Testing the effect of CO2 concentration on the dynamics of marine heterotrophic bacterioplankton

To date no study exists that directly addresses changes in dynamics of heterotrophic bacteria in surface waters in relation to partial pressure of CO 2 (pCO2). Therefore, we studied the effect of changes in pCO2 on bacterial abundance and activities by using mesocosms with different pCO2 levels (;190, ;370, and ;700 ppmV, representing past, present-day, and future atmospheric pCO2, respectively). Abundance of total bacteria did not differ with increasing pCO2 throughout the whole study period, whereas bacterial protein production (BPP) was highest at highest pCO2. This effect was even more pronounced for cell-specific production rates, especially those of attached bacteria, which were up to 25 times higher than those of free bacteria. During the breakdown of the bloom, however, the abundance of both free and attached bacteria was significantly increased with pCO 2. Differences in bacterial growth rate (m) were smaller than those of BPP, but both m and BPP of attached bacteria were elevated under high pCO2. Averages of total protease as well as a- and b-glucosidase activities were highest at elevated pCO 2 levels, but a statistically significant dependence on pCO 2 was only evident for protease activity. There is a measurable but indirect effect of changes in pCO2 on bacterial activities that are mainly linked to phytoplankton and presumably particle dynamics.