Marine dissolved organic matter (DOM) contains as much carbon as the Earth's atmosphere, and represents a critical component of the global carbon cycle. To better define microbial processes and activities associated with marine DOM cycling, we analyzed genomic and transcriptional responses of microbial communities to high-molecularweight DOM (HMWDOM) addition. The cell density in the unamended control remained constant, with very few transcript categories exhibiting significant differences over time. In contrast, the DOM-amended microcosm doubled in cell numbers over 27 h, and a variety of HMWDOM-stimulated transcripts from different taxa were observed at all time points measured relative to the control. Transcripts significantly enriched in the HMWDOM treatment included those associated with two-component sensor systems, phosphate and nitrogen assimilation, chemotaxis, and motility. Transcripts from Idiomarina and Alteromonas spp., the most highly represented taxa at the early time points, included those encoding TonB-associated transporters, nitrogen assimilation genes, fatty acid catabolism genes, and TCA cycle enzymes. At the final time point, Methylophaga rRNA and non-rRNA transcripts dominated the HMWDOM-amended microcosm, and included gene transcripts associated with both assimilatory and dissimilatory single-carbon compound utilization. The data indicated specific resource partitioning of DOM by different bacterial species, which results in a temporal succession of taxa, metabolic pathways, and chemical transformations associated with HMWDOM turnover. These findings suggest that coordinated, cooperative activities of a variety of bacterial "specialists" may be critical in the cycling of marine DOM, emphasizing the importance of microbial community dynamics in the global carbon cycle.icrobial activities drive most of Earth's biogeochemical cycles. Many processes and players involved in these planetary cycles, however, remain largely uncharacterized, due to the inherent complexity of microbial community processes in the environment. Cycling of organic carbon in ocean surface waters is no exception. Though marine dissolved organic matter (DOM) is one of the largest reservoirs of organic carbon on the planet (1), microbial activities that regulate DOM turnover remain poorly resolved (2).Marine DOM is an important substrate for heterotrophic bacterioplankton, which efficiently remineralize as much as 50% of total primary productivity through the microbial loop (3-6). Though some DOM is remineralized on short timescales of minutes to hours, a significant fraction escapes rapid removal. In marine surface waters, this semilabile DOM transiently accumulates to concentrations 2-3 times greater than are found in the deep sea (7), and represents a large inventory of dissolved carbon and nutrients that are potential substrates for marine microbes. Timeseries analyses of semilabile DOM accumulation in temperate and subtropical upper ocean gyres show an annual cycle in DOC inventory with net accumulation following the ...

Mesoscale eddies may play a critical role in ocean biogeochemistry by increasing nutrient supply, primary production, and efficiency of the biological pump, that is, the ratio of carbon export to primary production in otherwise nutrient-deficient waters. We examined a diatom bloom within a cold-core cyclonic eddy off Hawaii. Eddy primary production, community biomass, and size composition were markedly enhanced but had little effect on the carbon export ratio. Instead, the system functioned as a selective silica pump. Strong trophic coupling and inefficient organic export may be general characteristics of community perturbation responses in the warm waters of the Pacific Ocean.