Abstract
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 (ΦPSII), steady-state light response curves (LCs)) and in situ incorporation of 14C (photosynthesis-irradiance curves). The ΦPSII and rETRm,LC (relative maximum electron transport rate) clearly reduced under acidification, in particular, when phytoplankton were exposed to high light levels. However, PBmax (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 PBmax and α (photosynthetic efficiency) under greenhouse condition were significantly higher than those under ambient condition without any difference in ΦPSII, rETRm,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.
Highlights
The ocean has been recognized as an anthropogenic CO2 reservoir since the Industrial Revolution, but ocean acidification has only recently been considered to be a critical problem
Kim et al.: Phytoplankton carbon assimilation in future ocean could be positively affected by elevated CO2 along with CO2 affinity changes and downregulation of CCMs (Rost et al, 2003; Giordano et al, 2005; Sobrino et al, 2008; Hopkinson et al, 2011)
Previous mesocosm studies have shown that cell growth and primary production are promoted by an enhanced organic carbon assimilation rate alongside the community succession of phytoplankton (Riebesell et al, 2007). These results possibly suggest that the downregulation of CCMs could positively influence the biomass and primary production of natural phytoplankton assemblages
Summary
The ocean has been recognized as an anthropogenic CO2 reservoir since the Industrial Revolution, but ocean acidification has only recently been considered to be a critical problem. Changes in carbon chemistry influence phytoplankton physiology, because photosynthesis and respiration are affected by increases in CO2 and HCO−3 concentrations. Kim et al.: Phytoplankton carbon assimilation in future ocean could be positively affected by elevated CO2 along with CO2 affinity changes and downregulation of CCMs (Rost et al, 2003; Giordano et al, 2005; Sobrino et al, 2008; Hopkinson et al, 2011). Some phytoplankton species that have already permanently adjusted to limited CO2 environments, are not affected by oceanic CO2 increases with active CCMs (Beardall and Raven, 2004; Giordano et al, 2005). Physiological changes of phytoplankton might be complicated due to the various adaptation mechanisms under high CO2 conditions, and they can heavily depend upon species-specific physiological characteristics of phytoplankton
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