By shunting material out of the predatory pathway toward detritus and dissolved material, viruses are believed to have an important impact on biogeochemical functions of the pelagic microbial food web. To include viruses as a single plankton functional type (PFT) in dynamic food web models is, however, not trivial since they will then compete with predators for the same host/prey community as a shared limiting resource. As rec...

In food webs, interactions between competition and defence control the partitioning of limiting resources. As a result, simple models of these interactions contain links between biogeochemistry, diversity, food web structure and ecosystem function. Working at hierarchical levels, these mechanisms also produce self‐similarity and therefore suggest how complexity can be generated from repeated application of simple underlying pr...

Interactions of bottom‐up factors such as the availability of mineral and organic nutrients, and top‐down factors like predation and viral infection affect microbial communities of the pelagic food web. Hypothesis derived from previous experimental and modeling work suggest some general mechanisms on how these factors may be linked and call for an experiment to test interactions between (1) nitrogen source and diatom cell size...

Summary Viruses have recurrently been hypothesized as instrumental in driving microbial population diversity. Nonetheless, viral mediated co‐existence of r/k‐strategists, predicted in the Killing‐the‐Winner (KtW) hypothesis, remains controversial and demands empirical evidence. Therefore, we measured the life strategy parameters that characterize the relevant system Micromonas ‐Micromonas Virus (MicV). A large number of host a...

Summary For more than 25 years, virus‐to‐bacteria ratios (VBR) have been measured and interpreted as indicators of the importance of viruses in aquatic ecosystems, yet a generally accepted theory for understanding mechanisms controlling VBR is still lacking. Assuming that the denominator (total bacterial abundance) is primarily predator controlled, while viral lysis compensates for host growth rates exceeding this grazing loss...

Summary By adding a generic description of cost of resistance ( COR ) to the existing ‘killing‐the‐winner’ model, we show how this expands the model's explanatory power to include rank‐abundance relationships in the host population. The model can predict a counter‐intuitive relationship previously suggested in the literature, where abundant viruses are associated with rare hosts and vice versa . The model explains the observed...

The microbial part of the pelagic food web is believed to be a highly dynamic and tightly coupled network in which system behavior emerges from organism properties and interactions. A central goal in microbial ecology is a quantitative understanding of these interactions. A key aspect of these linkages is again the efficiency of food acquisition in the different groups of planktonic organisms. Here we estimate osmotrophic nutr...

Summary The use of inorganic phosphate (Pi) and dissolved organic phosphorus (DOP) by different bacterial groups was studied in experimental mesocosms of P‐starved eastern Mediterranean waters in the absence (control mesocosms) and presence of additional Pi (P‐amended mesocosms). The low Pi turnover times in the control mesocosms and the increase in heterotrophic prokaryotic abundance and production upon Pi addition confirmed...

Summary We demonstrate here results showing that bottom‐up and top‐down control mechanisms can operate simultaneously and in concert in marine microbial food webs, controlling prokaryote diversity by a combination of viral lysis and substrate limitation. Models in microbial ecology predict that a shift in the type of bacterial growth rate limitation is expected to have a major effect on species composition within the community...

We measured the turnover of phosphorus (P) from radioactive‐labeled dissolved deoxyribonucleic acid (dDNA), adenosine triphosphate (ATP), and orthophosphate, and the partitioning of P from these sources into different size fractions of algae and bacteria in nutrient‐manipulated mesocosms. There was a transition from uptake dominated by larger organisms during balanced enrichment toward uptake dominated by smaller organisms dur...

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 µg L −1 , 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...

Coexistence of two organisms competing for the same nutrient is possible if one is an ‘uptake’, and the other a ‘predation defence’ specialist. In pelagic food webs this principle has been linked to cell size. Small osmotroph cells, with their high surface : volume ratio, have been argued to be uptake specialists, while larger osmotrophs avoiding the intense grazing pressure from small protozoan predators might represent ‘pred...

Previous studies have suggested that Mediterranean surface water becomes phosphorus limited for both bacteria and phytoplankton during stratified periods and that orthophosphate uptake in these situations was close to diffusion limitation for both cyanobacteria and autotrophic nanoflagellates. In order to better understand vertical and seasonal variations of this system, we measured orthophosphate uptake by bacteria and phytop...

Using 32 >P, uptake and transfer of phosphorus in the microbial food web were studied in surface water from Villefranche Bay (northwestern Mediterranean) from September to December 2001. During the study, the thermocline gradually declined and vertical mixing started, leading to a transition from a nutrient‐depleted period to a nutrient‐replete period. Before vertical mixing started, the orthophosphate turnover time ranged fro...

The effects of organic and inorganic nutrient enrichments on algal‐bacterial competition were investigated using mesocosms. Interactions were followed over 10 d in 12, 3−m 3 seawater mesocosms in the Isefjord, Denmark. Two sets of four mesocosms were given the same daily addition of “phytoplankton nutrients” (phosphate and nitrate) but received different amounts of glucose, and one set was kept in excess with respect to silica...

Summary Bacterial and archaeal assemblages have been studied in a multipond solar saltern using a range of microbial ecology techniques by four laboratories simultaneously. These include 16S rDNA sequencing from both denaturing gradient gel electrophoresis (DGGE) and clone libraries, and culturing methods. Water samples from eight ponds were analysed, covering a salinity range from near sea water (4% salt) to saturated sodium...

Mechanisms controlling virus abundance and partitioning of loss of bacterial production between viral lysis and protozoan predation are discussed within the framework of an idealized Lotka‐Volterra–type model. This combines nonselective protozoan predation with host‐selective viral lysis of bacteria. The analysis leads to a reciprocal relationship between bacterial diversity and viruses, in which coexistence of competing bacte...

Surface‐water microbial populations were investigated in the northwest Mediterranean for possible indicators of phosphate deficiency and limitation. Low phosphorus availability was suggested by short turnover time (min. observed 0.68 h), high‐alkaline phosphatase activity ( V max = 28 nM hydrolyzed h −1 ), subsaturation of phosphate uptake (2.6–9% of V max ), and high‐pulse uptake capacity of added orthophosphate. Based on hig...

Recent literature indicates that dissolved organic carbon (DOC) may accumulate in productive surface waters. Such accumulation will allow export of DOC to the aphotic zone by diffusion and downwelling. As an alternative to models based on low degradability, we here propose a mechanism where bacterial carbon consumption is restricted due to food web mechanisms controlling both growth and biomass of the bacteria: growth rate is...