic gyres are expanding in areal extent, and this trend has been attributed to an increase in stratification of the surface ocean (Gregg et al., 2005; Polovina et al., 2008; Irwin and Oliver, 2009). Global warming is expected to enhance vertical stratification especially in the tropics and subtropics (Sarmiento et al., 2004). It is important to determine the impact of increased stratification not only on the quantity of phytoplankton pigment, but also on the community structure of the phytoplankton. Numerically, as well in biomass units, picophytoplankton, cells less than 3 mm in diameter, dominate the subtropical gyres. This size class includes both prokaryotes (represented by cyanobacteria) and eukaryotic cells, and it is believed the fate of carbon fixed in the upper ocean depends on which of these two groups dominate (Karl et al., 2001; Corno et al., 2007). Cyanobacteriadominated systems tend to be more efficient at recycling carbon within the surface ocean, whereas eukaryotic cells are believed to play a significant role in carbon export (Corno et al., 2007). Results and discussion Since the Blue EArth GLobal Expedition (BEAGLE) sampled each subtropical basin during a different season (Bouman et al., 2006), a range of environmental conditions in the surface mixed layer was observed. For example, within the South Pacific Subtropical Gyre (SPSG) province, where deep winter mixing is known to occur at latitudes south of 30 S (Longhurst, 1998), mixedlayer depths were matching and in some cases exceeding 2011 Society for Applied Microbiology and Blackwell Publishing Ltd

2 2 H. A. Bouman et al. the euphotic depth and temperatures averaged 16.8 C. In contrast, sampling in the Atlantic Subtropical Gyre province occurred after the onset of vertical stratification, resulting in a markedly shallower mixed layer and intermediate temperatures of around 20.0 C. In the Indian Ocean, the strongly stratified summer conditions led to shallow and strongly stratified mixed layers with high surface temperatures, averaging 26.4 C. We examined the change in phytoplankton community structure within the surface layer (top 10 m) of the Southern subtropical gyres using both pigment markers and dot-blot hybridization technology. Although the two methods are able to determine community structure of natural assemblages in an efficient manner, both techniques have their potential biases. Molecular probes used may not be broad enough to detect all members of the target population and, in the case of pigment markers, many diagnostic pigments span more than one algal class, and therefore interpretation of pigment data must account for their complex distribution among algal taxa (Wright and Jeffrey, 2006). Despite the potential shortcomings of specificity of both pigment markers and molecular probes, they offer the best approach of mapping the relative abundance of phytoplankton groups. Variability in picophytoplankton community structure across gradients of vertical mixing Prokaryotes versus eukaryotes. We examined the change in picophytoplankton community structure within the surface layer by comparing the abundance of picophytoplankton groups, starting at the level of cyanobacteria versus eukaryotes at different stations spanning the three subtropical basins of the Southern Hemisphere. Figure 1 shows that the abundance of picocyanobacteria obtained by flow cytometry counts was negatively correlated with nutrient concentration (NO 3 + NO 2) and the ratio of the depth of the mixed layer to the euphotic depth (zm/zeu) that Fig. 1. Relationship between the abundance of picocyanobacteria and photosynthetic picoeukaryotes and environmental variables: nitrate and nitrite concentration, temperature, the mixed layer depth (z m) divided by the depth of the euphotic zone (z eu), and Brunt Väisälä frequency (N 2 ) along the BEAGLE transect spanning the three subtropical basins (Pacific, Atlantic and Indian Oceans). Measurements of phytoplankton abundance were made using a Becton Dickinson FACsort flow cytometer as described in Li (1995). Cell abundances of the three picophytoplankton (< 3 mm) groups Prochlorococcus, Synechococcus and photosynthetic picoeukaryotes were obtained for each surface sample. The depth of the mixed layer (z m) and the strength of the vertical density gradient (N 2 ) were calculated from vertical profiles of density measured using a conductivity temperature depth (CTD) sensor (Sea-Bird Electronics, USA). The z m was defined at the depth at which r decreased (in sigma-t units) from its surface reference value. Nitrate (NO 3- ) and nitrite (NO 2- ) concentrations were obtained using a modification of the method of Grasshoff (1970), as described in Uchida and Fukasawa (2005a,b).

3 Effect of stratification on marine picophytoplankton 3 may be considered an index of light availability in the mixed layer, and positively correlated with the sea surface temperature, and the strength of water-column stratification (N 2 ). Photosynthetic picoeukaryote (PPE) abundances showed the opposite relationship: abundances were highest in the deeply mixed, weakly stratified water column with moderate to high concentrations of inorganic nitrogen. These patterns were verified using pigment indices for the relative contribution of cyanobacteria and eukaryotes to total chlorophyll a biomass (Fig. 2). The photoprotective carotenoid zeaxanthin (Zeax), a diagnostic pigment of marine cyanobacteria, was dominant in highly stratified conditions whereas the photosynthetic carotenoids 19 -hexanoyloxyfucoxanthin (19 -Hex) and 19 - butanoyloxyfucoxanthin (19 -But), pigment markers for Haptophyceae and Chrysophyceae, respectively, constituted most of the accessory pigment complement in the well-mixed stations (Fig. S1). Note that in the hyperoligotrophic waters of the Indian Ocean, the diagnostic Fig. 2. Relationship between the percentage contribution of picocyanobacteria and eukaryotes to total chlorophyll a concentration and environmental variables described in Fig. 1. High performance liquid chromatography pigment analysis was conducted on filtered seawater samples collected within the top 10 m of the water column as described in Barlow and colleagues (2007). Seawater samples were not pre-screened prior to filtration, and thus contribution of larger phytoplankton (e.g. diatoms) was observed in coastal regions. Zeaxanthin (Zeax) is used as a diagnostic pigment of marine picocyanobacteria, and 19 -hexanoyloxyfucoxanthin (19 -Hex) and 19 -butanoyloxyfucoxanthin (19 -But) are used as pigment markers for Haptophyceae and Chrysophyceae respectively. For detailed information on diagnostic pigments used and their taxonomic specificity please see Appendix S1 and Table S1.

4 4 H. A. Bouman et al. pigment associated with chrysophytes (19 -But) was below the limit of detection. It is likely that this was a function of the low biomass limiting our ability to detect the less abundant photosynthetic carotenoids and does not imply that this group is absent in these waters. Ordination analysis of phytoplankton types provides an objective representation of the relationship between phytoplankton community structure and the marine environment (Margalef, 1979). Canonical correspondence analysis (CCA; Ter Braak, 1994) was used to map phytoplankton community structure in ecological space. Environmental variables explained 70% of the variation in the flow cytometry and HPLC pigment data, of which 67% was explained by the first two axes (Fig. 3A). Stations on the left of the plot represent well-mixed assemblages, acclimated to low light intensities and high nutrient conditions with moderate to high (meso-to-eutrophic) chlorophyll a concentrations. The fractions of the diagnostic pigments associated with haptophytes (19 -Hex), chrysophytes (19 -But) and chlorophytes (chlorophyll b) are high in this well-mixed, mesotrophic regime. Stations to the right of the origin are strongly stratified (high surface temperatures and high Brunt Väisälä frequencies), are nutrient poor and have low pigment biomass (oligotrophic). In this ecological space, diagnostic pigments associated with cyanobacteria (Zeax) and Prochlorococcus (divinyl chlorophyll a, DV-Chla) are dominant, and the abundance of Prochlorococcus cells is high. The additional variation explained by Axis 2 tends to separate the eutrophic, upwelling stations from those sampled in more mesotrophic conditions. Picocyanobacteria. Canonical correspondence analysis (Ter Braak, 1994) was also used to examine relationships between environmental variables and the phylogenetic composition of picocyanobacterial communities. Indices of community structure used in this analysis were the relative abundance of Prochlorococcus ecotypes and Synechococcus clades determined by dot blot hybridization (Fig. S2). Environmental variables explained 74% of the variation in the data, with 60% explained by the first two axes (Fig. 3B). Similar to Fig. 3, stations located to the left of the origin represent well-mixed assemblages. Synecho- A B Fig. 3. A. CCA ordination diagrams of percent contribution of phytoplankton groups to total carbon and chlorophyll a biomass with corresponding environmental variables: T = temperature, N 2 = Brunt Väisälä frequency, DV/cell = DV Chl-a per Prochlorococcus cell, z m/z eu = ratio of mixed layer depth to photic depth. Euk = picophytoeukaryotes, Chrys = chrysophytes, Hapt = haptophytes, Chlor = chlorophytes, Pro = Prochlorococcus, Syn = Synechococcus, Cyan = Cyanobacteria (Pro+Syn). To estimate the contribution of these three picophytoplankton groups to carbon biomass, carbon-conversion factors for Prochlorococcus, Synechococcus and picophytoeukaryotes were applied to cell counts as described in Grob and colleagues (2007). We adopted the approach of Uitz and colleagues (2006) and Liu and colleagues (2009) to assess the contribution of various phytoplankton taxa to total chlorophyll a concentration using pigment markers (see Appendix S1). Red arrows and labels denote fractional contribution of phytoplankton groups to total phytoplankton carbon by flow cytometry. Blue arrows and labels denote relative contribution of phytoplankton to total chlorophyll a concentration determined by HPLC analysis. Yellow triangles denote coastal stations; white circles denote open ocean stations. B. Percentage relative hybridization of picocyanobacterial lineage-specific probes plotted against the same environmental variables as (A). Arrows pointing in roughly the same direction indicate a high positive correlation, arrows crossing at right angles indicate a near-zero correlation and arrows pointing in the opposite direction have a high negative correlation.

5 Effect of stratification on marine picophytoplankton 5 coccus clade II was dominant in the eutrophic upwelling station, whereas clades I and IV were abundant in the well-mixed offshore stations. These results are consistent with a global synthesis of Synechococcus biogeography (Zwirglmaier et al., 2008), which shows that clade II tends to dominate in low-latitude (tropical/ subtroptical) coastal regions. At latitudes greater than 30, this group is virtually absent, which suggests that temperature may play a role in governing its distribution. The high-light (HL) adapted HLII Prochlorococcus ecotype favoured strongly stratified waters, whereas the high-light adapted subclade HLI tended to co-dominate with other Synechococcus clades in waters of intermediate mixing. Low-light (LL) adapted ecotypes of Prochlorococcus tend to fall between the strongly mixed and well-stratified regimes. Stratification and its impact on picophytoplankton community structure Water-column stratification has been implicated as the principal factor governing the seasonal succession of marine phytoplankton species in coastal, temperate waters (Cushing, 1989). The link between the physical environment and phytoplankton community structure was conceptualized by Margalef s phytoplankton mandala, which separates the dominant groups of phytoplankton based on the levels of turbulence and nutrient availability (Margalef, 1979; Cullen et al., 2002). Although Margalef s conceptual model focused principally on the large phytoplankton (diatoms and dinoflagellates), it has been shown in other studies that temperature and water column stability explain the spatial and temporal patterns in the overall size structure of phytoplankton communities (Li, 2002; Bouman et al., 2003). Another proxy of water column stability, nitricline depth, has also been shown to explain basin-scale changes in the relative contribution of diatoms (microphytoplankton) and coccolithophores (nanophytoplankton) to phytoplankton biomass (Carmeño et al., 2008). Yet, in the open ocean, it is the picophytoplankton size class that dominates the phytoplankton standing stock. Our results show that shifts in the community structure of marine phytoplankton in response to vertical stratification are observed over a variety of taxonomic levels, as shown by the relative contribution of picocyanobacteria to total phytoplankton pigment biomass, to within the picocyanobacterial population, which is quantified using flow cytometry and molecular probes. The strong correlation between upper ocean stratification and the relative contribution of cyanobacteria to phytoplankton standing stock is consistent with observations from the ALOHA timeseries station (Corno et al., 2007). Matsumoto and Ando (2009) also show a strong correlation between the ratio of Zeax (a pigment marker of the picocyanobacteria Prochlorococcus and Synechococcus) to chlorophyll a (found in all phytoplankton groups) and direct measurements of turbulent kinetic energy, although in our study the change in Zeax:Chl-a is likely due to both the photoacclimatory status of Prochlorococcus and compositional change in the phytoplankton assemblage. The increase in the relative abundance of eukaryotes with increased mixing of the surface layer is also in agreement with other studies that show picoeukaryotes being better adapted to grow in physically dynamic environments than marine cyanobacteria, often exceeding the contributions of both Prochlorococcus and Synechococcus to total picophytoplankton biomass (Lindell and Post, 1995; Campbell et al., 1998; Steinberg et al., 2001). However, deep mixing does not always lead to a domainshift from a cyanobacteria-dominated to a eukaryoticdominated surface assemblage. For example, Letelier and colleagues (1993) examined the pigment composition of surface phytoplankton assemblages over a three-year period in the NPSG and found no significant change in the relative concentration of diagnostic pigments. Apparently, occurrence of a domain-shift depends on the strength of entrainment of nitrate from depth to the mixed layer (Corno et al., 2007). As the nitricline deepens, flux of new nitrogen from wind-induced vertical mixing is reduced, and this favours a Prochlorococcus-dominated system that is well suited to using regenerated forms of nitrogen (Karl et al., 2001). Role of stratification in governing picocyanobacteria phylogeography Our data also show that within the picocyanobacterial population, there is a further ecological partitioning of the two major genera. Prochlorococcus numbers increased with stratification and were negatively correlated with NO 3 + NO 2 concentration, whereas the abundance of its sister taxon Synechococcus shows the opposite, although weaker, trend (Fig. 3A). These relationships are consistent with the observations at Station ALOHA (Campbell et al., 1997) and in the western North Atlantic (Zinser et al., 2007), where Synechococcus tended to dominate during periods of vertical mixing coincident with a shoaling of the nitracline and Prochlorococcus, on the other hand, tended to be more abundant under highly stratified, nutrient-deplete conditions. Several hypotheses have been put forward to explain the ecological success of Prochlorococcus over other picophytoplankton under oligotrophic conditions. Many centre on the notion that Prochlorococcus is better able to acquire nutrients under extreme nutrient limitation. Prochlorococcus has the highest surface area to volume ratio of all marine oxygenic photoautotrophs (Raven et al.,

6 6 H. A. Bouman et al. 2005; Partensky and Garczarek, 2010). It is widely believed that genome streamlining during the evolution of Prochlorococcus has resulted in a minimal cell size that allows them to outcompete other phytoplankton under extreme nutrient limitation (Dufresne et al., 2005). The selective loss of genes has led to the differentiation of two genetically and physiologically defined ecotypes: one adapted to HL and another to LL. Interestingly, when we examine specific genetic lineages of picocyanobacteria, assessed through dot blot hybridization analyses, we observe that the Prochlorococcus HLII ecotype, containing cultured isolates with the smallest genomes, dominate under conditions of strong stratification, whereas those with larger genomes (LL ecotypes and Synechococcus) tend to be more prevalent under conditions of deep vertical mixing (Fig. 3B). The shift towards LL ecotypes and Synechococcus clades with increased surface mixing and thus increased flux of new nitrogen from depth is also consistent with genomic studies, which have shown that many HL cultured isolates of Prochlorococcus are unable to use both nitrite and nitrate as a nutrient source (Moore et al., 2002; Rocap et al., 2003), although there is evidence in metagenomic libraries that uncultured strains of HL and LL Prochlorococcus exist that have nitrite and nitrate assimilation genes (Martiny et al., 2009). Nevertheless, the omission of nitrogen assimilation genes in several Prochlorococcus strains points at the advantages of specialization through genome streamlining to oligotrophic conditions within a stable water column. In addition to influencing the supply of nutrients from depth, stratification also regulates the light environment of cells within the mixed layer. A decrease in the average light intensity in the mixed layer and a change in the spectral quality of the underwater light field may also favour the LL strains rich in divinyl chlorophyll b that are able to harvest low light with great efficiency (Moore and Chisholm, 1999; Moore et al., 1995; Sathyendranath and Platt, 2007; Stomp et al., 2007). Vertical mixing also leads to rapid changes in light intensity. Culture experiments have shown that HL ecotypes can tolerate rapid changes in light better than their LL counterparts (Bailey et al., 2005; Six et al., 2007). However, results obtained in this study show the opposite: that LL ecotypes out-compete their HL counterparts in well-mixed surface waters (see also Bouman et al., 2006). The likely reason for this apparent discrepancy is that the LL strain used in the aforementioned culture studies was SS120: a strain that has been shown to be sensitive to rapid changes in light conditions (Six et al., 2007). However, in the deeply mixed waters of the SPSG, the abundance of SS120 was low based on the weak hybridization of this probe to surface samples (Fig. S2). Zinser and colleagues (2007) found that within the LL clade, there still exists a significant degree of physiological diversity and that some LL strains, such as enatl, can flourish under rapid fluctuations in irradiance, whereas others, such as SS120, cannot. A recent review by Partensky and Garczarek (2010) supports this view: that the LL strain enatl can be considered as an intermediate between the HL and LL ecotypes, possessing many more genes to protect against light and UV damage than other LL strains. Thus, it may be that the enatl clade is dominating the LL-dominated waters in our study. Results from dot-blot hydridization analyses suggest that niche partitioning among Synechococcus lineages may also be linked to stratification (Fig. 3B). Clades I and IV, which tend to be abundant in more productive coastal systems (Zwirglmaier et al., 2007; 2008; Scanlan et al., 2009), were predominant in regions of high nutrient supply and vertical mixing, whereas clade III favours well-stratified oligotrophic waters. Synechococcus clades V VII tend to be present over a wide range of oceanographic conditions and thus cosmopolitan in nature (Zwirglmaier et al., 2008), and this pattern is also evident from the dot-blot hydridization data (Fig. S2). Vertical mixing and the dominance of photosynthetic picoeukaryotes Although PPEs are not as numerous as picocyanobacteria, as a group they can constitute a significant fraction of the global inventory of algal biomass (Grob et al., 2007; Liu et al., 2009) and consequently primary productivity (Li, 1994; Jardillier et al., 2010). The tendency for PPE and Synechococcus to covary and show opposite patterns of abundance with Prochlorococcus exhibited in the BEAGLE data set has been reported across a range of environmental gradients (Campbell et al., 1998; DuRand et al., 2001; Shalapyonok et al., 2001). The high contribution of 19 -Hex, 19 -But and chlorophyll b in the winter-mixed surface waters of the Pacific Basin highlights the importance of the small eukaryotic phytoplankton that harbour these pigments. 19 -Hex is the dominant photosynthetic carotenoid in the open ocean (Liu et al., 2009) and is largely found within the Haptophyta, although may also be present in other heterokont algae (Wright and Jeffrey, 2006). The dominance of chlorophyll b containing PPE at the bottom of the photic zone below the maximum concentration of Prochlorococcus cells suggests that these cells can harvest light with high efficiency (Hickman et al., 2010), which may in part explain the occurrence of this pigment in the well-mixed waters of the Pacific Basin. Using 18S rrna sequences from cells sorted by flow cytometry, Shi and colleagues (2009) found that chlorophyll b containing prasinophytes dominated the more mesotrophic and slightly oligotrophic conditions, whereas chrysophytes tended to dominate in the central gyre. Using dot-blot hybridizations with 16S

7 Effect of stratification on marine picophytoplankton 7 Fig. 4. Genome size for representative phytoplankton classes observed in this study and their corresponding ecological niches. rrna oligonucleotide probes, Lepère and colleagues (2009) found that the classes Prymnesiophyceae and Chrysophyceae dominated over the same transect. Unfortunately, few cultured representatives of the PPE exist to provide insight as to which of their physiological attributes allow these cells to outcompete their prokaryotic counterparts under conditions of turbulence. It could be argued that their phylogenetic diversity and larger genomes may allow this group to rapidly acclimate to a more dynamic environment. In the geological record, the size of diatoms has been shown to be negatively correlated with water-column stability (Falkowski and Oliver, 2007). Given that cell size is generally positively correlated with genome size (Veldhuis et al., 1997; Connolly et al., 2008), it has been proposed that ocean physics can alter the genomic structure of marine eukaryotic phytoplankton (Falkowski and Oliver, 2007). Figure 4 provides a conceptual diagram of how genome size varies among representative phytoplankton classes that were dominant across the sampling transect and their corresponding niches. The trend shows a clear relationship between turbulence and genome size, in particular for the small cyanobacteria, which supports the view that phytoplankton with a minimal genome size tends to dominate in stable, oligotrophic waters where a minimal cell size favours the acquisition of limiting nutrients (Raven et al., 2005). Liu and colleagues (2009) have proposed an alternative explanation for the ecological success of haptophytes: that the cells can adopt a mixotrophic lifestyle under growth conditions that are suboptimal with respect to both light and nutrients. Yet the utilization of organic material to sustain growth cannot explain the ability of PPE to outcompete cyanobacteria, given that picocyanobacteria are also able to supplement their photosynthetic incorporation of inorganic elements with uptake of organic nutrients (Zubkov et al., 2003; Zubkov, 2009). More work on the nutritional modes of marine cyanobacteria and eukaryotic phytoplankton will provide valuable insight into how different trophic strategies are responsible for the large-scale distribution of these two phytoplankton groups. Concluding remarks Although there is an increasing awareness within the ocean modelling community of the importance of partitioning the phytoplankton community into functional types, such models often pool picophytoplankton into a single group (e.g. Le Quéré et al., 2005). However, the fate of fixed carbon in subtropical gyres is believed to be

8 8 H. A. Bouman et al. dependent on whether Prochlorococcus or small eukaryotes dominate the phytoplankton standing stock (Liu et al., 2009). This work shows that the relative importance of prokaryotic versus eukaryotic phytoplankton within the subtropical open ocean is intimately linked to the level of stratification of the upper ocean, which sets the three principal environmental factors governing algal growth (light, temperature and nutrients) within the surface mixed layer. Developing a deeper understanding of the ecological and physiological mechanisms responsible for the shift in phytoplankton community structure across gradients of vertical stability is fundamental to predicting the response of ocean systems to global climate change. Acknowledgements We thank the instructors and trainees who participated in the BEAGLE cruise, as well as the captain and crew of the R.V. Mirai. This research was funded by the DFO Strategic Science Fund, the Chilean National Commission for Scientific and Technological Research through the FONDAP Programme, and by NERC grant NE/G005125/1 (to D.J.S.). H.A.B. was supported by a NSERC postdoctoral fellowship, a Canadian Space Agency supplement and a Fundación Andes (Chile) postdoctoral fellowship. References Bailey, S., Mann, N.H., Robinson, C., and Scanlan, D.J. (2005) The occurrence of rapidly reversible nonphotochemical quenching of chlorophyll a fluorescence in cyanobacteria. FEBS Lett 579: Barlow, R., Stuart, V., Lutz, V., Sessions, H., Sathyendranath, S., Platt, T., et al. (2007) Seasonal pigment patterns of surface phytoplankton in the subtropical southern hemisphere. Deep Sea Res I 54: Bouman, H.A., Platt, T., Sathyendranath, S., Li, W.K.W., Stuart, V., Fuentes-Yaco, C., et al. (2003) Temperature as indicator of optical properties and community structure of marine phytoplankton: implications for remote sensing. 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