Effect of sulfate availability on phytoplankton stoichiometry.

Sulfur (S) is a key element in multiple metabolic pathways of phytoplankton cells. The effect of S availability on phytoplankton elemental quotas and stoichiometry has been addressed in few studies, using a limited number of species and with contradictory results. Using high-temperature combustion oxidation and X-ray fluorescence methods, we measured the concentrations of micro- and trace elements in monocultures of 20 marine phytoplankton species, grown with different sulfate concentrations representing those of early and modern oceans. We found that, independently from the sulfate concentration in the media, the red lineage species had higher S quotas than those of the green lineage, resulting in lower C:S (93) and higher S:P (1.06) than the green lineage species (226 and 0.76, respectively). This suggests a genetic constraint in the S quota and aligns with the sulfate facilitation hypothesis, shedding light on a metabolic basis for the expansion of the red lineage algae and their current dominance in ocean waters. We also have shown a physiological response of phytoplankton cells to different sulfate availability, by either decreasing phosphorus or increasing zinc quotas. The P response was more characteristic in the red lineage, with higher S requirements and metabolic S fluxes, while the Zn response was independent of genotypic constraints or plastid type.