Biodegradable dissolved organic matter in a temperate and a tropical stream determined from ultra–high resolution mass spectrometry

We investigated dissolved organic matter (DOM) metabolism by employing plug-flow biofilm reactors and ultra– high resolution mass spectrometry of DOM isolated by C18 extraction in two forested stream ecosystems, a low DOM tropical stream sampled at baseflow and a higher DOM temperate stream sampled during a storm. On passage through the bioreactors, DOM concentrations in the tropical stream sample declined by 22%, whereas they declined by 42% in the temperate stream sample. The extracted DOM was subjected to electrospray ionization coupled to Fourier transform ion cyclotron resonance mass spectrometry to obtain information on molecular weight distributions and elemental compositions for the thousands of compounds whose masses are calculated with sufficient accuracy to allow calculation of unique elemental formulas. In both streams, metabolism modifies DOM to lower molecular weight molecules, and oxygen-rich molecules are selectively biodegraded. Applying van Krevelen analyses for the unique elemental formulas of DOM constituents revealed that hydrogen-deficient molecules with low H : C ratios (assigned to black carbon–derived molecules) are present and generally not metabolized. Black carbon molecules are refractory to biodegradation compared with other components of DOM, supporting the suggestion that black carbon molecules in DOM flow to the ocean without experiencing significant microbial degradation. Dissolved organic matter (DOM) provides energy and carbon for microorganisms in aquatic ecosystems, especially heterotrophic bacteria. Understanding controls on the processes of DOM uptake and metabolism enhances our understanding of aquatic food webs. Similarly, describing the chemical composition of the biodegradable component of DOM (BDOM) helps clarify how heterotrophic bacteria obtain energy and nutrients. Consequently, there have been numerous studies on the utilization of DOM by microorganisms where bioassays, combined with bulk parameters and molecular-level parameters, have been used to study the biologically labile portion of DOM. Bulk parameters such as DOC concentration, elemental composition, oxidation state of DOM, and dissolved oxygen concentration, as well as molecular-level analyses such as 1 Present address: Korean Basic Science Institute, 52 Eoeun-dong, Yuseong-gu, Daejeon 305-333. 2 Corresponding author. Present address: Department of Chemistry, Old Dominion University, Norfolk, Virginia 23529 (phatcher@odu.