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

Abstract

Natural bacterial assemblages are not physiologically and phenotypically homogeneous. Development of new methodologies, such as flow cytometry, has allowed bacterial types with different degrees of activity (which have been called HighDNA and LowDNA bacteria) to be distinguished on the basis of their DNA content. Because previous data have suggested that HighDNA bacteria are really the active fraction of the community, we hypothesize that the dynamics of bacterial production (BP) and grazing losses should be linked to the 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 Mediterranean 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 transect we visited 'coastal' (on the continental platform), 'slope' and 'open sea' (> 2000 m) stations. Samples were collected at surface and at the deep chlorophyll maximum. Bac- terial 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 and grazing rates inside experimental bottles every 8 to 12 h for a total period of 44 h. Bacterial abun- dance was counted and cell volume was estimated by flow citometry, BP was determined by 3 H-leucine incorporation, and grazing rate was obtained by following the evolution inside the experimental bottles of added 5-((4, 6 dichlorotriazin-2yl) amino)-fluorescein-stained Pseudomonas diminuta as fluorescent- labeled bacteria. In situ, BP was higher in coastal and slope stations than in open sea stations. The percentage of HighDNA bacteria ranged between 25 and 87% and BP between 0.09 and 5.9 µg C l -1 d -1 , lower in the open sea and higher in the slope station of Palamos. Grazing loss rates followed a similar pattern, from 0.2 × 10 5 to 2.2 × 10 5 cells ingested ml -1 d -1 , again lower in the open sea and higher at the coastal station of Palamos. In most of the experiments, BP increased with time following the increase of HighDNA bacteria, while LowDNA bacteria remained practically constant during the whole period. Exponential bacterial growth appeared at 20 to 32 h. Grazing rates were maximal right after the expo- nential bacterial growth (at 32 to 44 h), suggesting that the increase of the HighDNA bacterial fraction in the previous period stimulated grazers to consume it. In both in situ and experimental samples, abun- dance and biomass of HighDNA bacteria were strongly correlated with BP and grazing rates, but no correlation was found between these variables and LowDNA bacterial abundance. Samples with the lowest percentage of HighDNA bacteria also had low BP and grazing rates. Indeed, the dynamics of BP and grazing losses were better related to changes in HighDNA bacteria, providing further evidence that they can be considered as the 'active' fraction of the whole bacterial community.