Bioavailability and Molecular Structure of Dissolved Organic Matter in South Texas Rivers and Adjacent Estuaries

Riverine dissolved organic matter (DOM) is a major source of reduced carbon exported from land to marine environments, and the inflow of riverine organic matter greatly affects biogeochemical cycling in estuaries and bays. However, molecular level information of DOM remains elusive, and some fundamental characteristics of DOM, such as its isomer information, are still unknown. In this study, natural DOM samples were collected from several south Texas rivers across a large temporal range in different flow conditions. Molecular structure, with the focus on isomer aspects, of riverine DOM was investigated using an Ion Mobility Quadrupole Time Of Flight (Q-TOF) Liquid Chromatography Mass Spectrometry (IM QTOF LC/MS). In addition, the bioavailability of riverine DOM in base-flow condition was assessed through microcosm incubations. Our results revealed that riverine DOM shifted from a protein-like and lipid-like dominated community at base flow condition, to a lignin, tannin and condensed aromatic structure dominated one during high flow event based on MS and tandem MS data. Furthermore, about 4 – 22% of DOM molecules contain structural isomers among different flow conditions. This low number of isomer percentages suggests that the number of isomers in DOM is highly constrained. In addition, the incubation results showed that bioavailable DOC (BDOC) and bioavailable dissolved organic nitrogen (BDON) ranged from 0 – 6% and 9 – 15%, respectively. MS and tandem MS data revealed the averaged O/C ratio of the assigned formulas decreased while the H/C ratio increased, changing the DOM community to be more “marine-like”. More importantly, isomer percentage (number of compounds possessing isomer/total number of compounds) decreased from ca. 7% before incubation to ca. 4% after incubation, indicating that DOM community becomes more homogenous during incubation with microbial metabolism, further confirming that the number of isomers in DOM is under certain regulation. Taken together, our study not only evaluated the bioavailability of riverine DOM in south Texas waters, but also have provided important information on the isomeric complexity of natural DOM.