Microbial cycling of marine high molecular weight dissolved organic matter
Author(s)Sosa, Oscar A.
KeywordsKnorr (Ship : 1970-) Cruise KN207-01
Kilo Moana (Ship) Cruise KM13-05
Kilo Moana (Ship) Cruise KM14-09
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AbstractSubmitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 2016
Microorganisms play a central role mediating biogeochemical cycles in the ocean. Marine
dissolved organic matter (DOM) – a reservoir of organic solutes and colloids derived from plankton is a
major source of carbon, nutrients, and energy to microbial communities. The biological transformation
and remineralization of DOM sustains marine productivity by linking the microbial food web to higher
trophic levels (the microbial loop) and exerts important controls over the cycles of carbon and bioessential
elements, such as nitrogen and phosphorus, in the sea. Yet insight into the underlying
metabolism and reactions driving the degradation of DOM is limited partly because its exact molecular
composition is difficult to constrain and appropriate microbial model systems known to decompose
marine DOM are lacking. This thesis identifies marine microorganisms that can serve as model systems to
study the metabolic pathways and biochemical reactions that control an important ecosystem function,
DOM turnover. To accomplish this goal, bacterial isolates were obtained by enriching seawater in
dilution-to-extinction culturing experiments with a natural source of DOM, specifically, the high
molecular weight (HMW) fraction (>1 kDa nominal molecular weight) obtained by ultrafiltration.
Because it is relatively easy to concentrate and it is fairly uniform in its chemical composition across the
global ocean and other aquatic environments, HMW DOM has the potential to serve as a model growth
substrate to study the biological breakdown of DOM. The phylogeny, genomes, and growth
characteristics of the organisms identified through this work indicate that HMW DOM contains
bioavailable substrates that may support widespread microbial populations in coastal and open-ocean
environments. The availability of ecologically relevant isolates in culture can now serve to test hypothesis
emerging from cultivation-independent studies pertaining the potential role of microbial groups in the
decomposition of organic matter in the sea. Detailed studies of the biochemical changes exerted on DOM
by selected bacterial strains will provide new insight into the processes driving the aerobic microbial food
chain in the upper ocean.
Financial support for this work was provided by the National Science Foundation Center
for Microbial Oceanography: Research and Education (award #EF0424599) and the Gordon and
Betty Moore Foundation (grant #492.01, #3777, and #3298).