Ecophysiological basis of spatiotemporal patterns in picophytoplankton pigments in the global ocean

Information on the intracellular content and functional diversity of phytoplankton pigments can provide valuable insight on the ecophysiological state of primary producers and the flow of energy within aquatic ecosystems. Combined global datasets of analytical flow cytometry (AFC) cell counts and High-Performance Liquid Chromatography (HPLC) pigment concentrations were used to examine vertical and seasonal variability in the ratios of phytoplankton pigments in relation to indices of cellular photoacclimation. Across all open ocean datasets, the weight-to-weight ratio of photoprotective to photosynthetic pigments showed a strong depth dependence that tracked the vertical decline in the relative availability of light. The Bermuda Atlantic Time-series Study (BATS) dataset revealed a general increase in surface values of the relative concentrations of photoprotective carotenoids from the winter-spring phytoplankton communities dominated by low-light acclimated eukaryotic microalgae to the summer and early autumn communities dominated by high-light acclimated picocyanobacteria. In Prochlorococcus-dominated waters, the vertical decline in the relative contribution of photoprotective pigments to total pigment concentration could be attributed in large part to changes in the cellular content of photosynthetic pigments (PSP) rather than photoprotective pigments (PPP), as evidenced by a depth-dependent increase of the intracellular concentration of the divinyl chlorophyll-a (DVChl-a) whilst the intracellular concentration of the PPP zeaxanthin remained relatively uniform with depth. The ability of Prochlorococcus cells to adjust their DVChl-a cell-1 over a large gradient in light intensity was reflected in more highly variable estimates of carbon-to-Chl-a ratio compared to those reported for other phytoplankton groups. This cellular property is likely the combined result of photoacclimatory changes at the cellular level and a shift in dominant ecotypes. Developing a mechanistic understanding of sources of variability in pigmentation of picocyanobacteria is critical if the pigment markers and bio-optical properties of these cells are to be used to map their biogeography and serve as indicators of photoacclimatory state of subtropical phytoplankton communities more broadly. It would also allow better assessment of effects on, and adaptability of phytoplankton communities in the tropical/subtropical ocean due to climate chang

Effect of photoacclimation on bio-optical properties of marine phytoplankton

Photoacclimation in marine phytoplankton is manifested through changes in pigment composition and has a significant impact on in vivo light absorption. Results obtained from this thesis work clearly show that the variability in the chlorophyll a (Chl-a)-specific absorption coefficient of phytoplankton at 440 nm (a*ph (440)) observed over a broad latitudinal gradient of the Atlantic Basin was influenced significantly by changes in the concentration of non-photosynthetic pigments (NPPs) relative to total pigments (the NPP index), whereas pigment indices of cell size showed a weaker correlation with a*ph (440). Spatial and seasonal changes in the NPP index were the combined result of photoacclimation and the photoadaptive variation in pigment composition of dominant prokaryotic and eukaryotic groups. Variability in the spectral shape (indexed by the spectral slope) of phytoplankton absorption could be largely explained by NPPs, particularly zeaxanthin (a marker for marine picocyanobacteria) which was highly abundant in the (sub)tropical oligotrophic gyres due to the predominance of the marine picocyanobacterium Prochlorococcus. In Prochlorococcus-dominated waters, depth-dependent variation in the NPP index was driven primarily by variation in the intracellular concentration of photosynthetic pigments (PSPs), rather than NPPs, which was the result of both photoacclimation and a shift in dominant ecotypes. Photoacclimation of this globally-significant genus was also clearly reflected in the strong vertical change in the intracellular concentration of the dominant light harvesting pigment, divinyl Chl-a (DVChl-a). The flexibility in the DVChl-a cell−1 in Prochlorococcus leads to a more highly variable carbon-to-DVChl-a ratio (θPro) observed in this genus than in other phytoplankton groups. θPro was shown to be highly correlated with both the NPP index and the spectral slope between 443 and 550 nm, which suggests that both of these indices may serve as useful predictors of θPro. In strongly-stratified, oligotrophic waters where the NPP index may be considered a proxy of ambient light levels, the NPP index could account for a significant fraction of the variability in the parameters describing the photosynthesis-irradiance (PE) response curve. Using estimates of θPro derived from its relationship with the NPP index, the maximum growth rates for surface Prochlorococcus populations (μm, Pro) were calculated from maximum Chl-a-specific photosynthetic rates measured at the Prochlorococcus-dominated stations, yielding an average μm, Pro value of 0.22 d−1 , which is consistent with field and culture studies. Thus, the NPP index can be considered a useful proxy of both the photoacclimatory status and photophysiological characteristics of phytoplankton in subtropical marine ecosystems.</p