Dilution reveals how viral lysis and grazing shape microbial communities

Abstract

Grazing by protists and viral infection are the main known sources of marine bacterial mortality, and both processes shape the structure and ecology of microbial communities. We diluted planktonic microbial communities to determine how decreasing the encounter rate between bacteria, grazers, and viruses affects community structure. In experimental treatments, sea water was diluted 10-fold in water from which protists and bacteria had been removed (0.2 lm filtered), or from which protists bacteria and viruses had been removed (0.02 lm filtered). In both dilution treatments, bacterial and protistan communities generally recovered within 3 d to near their original abundances, but viruses did not. The microbial community structure of bacteria in both diluted treatments was significantly different from untreated control communities, after 3 d, and significantly different between the 0.2 lm and 0.02 lm dilution treatments after 6 d. Diluted treatments were dominated by operational taxonomic units that are normally rare in near-surface seawater, including some normally more abundant in the deep water at this location such as SAR324 and SAR406, while abundant surface taxa including SAR11 did not increase in abundance after dilution. These results are consistent with the hypothesis that microbes face a tradeoff between fast growth and resistance to predation and infection, and that microbes that are normally most abundant in the California coastal surface waters (such as SAR11 and Actinobacteria) are both more grazer resistant and slower growing than potentially opportunistic organisms that are rare in surface water but capable of fast net growth when grazers or viruses are removed.

Simple models combining competition, defence and resource availability have broad implications in pelagic microbial food webs

“…Another experimentally confirmed prediction is a shift to dominance of previously rare host types when viral control is reduced (Bouvier & del Giorgio ; Cram et al . ).…”

“…If these low-active individuals starve, the theory above suggests this to be because they 'have locked the gates to keep the enemy out', rather than because 'there is no food out there' (Thingstad et al 2014). Another experimentally confirmed prediction is a shift to dominance of previously rare host types when viral control is reduced (Bouvier & del Giorgio 2007;Cram et al 2016).…”

Impact of grazing, resource availability and light on prokaryotic growth and diversity in the oligotrophic surface global ocean

“…It is important to note that the total amount of resources was not manipulated here; we only modified its availability by reducing prokaryotic abundance through dilution. grazers are present) seawater (Ferrera et al, 2011;Cram et al, 2016), which may be explained by the tight control exerted by grazers on fastgrowing bacteria. grazer-free) and diluted seawater short-term incubations with high throughput sequencing analyses in the oligotrophic ocean.…”

Grazers structure the bacterial and algal diversity of aquatic metacommunities

“…However, other mechanisms involving dilution effects are also possible, including changes in encounter rates between species (Cram et al. ) and rates of colonization via areal dispersal (Jones et al. ).…”

“…, Cram et al. ). However, it is difficult to generalize across these studies, because they varied widely in their duration (from a few days to months), were performed in vessels of different size (<1–500 L), and used a wide range of methods to quantify bacterial diversity (in situ hybridization, fingerprinting methods, next‐generation sequencing).…”