Spectral irradiance dependence of sunlight effects on plankton dimethylsulfide production

We investigated the short-term effects of variable solar irradiance and spectrum on the gross biological production of dimethylsulfide (DMS), a trace gas with potential climatic effects, in eight experiments performed at different times of the year in a northwest Mediterranean coastal site. Experimentally determined net community DMS production, DMS photolysis, and dark microbial DMS consumption rates were used to calculate gross community DMS production by budgeting. In addition, the composition of the bacterioplankton and phytoplankton communities in the initial samples, and the photoinhibition of bulk bacterial heterotrophic activity and phytoplankton photosynthetic efficiency were monitored. Our results show that: (1) gross DMS production is irradiance dependent, with a maximum short-term stimulation factor of 2- to 6-fold compared to dark incubations; (2) its spectral shape is variable but generally similar to that of phytoplankton photoinhibition or photodamage, with more effective stimulation at shorter ultraviolet wavelengths; and (3) stronger stimulation occurs when samples are overexposed with respect to their prior exposure. Remarkably, the photoresponse of gross DMS production was in most cases strong enough to (at least) compensate the photochemical DMS loss at the water subsurface. Such response would prevent DMS depletion in stratified and highly irradiated waters. Since the initial microbial communities were representative of meso-to oligotrophic conditions, our observations should apply to a wide variety of oceanic regimes.