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:Phytoplankton-bacteria interactions are pivotal in marine ecosystems, influencing primary production and biogeochemical cycles. Diatoms engage in diverse relationships with bacteria, ranging from mutualism to pathogenicity. This study explores the interaction between a novel Alteromonas macleodii strain from the Equatorial Pacific and the model Thalassiosira pseudonana across the diatom different growth phases. We demonstrate that A. macleodii’s algicidal activity depends on the diatom’s growth phase, defensive capacity, and nutrient availability. The algicidal effect manifests during the diatom’s stationary phase or with external nutrient supplementation, implicating organic matter availability as a key driver. Transcriptomic analysis reveals that A. macleodii shifts from motility-associated to growth-associated gene expression based on the diatom’s physiology and coculture duration. Filtrate assays and fluorescence microscopy suggest a two-stage infection model: initial bacterial motility and exudate secretion induce diatom death, followed by bacterial aggregation around debris. Comparative transcriptomics with other algal hosts highlights host-specific bacterial responses, underscoring the context-dependent nature of these interactions. Our findings provide a deeper understanding of the molecular mechanisms driving diatom-bacteria interactions, shedding light on their role in marine microbial ecology and ecosystem functioning.