|The impact of Pelagibacter on DOM under light and dark conditions|
Stephen J. Giovannoni (Oregon State University); Neil V. Blough and Rosanna Del Vecchio (University of Maryland);
Heterotrophic bacterial metabolism and photochemical degradation are two of the primary transformative mechanisms for dissolved organic matter (DOM) in the marine environment. Pelagibacter ubique, a member of the SAR11 clade of a-proteobacteria, is a ubiquitous marine bacterial species and can represent up to 25% of the heterotrophic bacterial population in the ocean. P. ubique has been isolated into laboratory culture and its genome has been sequenced. Nonetheless, the growth requirements of this important marine bacterium are not well constrained. P. ubique reaches high cell densities only in low-nutrient seawater (Oregon coastal waters) and apparent nutrient limitation is not alleviated by the addition of inorganic or organic supplements. We have examined the composition of marine dissolved organic matter before and after incubation with P. ubique using ultrahigh resolution Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Our preliminary work indicates that P. ubique incorporates specific compounds (inferred from unique m/z values) within marine DOM. Interestingly, P. ubique incorporates and produces different compounds when grown under light conditions than under dark conditions. This is surprising since P. ubique exhibits no growth differences under light and dark conditions, despite the presence of a proteorhodopsin proton pump. Here we propose to investigate the impact of photochemistry on the suite of compounds incorporated and produced by P. ubique during growth experiments. We will determine the molecular-level composition of DOM and its optical characteristics before and after incubation with P. ubique. We will examine whether heterotrophic bacterial metabolism is a source of chromophoric DOM (CDOM) and whether the products of photodegradation are more or less available for heterotrophic processes. We will compare these results to those gained from growth experiments using DOM from other sources, in particular DOM with increased terrestrial influence and photobleached DOM.
Funding: National Science Foundation, Chemical Oceanography Program
Personnel: Melissa Kido Soule (FT-MS analysis), Krista Longnecker (postdoc / lab manager)
Kujawinski, E. B., K. Longnecker, K. A. Barott, and M. C. Kido Soule. Submitted (Oct 2012). Protozoan grazing simultaneously influences bacterial diversity and dissolved organic matter composition in seawater. Proceedings of the National Academy of Sciences, USA.
Kujawinski, E. B., 2011. The impact of microbial metabolism on marine dissolved organic matter. Annual Reviews of Marine Science 3: 567-599.
Kido Soule, M. C., K. Longnecker, S. J. Giovannoni and E. B. Kujawinski. 2010. Impact of instrument and experiment parameters on reproducibility of ultrahigh resolution ESI FT-ICR mass spectra of natural organic matter. Organic Geochemistry. 41: 725-733.