Effect of elevated pCO2 on trace gas production during an ocean acidification mesocosm experiment

A mesocosm experiment was conducted in Wuyuan Bay (Xiamen), China, to investigate the effects of elevated p CO2 on the phytoplankton species Phaeodactylum tricornutum (P. tricornutum), Thalassiosira weissflogii (T. weissflogii) and Emiliania huxleyi (E. huxleyi) and their production ability of dimethylsulfide (DMS), dimethylsulfoniopropionate (DMSP), as well as four halocarbon compounds, bromodichloromethane ( CHBrCl2 ), methyl bromide ( CH3Br ), dibromomethane ( CH2Br2 ) and iodomethane ( CH3I ). Over a period of 5 weeks, P. tricornuntum outcompeted T. weissflogii and E. huxleyi, comprising more than 99 % of the final biomass. During the logarithmic growth phase (phase I), mean DMS concentration in high p CO2 mesocosms (1000 µ atm) was 28 % lower than that in low p CO2 mesocosms (400 µ atm). Elevated p CO2 led to a delay in DMSP-consuming bacteria concentrations attached to T. weissflogii and P. tricornutum and finally resulted in the delay of DMS concentration in the high p CO2 treatment. Unlike DMS, the elevated p CO2 did not affect DMSP production ability of T. weissflogii or P. tricornuntum throughout the 5-week culture. A positive relationship was detected between CH3I and T. weissflogii and P. tricornuntum during the experiment, and there was a 40 % reduction in mean CH3I concentration in the high p CO2 mesocosms. CHBrCl2 , CH3Br , and CH2Br2 concentrations did not increase with elevated chlorophyll a (Chl a) concentrations compared with DMS(P) and CH3I , and there were no major peaks both in the high p CO2 or low p CO2 mesocosms. In addition, no effect of elevated p CO2 was identified for any of the three bromocarbons.