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.

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.

Purification of X-prolyl dipeptidyl aminopeptidase from Lactobacillus casei subspecies.

Prolyl dipeptidylaminopeptidases from two subspecies of Lactobacillus casei were purified and biochemically characterized. L. casei ssp. casei UL21 (a debittering strain) and L. casei ssp. rhamnosus UL26 (a non-debittering strain) were the source bacteria for this study. Purification of the enzymes from both the sources was effected by a gel filtration step through Sephacryl S-300 followed by ion-exchange chromatography through DEAE Sephacel. This rendered an electrophoretically homogeneous enzyme preparation. The purified enzymes from both the sources showed similar temperature optimum (45 degrees C) and pH optimum (7.0). Their activity profiles on various substrates and the nature of inhibition by different inhibitors were also found to be similar, indicating that this enzyme is perhaps not significantly involved in the debittering process during the maturation of cheese.