Impact of solar radiation on the biological removal of dimethylsulfoniopropionate and dimethylsulfide in marine surface waters

Abstract

The effect of natural surface solar radiation on the biological removal of dimethylsulfo- niopropionate (DMSP) and dimethylsulfide (DMS) was determined and compared to the photochem- ical removal of DMSP and DMS. Natural bacterial assemblages (0.8 µm filtered seawater) from the northern Adriatic Sea and the coastal North Sea were exposed to surface solar radiation and incu- bated in the dark; the DMSP and DMS concentrations were measured concurrently. Photochemical removal rates were determined in 0.2 µm filtered seawater. Biological removal of DMSP in the light was 62 ± 14% lower than the biological removal rate obtained in the dark. High spatial and temporal variability in the biological removal rates was observed for the dark treatments, as well as for its sen- sivity to solar radiation, with rates for light treatments varying from 29 to 81% of those in the dark. The DMSP concentration above which no further increase of the biological DMSP removal rate was observed was substantially lower in the light treatments (~30 nM) than in the dark treatments (>80 nM). UV-B radiation only accounted for a minor inhibitory effect (~15% of total inhibition), whereas UV-A and PAR (photosynthetically active radiation) both contributed ~42% of total inhibi- tion. Biological DMS removal under solar radiation was only ~40 ± 14% of the biological DMS removal in the dark. Under surface solar radiation, photochemical removal was always higher than the dark biological removal. Our results indicate therefore, that the DMSP and DMS dynamics in the oceanic surface waters are severely influenced by solar radiation due to the partial inhibition of the microbial consortia responsible for DMSP and DMS turnover.