Project description:Prochlorococcus is found throughout the euphotic zone in the oligotrophic open ocean. Deep mixing and sinking in aggregates or while attached to particles can, however, transport cells below this sunlit zone, depriving them of light for extended periods of time and influencing their circulation via ocean currents. Viability of these cells over extended periods of darkness could shape the ecology and evolution of the Prochlorococcus collective. We have shown that when co-cultured with a heterotrophic microbe and subjected to repeated periods of extended darkness, Prochlorococcus cells develop a heritable dark-tolerant phenotype – through an apparent epigenetic mechanism – such that they survive longer periods of darkness. Here we examine this adaptation at the level of physiology and metabolism in co-cultures of dark-tolerant and parent strains of Prochlorococcus, each grown with the heterotroph Alteromonas under diel light:dark conditions. The relative abundance of Alteromonas is higher in dark-tolerant than parental co-cultures, while dark tolerant Prochlorococcus cells are also larger, contain less chlorophyll, and are less synchronized to the light:dark cycle. Meta-transcriptome analysis of the cultures further suggests that dark-tolerant co-cultures undergo a coupled shift in which Prochlorococcus uses more organic carbon and less photosynthesis, and Alteromonas uses more organic acids and fewer sugars. Collectively, the data suggest that dark adaptation involves a loosening of the coupling between Prochlorococcus metabolism and the light:dark cycle and a strengthening of the coupling between the carbon metabolism of Prochlorococcus and Alteromonas.
Project description:The fraction of dissolved dimethylsulfoniopropionate (DMSPd) converted by marine bacterioplankton into the climate-active gas dimethylsulfide (DMS) varies widely in the ocean, with the factors that determine this value still largely unknown. One current hypothesis is that the ratio of DMS formation:DMSP demethylation is determined by DMSP availability, with 'availability' in both an absolute sense (i.e., concentration in seawater) and in a relative sense (i.e., proportionally to other labile organic S compounds) being proposed as the critical factor. We investigated these models during an experimentally-induced phytoplankton bloom using an environmental microarray targeting DMSP-related gene expression in the Roseobacter group, a taxon of marine bacteria known to play an important role in the surface ocean sulfur cycle. The array consisted of 1,578 probes to 431 genes, including those previously linked to DMSP degradation as well as core genes common in sequenced Roseobacter genomes. The prevailing pattern of Roseobacter gene expression showed depletion of DMSP-related transcripts during the peak of the bloom, despite the fact that absolute concentrations and flux of DMSP-related compounds were increasing. A likely interpretation is that DMSPd was assimilated by Roseobacter populations in proportion to its relative abundance in the organic matter pool (the “relative sense” hypothesis), and that it is not taken up in preference to other sources of labile organic sulfur or carbon produced during the bloom. The relative investment of the Roseobacter community in DMSP demethylation did not predict the fractional conversion of DMSP to DMS, however, suggesting a complex regulatory process that may involve multiple fates of DMSPd.