Dynamics of dimethylsulphoniopropionate and dimethylsulphide under different CO₂ concentrations during a mesocosm experiment

Abstract. The potential impact of seawater acidification on the concentrations of dimethylsulfide (DMS) and dimethylsulfoniopropionate (DMSP), and the activity of the enzyme DMSP-lyase was investigated during a pelagic ecosystem CO₂ enrichment experiment (PeECE III) in spring 2005. Natural phytoplankton blooms were studied for 24 days under present, double and triple partial pressures of CO₂ (pCO₂; pH=8.3, 8.0, 7.8) in triplicate 25 m³ enclosures. The results indicate similar DMSP concentrations and DMSP-lyase activity (DLA) patterns for all treatments. Hence, DMSP and DLA do not seem to have been affected by the CO₂ treatment. In contrast, DMS concentrations showed small but statistically significant differences in the temporal development of the low versus the high CO₂ treatments. The low pCO₂ enclosures had higher DMS concentrations during the first 10 days, after which the levels decreased earlier and more rapidly than in the other treatments. Integrated over the whole study period, DMS concentrations were not significantly different from those of the double and triple pCO₂ treatments. Pigment and flow-cytometric data indicate that phytoplanktonic populations were generally similar between the treatments, suggesting a certain resilience of the marine ecosystem under study to the induced pH changes, which is reflected in DMSP and DLA. However, there were significant differences in bacterial community structure and the abundance of one group of viruses infecting nanoeukaryotic algae. The amount of DMS accumulated per total DMSP or chlorophyll-a differed significantly between the present and future scenarios, suggesting that the pathways for DMS production or bacterial DMS consumption were affected by seawater pH. A comparison with previous work (PeECE II) suggests that DMS concentrations do not respond consistently to pelagic ecosystem CO₂ enrichment experiments.