Sources and Biochemical Composition of Detrital Organic Matter In theSea

Detrital organic matter in the oceans is one of the largest and most dynamic reservoirs of reactive organic carbon on Earth. Despite the importance of this reservoir in the global cycle of carbon and associated bioreactive elements, not much is known about its sources, chemical composition and diagenetic processing. In this project we (1) use a novel multi-biomarker approach to trace bacterial remnants and provide estimates of bacterial contribution to marine organic matter, (2) investigate the chemical composition and diagenetic processing of suspended POM (>100 nm), high-molecular-weight dissolved organic matter (HMW DOM, 1-100 nm) and low-molecular-weight dissolved organic matter (LMW DOM, <1 nm), and (3) explore the composition and remineralization of semi-labile DOM in the upper mesopelagic (110-300 m) zone. Samples were collected at the US Joint Global Ocean Flux Study Program (JGOFS) time-series stations near Bermuda (BATS) and Hawaii (HOT) and analyzed for organic carbon, organic nitrogen, D- and L-amino acids, neutral sugars and amino sugars. A high-throughput microwave-assisted vapor-phase hydrolysis method was developed to measure D- and L-amino acids in seawater samples. Bacterial detritus was a major component of particulate organic matter (POM) and is an important source of submicron particles and colloids in the ocean. Peptidoglycan was a substantial component of POM but not of dissolved organic matter (DOM). Compositional differences between POM and DOM primarily reflected the selective incorporation of specific bacterial components into these reservoirs. Autotrophic and heterotrophic bacterial sources were not quantified separately, but the presence of D-aspartic acid (D-Asx) and D-serine (D-Ser) suggested that heterotrophic sources were substantial. The average reactivity of bacterial organic matter was comparable to that of the bulk organic carbon pool. Bacteria were important sources of labile, semilabile and refractory dissolved organic carbon. Bacterial organic matter accounted for ∼25 % of particulate and dissolved organic carbon and ∼50 % of particulate and dissolved organic nitrogen. These results demonstrate the importance of bacteria in regulating the ocean carbon and nitrogen cycles. Concentrations of amino acids, neutral sugar and amino sugars in unfiltered seawater sharply declined with depth at both stations, indicating an upper ocean source and rapid turnover of these components. Carbohydrates and amino acids were major reactive components of semi-labile DOM in the upper mesopelagic zone. The size distribution of organic matter was heavily skewed to smaller molecular sizes. Depth comparisons showed that larger size classes of organic matter were more efficiently removed than smaller size classes. Carbon-normalized yields of amino acids, neutral sugars and amino sugars decreased rapidly with depth and molecular size. Together these biochemicals accounted for 55% of organic carbon in surface POM but only 2% of the organic carbon in LMW DOM in deep water. Chemical compositions showed distinct differences between organic matter size classes indicating the extent of diagenetic processing increased with decreasing molecular size. These findings are consistent with the size-reactivity continuum model for organic matter in which bioreactivity decreases with decreasing molecular size and diagenetic processes lead to the formation of smaller components that are resistant to biodegradation.

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