Faculty Research Assistant Marie Johnson retrieves cultures from liquid nitrogen storage.

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 alphaproteobacteria, is a ubiquitous marine bacterial species and can represent up to 25% of the heterotrophic bacterial population in the ocean. 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 both before and after incubation with P. ubique (strain HTCC1062) using ultrahigh resolution Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) in collaboration with Dr. Elizabeth Kujawinski at the Woods Hole Oceanographic Institute (WHOI). 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.

10L carboy cultures of HTCC1062 and controls incubating with filtered air and exposed to a light cycle. (Photo by Joshua Kitner)

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 in collaboration with Drs. Neil Blough and Rossana Del Vecchio at the University of Maryland. 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 for this project is provided by NSF and the Gordon and Betty Moore Foundation.