The Response of Heterotrophic Prokaryote and Viral Communities to Labile Organic Carbon Inputs Is Controlled by the Predator Food Chain Structure.

Factors controlling the community composition of marine heterotrophic prokaryotes include organic-C, mineral nutrients, predation, and viral lysis. Two mesocosm experiments, performed at an Arctic location and bottom-up manipulated with organic-C, had very different results in community composition for both prokaryotes and viruses. Previously, we showed how a simple mathematical model could reproduce food web level dynamics observed in these mesocosms, demonstrating strong top-down control through the predator chain from copepods via ciliates and heterotrophic nanoflagellates. Here, we use a steady-state analysis to connect ciliate biomass to bacterial carbon demand. This gives a coupling of top-down and bottom-up factors whereby low initial densities of ciliates are associated with mineral nutrient-limited heterotrophic prokaryotes that do not respond to external supply of labile organic-C. In contrast, high initial densities of ciliates give carbon-limited growth and high responsiveness to organic-C. The differences observed in ciliate abundance, and in prokaryote abundance and community composition in the two experiments were in accordance with these predictions. Responsiveness in the viral community followed a pattern similar to that of prokaryotes. Our study provides a unique link between the structure of the predator chain in the microbial food web and viral abundance and diversity.

Effects of differences in organic supply on bacterial diversity subject to viral lysis.

Bacterial diversity is believed to be controlled both by bottom-up and top-down mechanisms such as nutrient competition, predation and viral lysis. We hypothesise that lytic viruses create trophic niches within bacterial communities, and thus primarily control richness and evenness, while substrate composition primarily controls community composition, that is, the inhabitants of these niches. To investigate this, we studied diversity of mixed bacterial communities subject to viruses under different regimes of organic matter supply. From a predator-free inoculum, bacterial communities were allowed to develop in batch cultures where the organic substrate was either a single compound [glucose (G)] or more complex mixtures produced by phytoplankton [Phaeocystis pouchetii (P) or Thalassiosira sp. (T)]. Throughout the experiment, c. 98% of the sequences in treatment G belonged to the Gammaproteobacteria class, which dominated also in the initial phase of the other treatments [T (c. 87%) and P (62%)]. In treatment T, the composition shifted to a dominance of Alphaproteobacteria (c. 37%), while in P, the proportion of Gammaproteobacteria remained stable. Richness increased with increasing substrate complexity, while evenness remained similar in the different treatments. The results suggest that both substrate composition (bottom-up) and viral lysis (top-down) operate simultaneously in the control of bacterial diversity. Despite the reduction in factors supposed to influence prokaryote diversity, the system was still complex if taken into account the potential synergistic interactions within and between the remaining factors.

Effect of increased pCO(2) on bacterial assemblage shifts in response to glucose addition in Fram Strait seawater mesocosms.

Ocean acidification may stimulate primary production through increased availability of inorganic carbon in the photic zone, which may in turn change the biogenic flux of dissolved organic carbon (DOC) and the growth potential of heterotrophic bacteria. To investigate the effects of ocean acidification on marine bacterial assemblages, a two-by-three factorial mescosom experiment was conducted using surface sea water from the East Greenland Current in Fram Strait. Pyrosequencing of the V1-V2 region of bacterial 16S ribosomal RNA genes was used to investigate differences in the endpoint (Day 9) composition of bacterial assemblages in mineral nutrient-replete mesocosms amended with glucose (0 µM, 5.3 µM and 15.9 µM) under ambient (250 µatm) or acidified (400 µatm) partial pressures of CO(2) (pCO(2)). All mesocosms showed low richness and diversity by Chao1 estimator and Shannon index, respectively, with general dominance by Gammaproteobacteria and Flavobacteria. Nonmetric multidimensional scaling analysis and two-way analysis of variance of the Jaccard dissimilarity matrix (97% similarity cut-off) demonstrated that the significant community shift between 0 µM and 15.9 µM glucose addition at 250 µatm pCO(2) was eliminated at 400 µatm pCO(2). These results suggest that the response potential of marine bacteria to DOC input may be altered under acidified conditions.