Department Seminar: Kaijun Lu

Title: Deciphering the molecular structure of dissolved organic matter in the ocean

Abstract: Dissolved organic matter (DOM) contains one thousand times more carbon (C) than all living organism in the oceans combined, representing one of the largest reduced C pools that can respond to changes in atmospheric CO2 on relevant time scales. However, our current understanding of the composition of natural DOM, particularly on molecular level, is still far from comprehensive. My research focuses on understanding the DOM composition using molecular level characterization coupled with laboratory incubations. We have investigated the isomers in DOM molecules, an important question given that thousands of molecular formulas have been assigned from advanced mass spectrometry. Using ion mobility mass spectrometry, our results demonstrated that isomer diversity is highly constrained (<10% of the formulas having isomers). We further showed that degradation decreases the isomeric diversity of DOM, as isomeric diversity declines. We also elucidated the formation of dissolved organic nitrogen (DON), a subset of DOM using 15N labeled short peptide and amino acids in laboratory incubations. Our results showed that the amount of refractory DOM formed is dependent on specific amino acids. Further analyses revealed that several N-containing DON molecules originated from the phenylalanine incubation also exist in natural deep-water samples, suggesting that some DOM molecules, formed during the early-stage organic matter decomposition, can potential last in millennial time scales.