The effect of climate change on the carbon balance between photosynthesis and respiration in Antarctic microalgae

The biological process of the carbon cycle in the Antarctic Ocean is controlled by thephotosynthetic activity of the primary producers. The amount of fixedcarbon does not only depend on the photosynthetic activity but also on the carbon lossesdue to respiration. Thus,the ratio photosynthesisto respiration (rP/R) is an importantparameter to predict the effect of climate change on the Antarctic ecosystem. Indeed, the ongoing changes in climate change are influencing the dynamics of environmental conditions,whichhas tremendous effects on the phytoplankton community. Therefore, two ecologically relevant species from the Southern Ocean were here investigated: the diatom Chaetocerossp.and the prymnesiophyte Phaeocystis antarctica, studying the changes in the rP/R under global climate change conditions.Three main parameters were examined i.e temperature, salinity and iron limitation. The P/R ratio was significantly affected bytemperature, while salinity hadonly a secondary importance, although with species-specific differences. More specifically, thevalues were ranging from 12.3 to 7.5 for Chaetocerossp. and from 12.4 to 2.5 forP. antarctica.The changes in this ratio were principally due to variations in respiration,rather than inphotosynthesis. Chaetocerossp. appears to be less flexible in the regulation of the extent of photoprotective mechanisms (non-photochemical quenchingand alternative electrons), but itsphotoprotective level was generally higherthan in P. antarctica. Regarding iron limitation, data were successfully collected only for Chaetocerossp..The P/R ratio, equal to 2.8,didnot changeunder iron limitation, with iron limited cells showinga very efficient acclimation to thelowered assimilatory metabolismby decreasing their respiratory losses.