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Microorganisms play a central role mediatingbiogeochemical cycles in the ocean. Marine dissolved organic matter(DOM) - a reservoir of organic solutes and colloids derived fromplankton is a major source of carbon, nutrients, and energy tomicrobial communities. The biological transformation andremineralization of DOM sustains marine productivity by linking themicrobial food web to higher trophic levels (the microbial loop)and exerts important controls over the cycles of carbon andbioessential elements, such as nitrogen and phosphorus, in the sea.Yet insight into the underlying metabolism and reactions drivingthe degradation of DOM is limited partly because its exactmolecular composition is difficult to constrain and appropriatemicrobial model systems known to decompose marine DOM are lacking.This thesis identifies marine microorganisms that can serve asmodel systems to study the metabolic pathways and biochemicalreactions that control an important ecosystem function, DOMturnover. To accomplish this goal, bacterial isolates were obtainedby enriching seawater in dilution-to-extinction culturingexperiments with a natural source of DOM, specifically, the highmolecular weight (HMW) fraction (>1 kDa nominal molecularweight) obtained by ultrafiltration. Because it is relatively easyto concentrate and it is fairly uniform in its chemical compositionacross the global ocean and other aquatic environments, HMW DOM hasthe potential to serve as a model growth substrate to study thebiological breakdown of DOM. The phylogeny, genomes, and growthcharacteristics of the organisms identified through this workindicate that HMW DOM contains bioavailable substrates that maysupport widespread microbial populations in coastal and open-oceanenvironments. The availability of ecologically relevant isolates inculture can now serve to test hypothesis emerging fromcultivation-independent studies pertaining the potential role ofmicrobial groups in the decomposition of organic matter in the sea.Detailed studies of the biochemical changes exerted on DOM byselected bacterial strains will provide new insight into theprocesses driving the aerobic microbial food chain in the upperocean.

Microbial cycling of marine high molecular weight dissolved organic matter