Viral lysis of specific bacterial populations has been suggested to be an important factor for structuring marine bacterioplankton communities. In the present study, the influence of bacteriophages on the diversity and population dynamics of four marine bacterial phage-host systems was studied experimentally in continuous cultures and theoretically by a mathematical model. By use of whole genome DNA hybridization toward community DNA, we analyzed the dynamics of individual bacterial host populations in response to the addition of their specific phage in continuous cultures of mixed bacterial assemblages. In these experiments, viral lysis had only temporary effects on the dynamics and diversity of the individual bacterial host species. Following the initial lysis of sensitive host cells, growth of phage-resistant clones of the added bacteria resulted in a distribution of bacterial strains in the phage-enriched culture that was similar to that in the control culture without phages after about 50-60 h incubation. Consequently, after a time frame of 5-10 generations after lysis, it was the interspecies competition rather than viral lysis of specific bacterial strains that was the driving force in the regulation of bacterial species composition in these experiments. The clonal diversity, on the other hand, was strongly influenced by viral activity, since the clonal composition of the four species in the phage-enriched culture changed completely from phage-sensitive to phage-resistant clones. The model simulation predicted that viral lysis had a strong impact on the population dynamics, the species composition, and the clonal composition of the bacterial community over longer time scales (weeks). However, according to the model, the overall density of bacteria in the system was not affected by phages, since resistant clones complemented the fluctuations caused by viral lysis. Based on the model analysis, we therefore suggest that viral lysis can have a strong influence on the dynamics of bacterial populations in planktonic marine systems.
Abundance, production and extracellular enzymatic activity of free-living and attached bacteria were measured during the development and collapse of a spring bloom in a eutrophic lake. Free-living bacteria accounted for most of the total bacterial production during the first part of the bloom. Their production had a significant positive correlation to chlorophyll (P < .01) and polysaccharide concentration (P < .02) and to potential β-glucosidase and aminopeptidase activity (P < .05), suggesting that algal release of dissolved polymeric compounds provided an important carbon source for bacterial production. As the bloom collapsed, we observed a change in the activity and structure of the microbial community. The mean contribution of attached bacteria to total bacterial production increased from 12% during the first part of the bloom to 26% at the end. Also, the extracellular enzymatic activity of attached bacteria increased as the bloom collapsed and constituted up to 75% of the total hydrolytic activity. An estimated disparity between hydrolytic activity and the corresponding carbon demand of attached bacteria suggested a net release of dissolved organic compounds from organic particles via polymer hydrolysis by attached bacteria.
We investigated the partitioning of carbon, nitrogen, and phosphorus between particulate and dissolved production using 11-m 3 marine mesocosms (bags) in a Norwegian fjord with a salinity of 28.3, a chlorophyll concentration of 0.6 mg L 21 , an even biomass among five algal groups, and nitrogen limitation as the initial conditions. The experiment lasted 21 days in August. Addition of silicate (+Si) resulted in diatom dominance, while a more diverse community was present in treatments with no added Si (2Si). Addition of inorganic nutrients in a N : P gradient from 64 to 4 either conserved the initial N limitation or forced the plankton communities to P limitation. Per added limiting nutrient, the diatom-dominated bags produced more particulate (POC) and dissolved organic carbon (DOC) than the other bags. However, the relative partitioning of net production to POC and DOC did not differ as a function of the plankton communities. Between 22% and 33% of the net production accumulated as new DOC. The higher values were found in the N-limited bags. The production of new dissolved organic nitrogen (DON) was variable over time, and short periods of positive production were followed by removal (negative production). Between 6% and 22% of the assimilated N was 1 Corresponding author (msondergaard@bi.ku.dk). AcknowledgmentsWe thank Anne J. Jacobsen, Louise Oriol, and Winnie Martinsen for their technical assistance. The thoughtful comments by two reviewers improved the final product. Bergen LSF is thanked for support (HPRI-CT-2002-00181) as is the European Union (EVK3-CT-2000-00034) and the Niels Bohr Foundation (TK).
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