DIEL VARIATIONS IN BACTERIAL HETEROTROPHIC ACTIVITY AND GROWTH IN THENORTHWESTERN MEDITERRANEAN SEA

Primary producers must respond to the diel changes in light availabili ty. Therefore, detection of diel cycles in bacterial activity would im ply tight coupling between the production of photosynthetic dissolved organic carbon (DOC) and its consumption by bacteria. Absence of diel cycles, on the contrary, would indicate that bacteria depend largely u pon allochthonous organic carbon and that bacteria are not tightly dep endent on photosynthetically produced autochthonous carbon. In 1993 an d 1994 we sampled 3 sites in the NW Mediterranean Sea several times a day, and measured several microbial parameters as well as the vertical profiles of DOC along the diel cycle. The sites were selected so that one was on the continental shelf and, thus, was more influenced by co astal runoff; a second one was over the shelf slope and a third, ocean ic one was located further offshore over a depth of 2000 m. We found c lear diel cycles in bacterial total and specific activity always in th e oceanic stations and sometimes in the shelf slope stations. Diel cha nges were detected as changes in both DNA and protein synthesis rates. These diel cycles were accompanied by diel changes in the distributio n of total DOG, and by diel changes in the proportion of bacteria cont aining visible nucleoids. Noon estimates of bacterial activity were mo re than twice the daily average in the oceanic site, but they were les s different in the other 2 sites. DOC changed daily by 15 mu M (5 to 1 5 % of the total stock). For bacterial activity to explain the diel ch anges in DOC concentration, bacteria should have growth efficiencies l ower than 10% in general, and lower than 2% in the oceanic station.