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

Abstract

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 change.

Determining drivers of phytoplankton carbon to chlorophyll ratio at Atlantic Basin scale

“…Veldhuis and Kraay (2004) give a range of 93 -122 for Prochlorococcus, and 38 -58 for Synechococcus which is within range of the values reported here. The range of 9 -499 for Prochlorococcus reported by Phongphattarawat et al (2023), based on a pigment-based analysis of a global dataset that included AMT data, is close to the range reported here. Figure 8 shows an intercomparison of Prochlorococcus C versus Chl between this work and previous literature values.…”

“…When data points are selected based on the dominant type present in the sample [i.e., >50% of the chlorophyll concentration can be attributable to Prochlorococcus (Figure 2)] regression statistics of slope: 0.672 ± 0.039, intercept: 1.392 ± 0.045, R 2 : 0.219 are obtained. This compares with slope: 0.843, intercept: 1.711, R 2 : 0.41 for Phongphattarawat et al (2023) using 49 fg C cell -1 (Cailliau et al, 1996) and slope: 0.788, intercept: 1.798, R 2 : 0.47 for Phongphattarawat et al ( 2023) using 52 and 158 fg C cell -1 for euphotic and sub-euphotic populations, respectively (Casey et al, 2013). Our values tend towards C:Chl = 145 (Sathyendranath et al, 2009) at low values (~0.01 mg m -3 ) of Chl, and diverge away (below) this at higher (~1 mg m -3 ) Chl.…”

“…Measurement of phytoplankton carbon in natural oceanographic environments has been notoriously difficult, with many existing methods relying on per-cell carbon content for different phytoplankton cells estimated based on laboratory culture experiments, e.g., Phongphattarawat et al (2023), or on expensive and labour-intensive ship-board measurements, e.g., Graff et al (2015). The method developed here employs differences in the scattering properties of cells, which can account for intra-species and inter-species differences in per-cell scattering and is demonstrated to function across multiple models of flow cytometers.…”