Impact of bacterial DMS production on [DMS/P] under ocean acidification (KOSMOS_2.0): insights from the subtropics.

Marine dimethyl sulfide (DMS), a ubiquitous atmospheric trace gas, comprises the largest source of sulphur to the atmosphere. So far, temperate and high-latitude ocean acidification (OA) mesocosm experiments point to a decrease of this precursor for cloud condensation nuclei, leading to fewer clouds, and resulting in an increased radiative force. To our knowledge no experiments have yet been carried out which address multiple forcings (temperature and pCO₂) in the subtropics. We thus joined the 55-day KOSMOS large mesocosm experiment on Gran Canaria to investigate if the observed decrease could be global. As subtropical and tropical oceans comprise a large proportion of the world’s oceans, we were i.a. interested if 1) increasing ocean acidification in a subtropical environment would also decrease [DMS], and if 2) bacterial DMS production could explain a large part of potential decreases.

Here we focus on the first phase (day 1-23), showing the impact of OA on [DMS] and [DMSP] (dimethylsulfoniopropionate). Bacteria are thought to be the main DMS producers, so we used ³⁵S-DMSP as a tracer to investigate the impact of bacterial DMS production on observed [DMS] decreases correlated with increasing OA.

[DMS] showed a strong inverse correlation with [H⁺] (-50% vs. ambient control). [DMSP_p] (-37%) and [DMSP_d] (-20%) also decreased with increasing [H⁺]. Our results support findings from higher latitude mesocosm experiments, thus suggesting the effect might be global. Bacterial DMS production rates, their rate constants, and yields during the peak in [DMS] were negatively correlated with [H⁺] on single days, while gross DMS-production was high enough to support observed [DMS] increases. Bacterial DMSP uptake rates and DMS production rates were not correlated with [H⁺] on any other day. Bacterial effects alone are thus not enough to explain observed changes in standing stocks. We will further explore the results by normalizing to bacterial protein production, cell abundance, and potential changes in gene expression, and finally link DMS/P metabolism to diversity (in progress).