Physiological Responses Of Phytoplankton Functional Groups To Elevated CO₂ Concentrations

There has been increasing interest in understanding the effects of increasing atmospheric CO₂ on biological processes in terms of ocean carbon cycling. Responses of marine functional groups of phytoplankton, such as silicifying or calcifying phytoplankton and cyanobacteria, to CO₂ concentration variations remain one of the central questions. However, few studies on marine diatoms and coccolithophorids, and almost no study on picocyanobacterial Synechococcus have been conducted in this regard so far. In the present study, physiological changes in a marine coccolithophorid Emiliania huxleyi, a diatom Skeletonema costatum, and a phycocyanin (PC)-rich Synechococcus strain Synechococcus cf_elongatus CCMP1379 and a phycoerythrin (PE)-rich strain Synechococcus sp.CCMP839 with increasing CO₂ concentrations (350, 600 and 800 ppm) were examined under manipulations using a plant growth chamber. The results showed that:1. E. huxleyi showed no significant change in growth rate over the entire experimental CO₂ range, while 23.6% increase in S. costatum was observed under 800 ppm CO₂. The RNA/DNA ratio of S. costatum cells correspondingly increased by 66.6% and positively correlated with growth rate (R2 = 0.764). 46.6% and 81.7% increase were observed in photosynthetic rate of E. huxleyi at 600 and 800 ppm CO₂, correspondingly 75.2% and 57.9% increase in carbohydrates, respectively. S. costatum showed no significant change in...