The anthropogenic increase of carbon dioxide (CO2) alters the seawater carbonate chemistry, with a decline of pH and an increase
in the partial pressure of CO2 (pCO2). Although bacteria play a major role in carbon cycling, little is known about the impact
of rising pCO2 on bacterial carbon metabolism, especially for natural bacterial communities. In this study, we investigated
the effect of rising pCO2 on bacterial production (BP), bacterial respiration (BR) and bacterial carbon metabolism during
a mesocosm experiment performed in Kongsfjorden (Svalbard) in 2010. Nine mesocosms with pCO2 levels ranging from ca. 180 to
1400 μatm were deployed in the fjord and monitored for 30 days. Generally BP gradually decreased in all mesocosms in an initial
phase, showed a large (3.6-fold average) but temporary increase on day 10, and increased slightly after inorganic nutrient
addition. Over the wide range of pCO2 investigated, the patterns in BP and growth rate of bulk and free-living communities
were generally similar over time. However, BP of the bulk community significantly decreased with increasing pCO2 after nutrient
addition (day 14). In addition, increasing pCO2 enhanced the leucine to thymidine (Leu : TdR) ratio at the end of experiment,
suggesting that pCO2 may alter the growth balance of bacteria. Stepwise multiple regression analysis suggests that multiple
factors, including pCO2, explained the changes of BP, growth rate and Leu : TdR ratio at the end of the experiment. In contrast
to BP, no clear trend and effect of changes of pCO2 was observed for BR, bacterial carbon demand and bacterial growth efficiency.
Overall, the results suggest that changes in pCO2 potentially influence bacterial production, growth rate and growth balance
rather than the conversion of dissolved organic matter into CO2.