Effect of CO² enrichment on bacterial metabolism in an Arctic fjord

International audienceThe anthropogenic increase of carbon dioxide (CO 2) alters the seawater carbonate chemistry, with a decline of pH and an increase in the partial pressure of CO 2 (pCO 2). Although bacteria play a major role in carbon cycling, little is known about the impact of rising pCO 2 on bacterial carbon metabolism, especially for natural bacterial communities. In this study, we investigated the effect of rising pCO 2 on bacterial production (BP), bacterial respiration (BR) and bacterial carbon metabolism during a mesocosm experiment performed in Kongsfjorden (Svalbard) in 2010. Nine meso-cosms with pCO 2 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 pCO 2 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 pCO 2 after nutrient addition (day 14). In addition, increasing pCO 2 enhanced the leucine to thymidine (Leu : TdR) ratio at the end of experiment, suggesting that pCO 2 may alter the growth balance of bacteria. Stepwise multiple regression analysis suggests that multiple factors, including pCO 2 , 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 pCO 2 was observed for BR, bacterial carbon demand and bacterial growth efficiency. Overall, the results suggest that changes in pCO 2 potentially influence bacterial production, growth rate and growth balance rather than the conversion of dissolved organic matter into CO 2