The effects of phototransformation of dissolved organic matter (DOM) on bacterial growth, production, respiration, growth efficiency, and diversity were investigated during summer in two lagoons and one oligotrophic coastal water samples from the Northwestern Mediterranean Sea, differing widely in DOM and chromophoric DOM concentrations. Exposure of 0.2-microm filtered waters to full sun radiation for 1 d resulted in small changes in optical properties and concentrations of DOM, and no changes in nitrate, nitrite, and phosphate concentrations. After exposure to sunlight or dark (control) treatments, the water samples were inoculated with the original bacterial community. Phototransformation of DOM had contrasting effects on bacterial production and respiration, depending on the water's origin, resulting in an increase of bacterial growth efficiency for the oligotrophic coastal water sample (120%) and a decrease for the lagoon waters (20 to 40%) relative to that observed in dark treatments. We also observed that bacterial growth on DOM irradiated by full sun resulted in changes in community structure of total and metabolically active bacterial cells for the three locations studied when compared to the bacteria growing on un-irradiated DOM, and that changes were mainly caused by phototransformation of DOM by UV radiation for the eutrophic lagoon and the oligotrophic coastal water and by photosynthetically active radiation (PAR) for the mesoeutrophic lagoon. These initial results indicate that phototransformation of DOM significantly alters both bacterial metabolism and community structure in surface water for a variety of coastal ecosystems in the Mediterranean Sea. Further studies will be necessary to elucidate a more detailed appreciation of potential temporal and spatial variations of the effects measured.

The high content in nutrients of freshwater outflows induces highly productive and buoyant plumes spreading over marine waters (MW). As a consequence, the growth of organisms developing in these low-salinity waters (LSW) might be potentially affected by UV-R (280-400 nm). This study investigated the penetration of UV-R and its impact on net community production (NCP) and bacterial protein (B(PROT)S) and DNA (B(DNA)S) synthesis in mesotrophic-LSW formed from the Rhône River and in oligotrophic MW of the Northwestern Mediterranean Sea (Gulf of Lions) in May 2006. High concentrations of chlorophyll a (up to 8 microg L(-1)) measured in the LSW (<37.8 psu, 0-10 m) were the main factor influencing the diffuse attenuation coefficients (K(d)) of both UV-R and photosynthetically active radiation (PAR). The mean ratio of the K(d) measured between the LSW and the MW increased with wavelength from 2.4 at 305 nm to 2.9 at 380 nm and 3.1 for PAR indicating more similarity in the UV region. NCP was severely inhibited by UV-R at the surface of the LSW, whereas no effect was measured in the surrounding MW. In contrast, B(PROT)S and B(DNA)S were affected deeper by UV-R in the MW (up to 8 m depth) compared to the LSW where inhibition was only observed at the surface. Differences in response of bacteria in LSW and MW are largely explained by differences in UV-R transparency; however, transplant experiments indicate that bacterial assemblages from the MW were also more sensitive to UV-R than those present in the LSW. We also observed that higher activity of bacteria after nutrient additions increased their sensitivity to UV-R during the day, but favored their recovery during the night incubation period for both LSW and MW. Results suggest that riverine and nutrient inputs may alter the effects of UV-R on microbial activity by attenuating the UV-R penetration and by modifying the physiology of bacteria.