Ocean dissolved free amino acids

Pocklington R (1970) A new method for the determination of amino acids in sea water and an investigation of the dissolved free amino acids of North Atlantic Ocean waters. PhD Dissertation. Dalhousie University, pp 1-102... 

Table 7.1 Summary of selected nitrogen (NOj", NO2, NH +, urea, dissolved free amino acids [DFAA], DON) uptake rates from the major ocean basins, and upwelling, coastal and estuarine systems using tracer techniques. Rates were converted to common units (nmol N L" using data provided in the original publication or using conversion factors specified in the footnotes... 

The capabilities of the analytical technique are sufficient to monitor dissolved free amino acids in oceanic samples, e.g. from the Sargasso Sea, where average concentrations as low as 25 nmole T have been reported for total DFAA (Lee and Bada, 1977 Liebezeit et al., 1980). 

Simply on the basis of the normal composition of marine organisms, we would expect proteins and peptides to be normal constituents of the dissolved organic carbon in seawater. While free amino acids might be expected as products of enzymic hydrolysis of proteins, the rapid uptake of these compounds by bacteria would lead us to expect that free amino acids would normally constitute a minor part of the dissolved organic pool. This is precisely what we do find the concentration of free amino acids seldom exceeds 150 xg/l in the open ocean. It would be expected that the concentration of combined amino acids would be many times as great. There have been relatively few measurements of proteins and peptides, and most of the measurements were obtained by measuring the free amino acids before and after a hydrolysis step. Representative methods of this type have been described [245-259]. Since these methods are basically free amino acid methods, they will be discussed next in conjunction with those methods. 

Pocklington, R., 1971. Free amino acids dissolved in North Atlantic Ocean waters. Nature, 230 374-375. 

Early studies looked at both dissolved free and combined amino acids (DFAA and DCAA, respectively) in total DOM, where HMWDOM proteins are a subset of the DCAA fraction. Lee and Bada (1975) first reported DFAA concentrations in the range of 40—50 nM in surface Pacific waters and total hydrolysable amino acid (THAA = DFAA + DCAA) concentrations that were often 10 times higher. To date, the observed range of TFIAA is approximately 250—650 nM in surface waters (Dittmar et al., 2001, 2004 Hubberten et al., 1995 Yamashita and Tanoue, 2003). Lee and Bada (1975) observed an approximate three-fold decrease in THAA below the euphotic zone (at these depths DFAA became negligible). This trend was also seen in more recent studies where amino acid concentrations decreased to between 160 and 360 nM in mid-depth waters (Hubberten et al., 1995 Yamashita and Tanoue, 2003). McCarthy et al. (1996) found that HMWDOM-DCAA concentrations were 178 and 278 nM in surface waters of the Sargasso Sea and North Pacific Ocean, respectively. 

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