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

[摘要] A mesocosm experiment was conducted to evaluate the influence ofphotosynthetic performance on the energetic balance of coastalphytoplankton, in relation to community production and autotrophicphytoplankton biomass in future coastal oceans. Natural phytoplanktonassemblages were incubated in field mesocosms under ambient condition(control: ca. 400 μatm CO₂ and ambient temperature), and twosets of potential future ocean conditions (acidification: ca. 900 μatmCO₂ and ambient temperature; greenhouse: ca. 900 μatm CO₂and 3 °C warmer). The photosynthetic performances were estimated by invivo fluorometry (effective quantum yield (Φ_PSII), steady-statelight 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 underacidification, in particular, when phytoplankton were exposed to high lightlevels. However, P^B_max (maximum photosynthetic rate) was the samein the ambient and acidification conditions. Thus, phytoplankton utilizedless light under acidification condition, but could still assimilate asimilar amount of carbon compared to the ambient condition. TheP^B_max and α (photosynthetic efficiency) under greenhousecondition were significantly higher than those under ambient conditionwithout 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 otherconditions. Nevertheless, the community production did not change betweenthe experimental treatments. The main reason for the lack of a change inprimary production under future climate conditions is the control ofautotrophic phytoplankton biomass by grazing. Consequently, acidificationand greenhouse environments have a potential to increase growth and primaryproduction of phytoplankton by enhancing inorganic carbon assimilationefficiency when top-down regulation is negligible.

[学科分类] 地球化学与岩石