The study subject of my master’s is dimethylsulfide (better known as DMS), which is a climatically active gas. It is produced in seawater by phytoplankton and bacteria. Once released to the atmosphere, it is oxidized and ultimately ends up as sulfate. Sulfate serves as cloud condensation nuclei, which makes clouds denser. Denser clouds reflect more light back to space, so less light gets to the Earth’s surface to warm it. In this way, DMS is the opposite of a greenhouse gas; I’m not too sure what they’re called in English, but in French we call them umbrella-effect gases.

In addition to this, a theory has given much interest to DMS. The CLAW hypothesis is named after its authors, Charlson, Lovelock (the same one who came up with Gaïa), Anderson and Watson. This hypothesis states that DMS acts as a global regulator of Earth’s climate. The proposed mechanism is as follows: a warming world will mean more light reaching the oceans. Phytoplankton receiving more light will produce more DMS. More DMS will cause more clouds and dim the sun’s light reaching the oceans. Phytoplancton lacking light will produce less DMS, and so the feedback loop is closed. So far, that hypothesis has been pretty hard to test and verify, but the existing data indicate that the story is much more complicated than the simple feedback loop described by Charlson et al. So, basically, so far we don’t know if there’s a feedback loop between DMS and the Earth’s global climate.

Even without all the interest generated by the CLAW hypothesis, DMS is a very important regulator of the Earth’s climate. DMS is the most important source of natural sulfate aerosols to the atmosphere (from memory, DMS contributes about half). In a warming world, the question is whether DMS production will become greater or lesser. So I am studying the controls on production of DMS from DMSP (diméthylsulfoniopropionate), the substance DMS is made from. Using the 35S technique you’ve seen me do, I trace the different pathways that DMSP takes in seawater – how much is uptaken by bacteria, what proportion do they use to build up their own molecules, and what proportion do they release as DMS? I try to piece that information together with other data (water masses, sea surface temperature, phytoplankton abundance, bacterial communities*) to really understand what controls the production of DMS.