Formation of Modified Biochemicals and their Contribution to Dissolved Organic Nitrogen in Marine Estuaries

Dissolved organic nitrogen (DON) is the largest reservoir of reduced nitrogen in the oceans, yet its composition and recycling remain poorly defined (e.g. review by Anita et al., 1991) A recent ¹⁵N-NMR analysis of the high molecular weight (>.1kDa- HMW) fraction by McCarthy et al. (1997) provides one important clue about the origin and composition of DON. They observed that HMW DON was almost exclusively as the amide linkage (R-NH-C=O) and they suggested that protein or perhaps chitin makes up as much of the HMW DON with little nitrogen present as other forms such as indoles or heterocycles. In some respects, the dominance of protein and perhaps chitin in DON is not surprising. Protein is typically 60% of cellular mass and up to 90% of cellular nitrogen for many organisms. Chitin is the second most abundant biopolymer on earth after cellulose.

Although much progress has been made in characterizing HMW dissolved organic material (DOM), much of it remains to be identified. An important paradox is that while NMR evidence indicates that protein or protein like material is a major fraction of DON, less than 30% of organic nitrogen can be recovered as amino acids using standard hydrolysis techniques. McCarthy et al. (1998) recently found that a portion of this DON may be remnant peptidoglycan, a structural polysaccharide in bacterial cell walls, which also contains amide nitrogen. Even with the inclusion of peptidoglycan, however, about half of the DON remains to be identified.