Interactive effects of vertical mixing, solar radiation and microbial activity on oceanic dimethylated sulfur cycling

The production and subsequent emission of volatile compounds is one of the numerous ways by which microbial plankton participate in the cycling of elements and influence the Earth's climate. Dimethylsulfide (DMS), produced by enzymatic decomposition of the algal intracellular compound dimethylsulfoniopropionate (DMSP), is the more abundant organic volatile in the upper ocean. Its global emission amounts ca. 28 Tg S per year, and represents the main biogenic source of sulfur to the troposphere and about 30% of the total S emission (anthropogenic, biogenic and volcanic). Atmospheric oxidation of DMS contributes to atmospheric acidity, and is believed to promote the formation and growth of aerosols. Furthermore, DMSderived sulfate aerosols have been suggested to cool the climate by reducing the amount of shortwave solar radiation reaching the Earth's surface through two mechanisms: by scattering solar radiation and, more important, by acting as cloud condensation nuclei, thus making clouds brighter and longerlived. The `CLAW' hypothesis postulates that, if oceanic DMS emission was in turn stimulated by solar radiation, a regulatory feedback mechanism could operate between marine plankton and the radiative budget over the oceans. However, the relationship between DMS emission and solar radiation is not straightforward, since a number of biochemical and photochemical transformations come into action from the moment DMSP is synthesized by phytoplankton until DMS is emitted. These transformations are intimately linked to the physical environment, the ecological setting and the microbial interactions, rendering the picture of dimethylated sulfur cycling a lot more complicated. Surprisingly, though, the seasonal cycle of seawater DMS concentration seems to follow that of solar radiation in the majority of oceanic regions, regardless their productivity regimes. The premise of this thesis is that, to understand this emerging pattern, we need to understand what regulates the DMS production and consumption processes and their balance (that is, DMS budgets). To this end, we have studied the response of biotic and abiotic DMS cycling to solar radiation by means of incubation experiments. At another level, we have studied the response of ecosystem DMS budgets to different radiation climates. Since the depth of the upper mixed layer regulates the amount and spectral composition of the `light' seen by the cells and molecules, our studies have been backed by a careful characterization of underwater radiation fields and vertical mixing dynamics. Our results show that solar radiation has a stimulating effect on gross DMS production. Moreover, the stimulation is more effective at shorter and more energetic wavelengths in the ultraviolet (UV) region. Direct DMS production and/or increased DMSP release by UVstressed phytoplankton is the most plausible explanation for this observation, leaving a secondary role for DMS production mechanisms related to bacterial metabolism and microzooplankton grazing. At the ecosystem level, we have shown that vertical mixingmediated solar exposure regulates whether DMS is preferentially oxidized by bacteria or by photochemical reactions. The outcome of this competition between DMS sinks is that total DMS loss rate constants vary little across oceanic biomes. As a result, the seasonal and also the shortterm variability in DMS concentrations respond mainly to gross DMS production. The stress response occurs at different temporal scales: seasonally, through the succession of microbial communities towards stronger DMSP producers and higher DMS yields in summer stratified waters; and across daynight cycles, where shortterm radiative stress modulates DMSP to DMS conversion yields. By means of a literature metaanalysis, we have improved the current understanding of the different DMS(P) cycling regimes and their links with the ecological geography of the sea