Dynamics of whole community bacterial production and grazing losses in seawater incubations as related to the changes in the proportions of bacteria with different DNA content

Natural bacterial assemblages are not physiologically and phenotypically ho mogeneous. Development of new methodologies, such as flow cytometry, has al lowed bacterial types with different degrees of activity (which have been c alled HighDNA and LowDNA bacteria) to be distinguished on the basis of thei r DNA content. Because previous data have suggested that HighDNA bacteria a re really the active fraction of the community, we hypothesize that the dyn amics of bacterial production (BP) and grazing losses should be linked to t he changes of this fraction rather than to changes in the whole community. To test our hypothesis we took samples during a cruise in the NW Mediterran ean Sea in March 1999 from 6 selected stations placed along 2 transects, 1 perpendicular to the city of Barcelona and the other to Palamos, In each tr ansect we visited 'coastal' (on the continental platform), 'slope' and 'ope n sea' (> 2000 m) stations. Samples were collected at surface and at the de ep chlorophyll maximum. Bacterial abundance (total, HighDNA and LowDNA) and BP were determined in in situ samples. Also, 12 experiments were performed to survey the dynamics of HighDNA bacteria (percentage and biomass), BP an d grazing rates inside experimental bottles every 8 to 12 h for a total per iod of 44 h. Bacterial abundance was counted and cell volume was estimated by flow citometry, BP was determined by H-3-leucine incorporation, and graz ing rate was obtained by following the evolution inside the experimental bo ttles of added 5-([4,6 dichlorotriazin-2yl) amino] -fluorescein-stained Pse udomonas diminuta as fluorescent-labeled bacteria. In situ, BP was higher i n coastal and slope stations than in open sea stations. The percentage of H ighDNA bacteria ranged between 25 and 87 % and BP between 0.09 and 5.9 pg C l(-1) d(-1), lower in the open sea and higher in the slope station of Pala mos. Grazing loss rates followed a similar pattern, from 0.2 x 10(5) to 2.2 x 10(5) cells ingested ml(-1) d(-1), again lower in the open sea and highe r at the coastal station of Palamos. In most of the experiments, BP increas ed with time following the increase of HighDNA bacteria, while LowDNA bacte ria remained practically constant during the whole period. Exponential bact erial growth appeared at 20 to 32 h. Grazing rates were maximal right after the exponential bacterial growth (at 32 to 44 h), suggesting that the incr ease of the HighDNA bacterial fraction in the previous period stimulated gr azers to consume it. In both in situ and experimental samples, abundance an d biomass of HighDNA bacteria were strongly correlated with BP and grazing rates, but no correlation was found between these variables and LowDNA bact erial abundance. Samples with the lowest percentage of HighDNA bacteria als o had low BP and grazing rates. Indeed, the dynamics of BP and grazing loss es were better related to changes in HighDNA bacteria, providing further ev idence that they can be considered as the 'active' fraction of the whole ba cterial community.