Abstract：CO2 fixation by planktonic algae and subsequent assimilation of algae-derived organics by planktonic bacteria, and the coupling relationship between them were studied at elevated CO2 levels. The elevated CO2 levels were achieved in three sets of mesocoms, each having a volume faction of 400×10-6 (P1), 800×10-6 (P2), and 1200×10-6 (P3), respectively. Results showed that elevated CO2 supplies promoted algal growth, leading to biomass of 9.4×106cells/mL, 1.1×107cells/mL, and 1.5×107cells/mL at P1, P2, and P3 levels. Algal esterase activity was also reduced at these CO2 levels, but the time to reach peak biomass was not affected. For planktonic bacteria in the same mesocoms, they reached peak biomass on day 12 (P1), 14 (P2), and 20 (P3), leading to cell density of 2.10×106cells/mL (P1), 1.94×106cells/mL (P2), and 1.70×106cells/mL (P3). In the first seven days, the growth rate of planktonic bacteria correlated with increasing CO2 levels: vP3>vP2>vP1, as was the bacterial activity. At elevated CO2 levels, algae showed high biomass with low metabolic activity, while bacterial showed relative low biomass with high metabolic activity; this contrast reflected differences in the response mechanism of algae and bacteria to changes in CO2levels. Therefore, the high biomass resulting from elevated CO2under global change may not effectively transform into bacterial biomass.
