No abstract

Host-virus interactions structure microbial communities, drive biogeochemical cycles, and enhance genetic diversity in nature 1 , 2 . Hypotheses proposed to explain the range of interactions that mediate these processes often invoke lysogeny 3 – 6 , a latent infection strategy used by temperate bacterial viruses to replicate in host cells until an induction event triggers the production and lytic release of free viruses. Most cultured bacteria harbor temperate viruses in their genomes (prophage) 7 . The absence of prophages in cultures of the dominant lineages of marine bacteria has contributed to an ongoing debate over the ecological significance of lysogeny and other viral life strategies in nature 6 , 8 – 15 . Here we report the discovery of prophages in cultured SAR11, the ocean’s most abundant clade of heterotrophic bacteria 16 , 17 . We show the concurrent production of cells and viruses, with enhanced virus production under carbon-limiting growth conditions. Evidence that related prophages are broadly distributed in the oceans suggests that similar interactions have contributed to the evolutionary success of SAR11 in nutrient limited systems.

Phytoplankton and bacteria form the base of marine ecosystems and their interactions drive global biogeochemical cycles. The effects of bacteria and bacteria-produced compounds on diatoms range from synergistic to pathogenic and can affect the physiology and transcriptional patterns of the interacting diatom. Here, we investigate physiological and transcriptional changes in the marine diatom Thalassiosira pseudonana induced by extracellular metabolites of a known antagonistic bacterium Croceibacter atlanticus. Mono-cultures of C. atlanticus released compounds that inhibited diatom cell division and elicited a distinctive morphology of enlarged cells with increased chloroplast content and enlarged nuclei, similar to what was previously observed when the diatom was co-cultured with live bacteria. The extracellular C. atlanticus metabolites induced transcriptional changes in diatom pathways that include recognition and signaling pathways, cell cycle regulation, carbohydrate and amino acid production, as well as cell wall stability. Phenotypic analysis showed a disruption in the diatom cell cycle progression and an increase in both intra- and extracellular carbohydrates in diatom cultures after bacterial exudate treatment. The transcriptional changes and corresponding phenotypes suggest that extracellular bacterial metabolites, produced independently of direct bacterial-diatom interaction, may modulate diatom metabolism in ways that support bacterial growth.