Abstract
Marine microbes are critical players in the global carbon cycle, affecting both the reduction of inorganic carbon and the remineralization of reduced organic compounds back to carbon dioxide. Members of microbial consortia all depend on marine dissolved organic matter (DOM) and in turn, affect the molecules present in this heterogeneous pool. Our understanding of DOM produced by marine microbes is biased towards single species laboratory cultures or simplified field incubations, which exclude large phototrophs and protozoan grazers. Here we explore the interdependence of DOM composition and bacterial diversity in two mixed microbial consortia from coastal seawater: a whole water community and a <1.0-μm community dominated by heterotrophic bacteria. Each consortium was incubated with isotopically-labeled glucose for 9 days. Using stable-isotope probing techniques and electrospray ionization Fourier-transform ion cyclotron resonance mass spectrometry, we show that the presence of organisms larger than 1.0-μm is the dominant factor affecting bacterial diversity and low-molecular-weight (<1000 Da) DOM composition over this experiment. In the <1.0-μm community, DOM composition was dominated by compounds with lipid and peptide character at all time points, confirmed by fragmentation spectra with peptide-containing neutral losses. In contrast, DOM composition in the whole water community was nearly identical to that in the initial coastal seawater. These differences in DOM composition persisted throughout the experiment despite shifts in bacterial diversity, underscoring an unappreciated role for larger microorganisms in constraining DOM composition in the marine environment.
Highlights
Dissolved organic carbon in the oceans constitutes a significant global reservoir of carbon, comparable to atmospheric carbon dioxide (700 GT; Hedges, 2002), and its cycling affects the flux of carbon between the oceans and the atmosphere
We conclude that isotopic incorporation of 13C from glucose could not be clearly discerned in the dissolved organic matter (DOM) in this experiment, and further discussion focuses on glucose as a carbon source, rather than on 13C as a tag for recent biological synthesis
This study provides an opportunity to simultaneously evaluate bacterial diversity and DOM composition as a function of the presence or absence of larger organisms, including protozoan grazers
Summary
Dissolved organic carbon in the oceans constitutes a significant global reservoir of carbon, comparable to atmospheric carbon dioxide (700 GT; Hedges, 2002), and its cycling affects the flux of carbon between the oceans and the atmosphere. Bulk compositional analysis supports this contention (e.g., presence of bacterialderived D-amino acids in deep ocean DOM; Kaiser and Benner, 2008) recent molecular-level investigations suggest that bacterial-derived refractory material differs structurally from refractory organic matter in the oceans (Osterholz et al, 2015). In these studies, bacteria likely dominated the incubations due to dilution, filtration or removal of light. The impact of other microbes on DOM composition may have been underestimated, phytoplankton who provide bacterial growth substrates (Amin et al, 2012) and grazers who prey on bacteria (Sherr and Sherr, 1994)
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