Publications (2)2.91 Total impact
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Article: Effects of photochemical transformations of dissolved organic matter on bacterial metabolism and diversity in three contrasting coastal sites in the Northwestern Mediterranean Sea during summer.
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ABSTRACT: 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.Microbial Ecology 03/2008; 55(2):344-57. · 2.91 Impact Factor -
Article: Distribution and bacterial availability of dissolved neutral sugars in the South East Pacific
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ABSTRACT: The distribution and bacterial availability of dissolved neutral sugars were studied in the South East Pacific from October to December 2004 during the BIOSOPE cruise. Four contrasted sites were investigated: Marquesas Islands (MAR), the hyper-oligotrophic South Pacific Gyre (GYR), the eastern part of the Gyre (EGY), and the coastal waters associated to upwelling of Chile (UPW). Total (free and combined) dissolved neutral sugar (TDNS) concentrations were higher in UPW (149–329 nM) and MAR (111–540 nM), than in GYR (79–390 nM) and EGY (58–492 nM). Nevertheless, their contribution to dissolved organic carbon (TDNS-C/DOC%) was generally low for all sites varying from 0.5% to 4% indicating that our South East Pacific surface waters were relatively poor in neutral sugars. Free dissolved neutral sugar (FDNS; e.g. sugars analyzed without hydrolysis) concentrations were very low within the detection of our method (5–10 nM) accounting <5% of the TDNS. In general, the predominant sugars within the TDNS pool were glucose, xylose, arabinose, and galactose while in the FDNS pool only glucose was present. TDNS stock to bacterial production ratios (integrated values from the surface to the deep chlorophyll maximum) were relatively high in GYR with respect to the low primary production, whereas the opposite trend was observed in the highly productive area of UPW. Intermediate situations were observed for MAR and EGY. Bioavailability of dissolved organic matter (DOM) exposed to natural solar radiation was also experimentally studied and compared to dark treatments. Our results showed no or little detectable effect of sunlight on DOM bacterial assimilation in UPW and in GYR while a significant stimulation was found in MAR and EGY. The overall results clearly suggest the semi-labile character of DOM in GYR compared to the labile of UPW and are consistent with dissolved organic carbon accumulation and the elevated C/N ratios reported by Raimbault et al. (2007).
