Changes in the Rubisco to photosystem ratio dominates photoacclimation across phytoplankton taxa.

When growth irradiance changes, phytoplankton acclimates by changing allocations to cellular components to re-balance their capacity to absorb photons versus their capacity to use the electrons from the oxidation of water at photosystem II. Published changes in the cellular allocations resulting from photoacclimation across algal groups highlight that algae adopt different strategies. We examined the photoacclimation of the photosynthetic apparatus of six marine phytoplankters under near-natural diel irradiance patterns. For most of the phytoplankters, Chl a per structural photosystem II unit decreased with increasing growth irradiance, but a parallel decline in optical packaging effect allowed cells to maintain their functional absorption cross section serving active photosystem II units (σ PSII). Furthermore, no significant changes were observed in the ratio of Chl a per photosystem I. The diatom Skeletonema marinoi proved an exception to this pattern as Chl a per photosystem II is stable and Chl a per photosystem I slightly decreased with light intensity. A clear decrease in the photosystem content per cell was observed for all species except for Thalassiosira oceanica and S. marinoi. Rubisco content per cell showed little variation with irradiance for most algae, except for a 3-fold increase in S. marinoi. A ~700 % increase in the Rubisco:photosystem ratio across species with increasing growth irradiance indicates this is a key cellular stoichiometric adjustment to balance photon absorption capacity and the carbon reduction capacity. Increasing the Rubisco:photosystem ratio occurs through a decrease in the photosystems per cell for most of the phytoplankters in this study, except in the case of S. marinoi where Rubisco per cell increased.