Impacts of elevated pCO₂ on trace gas emissions in two microalgae: Phaeocystis globosa and Nitzschia closterium

Environmental contextOcean acidification can affect marine microalgae, which can produce climate-active trace gases such as dimethylsulfide and various halocarbons. We conducted monoculture experiments simulating future ocean acidification, and showed that trace gas emissions are affected by elevated pCO₂ to different degrees. The responses of trace gases to elevated pCO₂ are compound- A nd species-specific. AbstractThe potential impacts of seawater acidification on the concentrations of dimethylsulfide (DMS), dimethylsulfoniopropionate (DMSP), dissolved acrylic acid (AAd) and various volatile halocarbons, including CH₃Cl, CHBr₃, CH₂Br₂, CHBr₂Cl, CHBrCl₂ and CH₃I, were examined during a laboratory CO₂ perturbation experiment for the microalgae Phaeocystis globosa and Nitzschia closterium. The microalgae were exposed to ambient CO₂ conditions (390-540 μatm; 1 μatm≤0.1Pa) and to projected concentrations for the end of the century (760-1000μatm, high carbon (HC)). The growth rate of the two species remained unaffected by elevated CO₂. Results showed a 48 and 37% decline in the DMS concentration normalised to cell density in P. globosa and N. closterium cultures in the HC treatment compared with the ambient treatment. No significant difference was observed for DMSPp and DMSPd in the two microalgae cultures between the two CO₂ levels. The mean AAd concentrations in the P. globosa culture showed a 28% decline in the HC treatment. By contrast, the cell-normalised concentrations of AAd in the HC treatment were 45% lower than in the ambient treatment in N. closterium cultures. No CO₂-induced effects were observed for CH₃Cl, CHBr₃, CHBr₂Cl, CHBrCl₂ and CH₃I, but cell-normalised concentrations of CH₂Br₂ in N. closterium cultures showed a 32% decline in the HC treatment relative to the ambient level. These results show that the metabolism processes responsible for the production of climate-active gases in phytoplankton may be affected by high CO₂ levels. There may be a potential delay in the responses of trace gas emissions to elevated pCO₂.