Published in Environmental Microbiology…

Identifying Labile DOM Components in a Coastal Ocean through Depleted Bacterial Transcripts and Chemical Signals 

A. Vorobev, S. Sharma, M. Yu, J. Lee, B. J. Washington, W. B. Whitman, F. Ballantyne IV, P. M. Medeiros, and M. A. Moran

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Understanding which compounds comprising the complex and dynamic marine dissolved organic matter (DOM) pool are important in supporting heterotrophic bacterial production remains a major challenge. We eliminated sources of labile phytoplankton products, advected terrestrial material, and photodegradation products to coastal microbial communitiesby enclosing water samples in situ for 24 h in the dark. Bacterial genes for which expression decreased between the beginning and end of the incubationand chemical formulae that were depleted over this same time frame were usedas indicators of bioavailable compounds, an approach that avoids augmenting or modifying the natural DOM pool. Transport- and metabolism-related genes whose relative expression decreased implicated osmolytes, carboxylic acids, fatty acids, sugars, and organic sulfur compoundsas candidate bioreactive molecules. FT-ICR MS analysis of depleted molecular formulae implicated functional groups ~30-40 Da in size cleaved from semi-polar components of DOM as bioreactive components. Both gene expression and FT-ICR MS analyses indicated higher lability of compounds with sulfur and nitrogen heteroatoms. Untargeted methodologies able to integrate biological and chemical perspectives can be effective strategies for characterizing the labile microbial metabolites participating in carbon flux.

Published in Environmental Microbiology…

Mosaic patterns of B‐vitamin synthesis and utilization in a natural marine microbial community

L. Gómez‐Consarnau, R. Sachdeva, S. M. Gifford, L. S. Cutter, J. A. Fuhrman, S. A. Sañudo‐Wilhelmy, M. A. Moran

Aquatic environments contain large communities of microorganisms whose synergistic interactions mediate the cycling of major and trace nutrients, including vitamins. B‐vitamins are essential coenzymes that many organisms cannot synthesize. Thus, their exchange among de novo synthesizers and auxotrophs is expected to play an important role in the microbial consortia and explain some of the temporal and spatial changes observed in diversity. In this study, we analyzed metatranscriptomes of a natural marine microbial community, diel sampled quarterly over one year to try to identify the potential major B‐vitamin synthesizers and consumers. Transcriptomic data showed that the best‐represented taxa dominated the expression of synthesis genes for some B‐vitamins but lacked transcripts for others. For instance, Rhodobacterales dominated the expression of vitamin‐B12 synthesis, but not of vitamin‐B7, whose synthesis transcripts were mainly represented by Flavobacteria. In contrast, bacterial groups that constituted less than 4% of the community (e.g., Verrucomicrobia) accounted for most of the vitamin‐B1 synthesis transcripts. Furthermore, ambient vitamin‐B1 concentrations were higher in samples collected during the day, and were positively correlated with chlorophyll‐a concentrations. Our analysis supports the hypothesis that the mosaic of metabolic interdependencies through B‐vitamin synthesis and exchange are key processes that contribute to shaping microbial communities in nature.