Community composition and activity of prokaryotes associated to detrital particles in two contrasting lake ecosystems.

The composition, distribution and extracellular enzyme activities of bacteria attached to small (2-50 microm in size) transparent exopolymer and Coomassie-stained proteinaceous particles (TEP and CSP) were examined in two lakes of different trophic status located in the Massif Central of France. TEP concentrations (10(4)-10(6) particle per L) were significantly higher in the more productive lake and were significantly related to chlorophyll a concentrations. The majority of TEP and CSP were colonized by bacteria that constituted 2.6% and 7.4% of the total 4',6-diamidino-2-phenylindole-stained bacteria in lakes Pavin and Aydat, respectively. In both lakes, the composition of particle-associated bacteria was different from that of free-living bacteria, the Betaproteobacteria and Bacteroidetes (i.e. former Cytophaga-Flavobacteria group) being the dominant groups on particles. We also found that 2-5 microm TEP were more colonized than 2-5 microm CSP in the two lakes, and that TEP colonization was higher in the less productive lake. Measurements of Leucine aminopeptidase and alpha-glucosidase activities in fractionated lake water (0.2-1.2, 1.2-5 and >5 microm fractions) indicated that proteolytic activity was always higher and that particle-associated bacteria have higher enzymatic activities than free-living bacteria. The glycolytic activities in the 1.2-5 and >5 microm fractions were related to the abundance of TEP. We conclude that small freshwater detrital organic particles constitute microhabitats with high bacterial activities in pelagic environments and, undoubtedly, present significant ecological implications for the prokaryotic community structure and function in aquatic ecosystems.

Incorporation of 3H-thymidine by different prokaryotic groups in relation to temperature and nutrients in a lacustrine ecosystem.

The incorporation of [3H-methyl] thymidine (3H-TdR) by Eubacteria, bacterial groups (alpha- and beta-Proteobacteria, Cytophaga-Flavobacter), and Archaea was measured according to temperature (7 and 17 degrees C) and nutrient levels (nitrogen, phosphorus, and carbon) in a lacustrine system (Sep, France). Short-term incubation was performed using a combination of microautoradiography and fluorescent in situ hybridization. Irrespective of the temperatures and nutrients studied, all the major phylogenetic bacterial groups assimilated 3H-TdR, and in most of the treatments studied, the proportion of beta-Proteobacteria taking up 3H-TdR was higher than those in the other bacterial groups. The proportion of Bacteria and different bacterial groups studied incorporating 3H-TdR were significantly increased, approximately 1.5-fold, by temperature except for alpha-Proteobacteria (7.6-fold). The nutrient effect was not the same for the different bacterial groups according to the temperatures studied. The proportions of alpha-Proteobacteria (at both temperatures) and Cytophaga-Flavobacter (at 7 degrees C) taking up 3H-TdR were significantly decreased and increased by adding N and P, respectively. Also, adding N, P, and C increased and decreased the percentage of beta-Proteobacteria incorporating 3H-TdR at 7 and 17 degrees C, respectively. The archaeal community showed a similar proportion of active cells (i.e., 3H-TdR) to the bacterial community, and uptake of 3H-TdR by Archaea was significantly increased (P < 0.05) by both temperature and nutrients. Thus, the assimilation of 3H-TdR by bacterial groups and Archaea in lacustrine system is significantly controlled by both temperature and nutrients.

Effects of viruses and predators on prokaryotic community composition.

Dialysis bags were used to examine the impact of predation and viral lysis on prokaryotic community composition (PCC) over a 5-day experiment in the oligomesotrophic Lake Pavin (France). The impact of the different predator communities (protists and metazoans) of prokaryotes was estimated by water fractionation (<5 microm: treatment filtered on 5 microm, without ciliates and metazoans; UNF: unfiltered treatment with all planktonic communities). Enrichments of natural viruses (<1.2 microm: with a natural virus concentration; <1.2 mum V and VV: with enrichment leading to a double or triple concentration of viruses, respectively) were used to indirectly assess the control of virioplankton. Viral activity was estimated from the frequency of visibly infected cells (FVIC). PCC was determined by fluorescence in situ hybridization (FISH) and terminal restriction fragment length polymorphism (T-RFLP). In this study, PCC was affected by the eukaryote communities (especially flagellates), and viruses to a lesser extent. Cyanobacteria declined significantly during the experiment and were highly correlated with the FVIC. In addition, the 503-bp terminal restriction fragment (T-RF) disappeared in treatments with virus enrichments, suggesting possible viral-associated mortality processes, whereas the 506-bp T-RF was not affected in these treatments. On one hand, these results suggest a control of the PCC: first, by viral lysis of some dominant phylotypes and second, by interspecific competition between resistant strains for the uptake of substrates released by this lysis. The increase of Archaea may suggest that these cells benefit such resources. On the other hand, the disappearance and the stable proportion of some dominant phylotypes suggested a selection pressure due to the predatory activity on prokaryotes. In conclusion, prokaryotic abundance appears to be mainly controlled by flagellate protists, which also affected PCC, whereas viruses seemed to be essentially responsible for profound changes in PCC via direct and indirect actions.