Search for: All records

Abstract Prochlorococcus is found throughout the euphotic zone in the oligotrophic open ocean. Deep mixing and sinking while attached to particles can, however, transport Prochlorococcus cells below this sunlit zone, depriving them of light for extended periods of time. Previous work has shown that Prochlorococcus by itself cannot survive extended periods of darkness. However, when co-cultured with a heterotrophic microbe and subjected to repeated periods of extended darkness, Prochlorococcus cells develop an epigenetically inherited dark-tolerant phenotype that can survive longer periods of darkness. Here we examine the metabolic and physiological changes underlying this adaptation using co-cultures of dark-tolerant and parental strains of Prochlorococcus, each grown with the heterotroph Alteromonas under diel light:dark conditions. The relative abundance of Alteromonas was higher in dark-tolerant than parental co-cultures, while dark-tolerant Prochlorococcus cells were larger, contained less chlorophyll, and were less synchronized to the light:dark cycle. Meta-transcriptome analysis revealed that dark-tolerant co-cultures undergo a joint change, in which Prochlorococcus undergoes a relative shift from photosynthesis to respiration, while Alteromonas shifts toward using more organic acids instead of sugars. Furthermore, the transcriptome data suggested enhanced biosynthesis of amino acids and purines in dark-tolerant Prochlorococcus and enhanced degradation of these compounds in Alteromonas. Collectively, our results demonstrate that dark adaptation involves a strengthening of the metabolic coupling between Prochlorococcus and Alteromonas, presumably mediated by an enhanced, and compositionally modified, carbon exchange between the two species. 

Abstract Prochlorococcus cells are the numerically dominant phototrophs in the open ocean. Cyanophages that infect them are a notable fraction of the total viral population in the euphotic zone, and, as vehicles of horizontal gene transfer, appear to drive their evolution. Here we examine the propensity of three cyanophages—a podovirus, a siphovirus, and a myovirus—to mispackage host DNA in their capsids while infecting Prochlorococcus, the first step in phage-mediated horizontal gene transfer. We find the mispackaging frequencies are distinctly different among the three phages. Myoviruses mispackage host DNA at low and seemingly fixed frequencies, while podo- and siphoviruses vary in their mispackaging frequencies by orders of magnitude depending on growth light intensity. We link this difference to the concentration of intracellular reactive oxygen species and protein synthesis rates, both parameters increasing in response to higher light intensity. Based on our findings, we propose a model of mispackaging frequency determined by the imbalance between the production of capsids and the number of phage genome copies during infection: when protein synthesis rate increase to levels that the phage cannot regulate, they lead to an accumulation of empty capsids, in turn triggering more frequent host DNA mispackaging errors.