Sources and Sinks Based on Chemical Information

DON source apportionment to particular marine biogeochemical processes such as N2-fixation (Meador et al, 2007). 

Cell membrane and cell wall components may also represent part of the accumulating DOM reservoir these compounds may be selectively preserved as a result 

The compositional studies discussed in this paper have provided some insights into the DOM cycle. For example, monomer level studies have shown that the chemical composition of HMWDOM is homogeneous in the surface ocean (Aluwihare et al, 1997 McCarthy et al, 1996). Compositional differences between the surface and deep ocean have demonstrated that polysaccharides are reactive and removed with depth (Aluwihare et al, 2002 Benner et al, 1992) furthermore, the relative abundance of amides resembhng those found in N-acetyl glucosamine also decreases with depth suggesting that these compounds are reactive and likely derived from surface ocean productivity (Aluwihare et al, 2005 Benner and Kaiser, 2003). Amino acid compositions and quantities exhibit no vertical trend and certain amino acids could have a common source throughout the ocean (McCarthy et al, 1998). D enantiomers of several amino acids are also ubiquitous in HMWDOM and indicate the presence of dissolved peptides derived from bacteria. 

Enzymatic assays can be applied in the marine environment to provide indirect information on dissolved compounds that are available to fuel bacterial production. Approaches that have been commonly appHed include measuring hydrolytic enzyme activities in seawater and monitoring degradation rates of model compounds. Protein hydrolysis in seawater is rapid as expressed by model protein studies (e.g., Nunn et al., 2003 Pantoja and Lee, 1999). This rapid and selective removal of dissolved proteins explains the relatively minor contribution from proteins to the accumulating DOM reservoir even though proteins are by far the most abundant intracellular biochemical. In an elegant study, Nunn and coworkers (2003) used matrix assisted laser desorption/ionization (MALDI) time of flight (TOP) mass 

Spectrometry (MS) to follow structural changes associated with the degradation of bovine serum albumin (BSA) in seawater. BSA was degraded rapidly in seawater and enzymatic attack occurred at a variety of sites on this protein. Based on these results the authors concluded that either non-specific proteases or a diverse collection of specific proteases were abundant in seawater. Consistent with these findings, the signature of HMWDOM and proteins isolated from HMWDOM indicate that dissolved proteins are turning over more rapidly than bulk HMWDOM (Meador et ai, 2007). 

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