Enhancement of photosynthetic carbon assimilation efficiency by phytoplankton in the future coastal ocean

Abstract. A mesocosm experiment was conducted to evaluate the influence of photosynthetic performance on the energetic balance of coastal phytoplankton, in relation to community production and autotrophic phytoplankton biomass in future coastal oceans. Natural phytoplankton assemblages were incubated in field mesocosms under ambient condition (control: ca. 400 μatm CO₂ and ambient temperature), and two sets of potential future ocean conditions (acidification: ca. 900 μatm CO₂ and ambient temperature; greenhouse: ca. 900 μatm CO₂ and 3 °C warmer). The photosynthetic performances were estimated by in vivo fluorometry (effective quantum yield (Φ_PSII), steady-state light response curves (LCs)) and in situ incorporation of ¹⁴C (photosynthesis-irradiance curves). The Φ_PSII and rETR_m,LC (relative maximum electron transport rate) clearly reduced under acidification, in particular, when phytoplankton were exposed to high light levels. However, P^B_max (maximum photosynthetic rate) was the same in the ambient and acidification conditions. Thus, phytoplankton utilized less light under acidification condition, but could still assimilate a similar amount of carbon compared to the ambient condition. The P^B_max and α (photosynthetic efficiency) under greenhouse condition were significantly higher than those under ambient condition without any difference in Φ_PSII, rETR_m,LC and α_,LC (electron transport efficiency) between the treatments. Therefore, phytoplankton utilized the same amount of light under greenhouse condition, but could assimilate more carbon than under ambient condition. As a result, Chl a normalized primary production was higher in greenhouse than in other conditions. Nevertheless, the community production did not change between the experimental treatments. The main reason for the lack of a change in primary production under future climate conditions is the control of autotrophic phytoplankton biomass by grazing. Consequently, acidification and greenhouse environments have a potential to increase growth and primary production of phytoplankton by enhancing inorganic carbon assimilation efficiency when top-down regulation is negligible.