Particulate-dissolved transformations as a sink for semi-labile dissolved organic matter: Chemical characterization of high molecular weight dissolved and surface-active organic matter in seawater and in diatom cultures

Abstract

Chemical characterizations of high molecular weight dissolved organic matter (HMW DOM) have shown that a compositionally well-defined family of acylated polysaccharides (APS) contributes a large fraction of DOM in the surface ocean. One process that might affect APS cycling is physical removal by aggregation. To investigate the importance of this mechanism, we compared the chemical composition and transparent exopolymer particle (TEP) abundances of seawater HMW DOM with surface-active organic matter (SAOM) that accumulates as standing sea surface foam during intense wind events. We further simulated the aggregation of SAOM using bubble-adsorption of HMW DOM isolated from laboratory cultures of the diatom Chaetoceros neogracile. 1H-NMR spectroscopy and molecular-level analysis of natural and laboratory-produced HMW DOM and SAOM show important contributions from polysaccharides, acetate, and aliphatic organic matter and similar yields and distributions of neutral sugars after hydrolysis. Microbial degradation of Chaetoceros neogracile DOM removes proteins and mannose-rich polysaccharides to produce HMW DOM with spectral and molecular-level characteristics that resemble seawater APS. These results imply that APS and hydrolysable sugars are resistant to microbial degradation over the time frame of our degradation experiment (40 days). As revealed by 1H-NMR spectroscopy and by the neutral monosaccharide data, there are important similarities in the composition of APS in HMW DOM and natural and laboratory-produced SAOM foamy material, rich in TEP. Although these samples are operationally classified in two distinct pools (dissolved vs. particulate) in seawater, they exhibit similar chemical and spectral characteristics. Finally, TEP concentration measurements in SAOM samples indicate that freshly-produced diatom DOM yields SAOM with TEP concentration similar to seawater and natural SAOM, but seven times higher than SAOM produced after bubble-adsorption of degraded HMW DOM.