Bacterial community composition responds to changes in copepod abundance and alters ecosystem function in an Arctic mesocosm study.

Combining a minimum food web model with Arctic microbial community dynamics, we have suggested that top-down control by copepods can affect the food web down to bacterial consumption of organic carbon. Pursuing this hypothesis further, we used the minimum model to design and analyse a mesocosm experiment, studying the effect of high (+Z) and low (-Z) copepod density on resource allocation, along an organic-C addition gradient. In the Arctic, both effects are plausible due to changes in advection patterns (affecting copepods) and meltwater inputs (affecting carbon). The model predicts a trophic cascade from copepods via ciliates to flagellates, which was confirmed experimentally. Auto- and heterotrophic flagellates affect bacterial growth rate and abundance via competition for mineral nutrients and predation, respectively. In +Z, the model predicts low bacterial abundance and activity, and little response to glucose; as opposed to clear glucose consumption effects in -Z. We observed a more resilient bacterial response to high copepods and demonstrate this was due to changes in bacterial community equitability. Species able to use glucose to improve their competitive and/or defensive properties, became predominant. The observed shift from a SAR11-to a Psychromonodaceae - dominated community suggests the latter was pivotal in this modification of ecosystem function. We argue that this group used glucose to improve its defensive or its competitive abilities (or both). Adding such flexibility in bacterial traits to the model, we show how it creates the observed resilience to top-down manipulations observed in our experiment.

Strong seasonality and interannual recurrence in marine myovirus communities.

The temporal community dynamics and persistence of different viral types in the marine environment are still mostly obscure. Polymorphism of the major capsid protein gene, g23, was used to investigate the community composition dynamics of T4-like myoviruses in a North Atlantic fjord for a period of 2 years. A total of 160 unique operational taxonomic units (OTUs) were identified by terminal restriction fragment length polymorphism (TRFLP) of the gene g23. Three major community profiles were identified (winter-spring, summer, and autumn), which resulted in a clear seasonal succession pattern. These seasonal transitions were recurrent over the 2 years and significantly correlated with progression of seawater temperature, Synechococcus abundance, and turbidity. The appearance of the autumn viral communities was concomitant with the occurrence of prominent Synechococcus blooms. As a whole, we found a highly dynamic T4-like viral community with strong seasonality and recurrence patterns. These communities were unexpectedly dominated by a group of persistently abundant viruses.

Counterintuitive carbon-to-nutrient coupling in an Arctic pelagic ecosystem.

Predicting the ocean's role in the global carbon cycle requires an understanding of the stoichiometric coupling between carbon and growth-limiting elements in biogeochemical processes. A recent addition to such knowledge is that the carbon/nitrogen ratio of inorganic consumption and release of dissolved organic matter may increase in a high-CO(2) world. This will, however, yield a negative feedback on atmospheric CO(2) only if the extra organic material escapes mineralization within the photic zone. Here we show, in the context of an Arctic pelagic ecosystem, how the fate and effects of added degradable organic carbon depend critically on the state of the microbial food web. When bacterial growth rate was limited by mineral nutrients, extra organic carbon accumulated in the system. When bacteria were limited by organic carbon, however, addition of labile dissolved organic carbon reduced phytoplankton biomass and activity and also the rate at which total organic carbon accumulated, explained as the result of stimulated bacterial competition for mineral nutrients. This counterintuitive 'more organic carbon gives less organic carbon' effect was particularly pronounced in diatom-dominated systems where the carbon/mineral nutrient ratio in phytoplankton production was high. Our results highlight how descriptions of present and future states of the oceanic carbon cycle require detailed understanding of the stoichiometric coupling between carbon and growth-limiting mineral nutrients in both autotrophic and heterotrophic processes.

Nature of phosphorus limitation in the ultraoligotrophic eastern Mediterranean.

Phosphate addition to surface waters of the ultraoligotrophic, phosphorus-starved eastern Mediterranean in a Lagrangian experiment caused unexpected ecosystem responses. The system exhibited a decline in chlorophyll and an increase in bacterial production and copepod egg abundance. Although nitrogen and phosphorus colimitation hindered phytoplankton growth, phosphorous may have been transferred through the microbial food web to copepods via two, not mutually exclusive, pathways: (i) bypass of the phytoplankton compartment by phosphorus uptake in heterotrophic bacteria and (ii) tunnelling, whereby phosphate luxury consumption rapidly shifts the stoichiometric composition of copepod prey. Copepods may thus be coupled to lower trophic levels through interactions not usually considered.

Are viruses important partners in pelagic fend webs?

Viruses have been assumed to play a rather negligible role as partners in microbial food web dynamics. However, recent discoveries suggest that the rate of virally induced lysis of marine microbial populations may be significant. This, in turn, may have important consequences for the developing conceptual framework of the microbial food web.

Viruses as partners in spring bloom microbial trophodynamics.

Population sizes of algae, bacteria, heterotrophic flagellates, and viruses were observed through the 1989 spring diatom bloom in Raunefjorden in western Norway. The culmination of the diatom bloom was followed by a peak in the concentration of bacteria and an increase in the concentration of heterotrophic flagellates, a pattern consistent with the concept of a food chain from photosynthetically produced organic material, through bacteria, to bacterivorous flagellates. The concentration of viruses varied through the spring bloom from 5 x 10 in the prebloom situation to a maximum of 1.3 x 10 viruses ml 1 week after the peak of the diatom bloom. Coinciding with the collapse in the diatom bloom, a succession of bacteria and viruses was observed in the mucous layer surrounding dead or senescent diatoms, with an estimated maximum of 23% of the total virus population attached to the diatoms. The dynamic behavior observed for the virus population rules out the possibility that it is dominated by inactive species, and the viruses are suggested to be active members of the microbial food web as agents causing lysis in parts of the bacterial population, diverting part of the bacterial production from the predatory food chain.

Mathematical description of competition between two and three bacterial species under dual substrate limitation in the chemostat: a comparison with experimental data.

A mathematical model is presented which describes the growth of two bacterial species in mixed chemostat cultures under dual substrate limitation. Competition experiments between a facultatively chemolithotrophic Thiobacillus and either a heterotroph or an obligately chemolithotrophic Thiobacillus served as an experimental model system [Gottschal, de Vries, and Kuenen, Archives of Microbiology, 121, 241-249 (1979)]. Furthermore, the introduction of Monod-type growth kinetics in the model allowed an assessment of the relative importance of the growth parameters for the outcome of the competition. In addition, it is shown how the results of the mathematical description of the two-membered mixed cultures can be used to predict the outcome of the competition between the three species competing for the two growth-limiting substrates acetate and thiosulfate in a three-membered mixed culture. In contrast to the experimental data of Gottschal, de Vries, and Kuenen it is predicted that two of the three species or only one of them (the "mixotroph") will survive in the culture. Within the framework of the proposed mathematical model, two possible explanations for the experimentally observed coexistence of three species are suggested: either the very slow dynamics of the system did not allow the attainment of a true steady state within the time scale of the present experiments or some parameters describing the mixed culture were extremely sensitive towards minor fluctuations in dilution rate. The results of the present mathematical model support the view that facultatively chemolithotrophic bacteria are able to survive under appropriate limiting mixed substrate conditions in the presence of more "specialized" heterotrophs and obligately chemolitotrophs, in spite of their relatively low specific growth rate.