Dissolved combined amino acids DCAA

Measurements of discrete water samples suffer the disadvantage that the measured peak heights depend upon the baseline level of the wash water. At low natural concentrations the purity of the wash water thus becomes a considerable problem. A continuous technique has been developed, therefore, which avoids all possible sources of contamination during sampling and transfer (see Fig. 26-3). This modification again is calibrated with dilutions of the glydne stock standard that are pumped instead of the sample. 

The technique, however, has some limitationa The inner diameter (i.d.) of the feed tube from the sampling site must be 1 mm, because tubes with smaller i.d. have increased flow resistances. Longer residence times of water in the feed tube should be avoided to prevent microbial growth and sample alteration. Thus, if the manifold of Fig. 26-3 is employed, the length of the feed tube should not exceed 40 m with an i.d. of 1 mm. The residence time will then be 5.23 min. which is well below the known microbial turn-over times of amino acids. 

Mixed reagent-. It should be made up daily as follows. To 1000 mL of sodium borate buffer add 10 mL of o-phthalaldehyde stock solution followed by 2mL of mercaptoethanol. After allowing to stand for a few minutes the reagent is ready for use. 

Dissolve 7.2 mg of glycine in 100 mL of 30 % (v/v) methanol-distilled water. This stock solution contains 1 mmol/L glycine. Exact dilutions of this stock standard with distilled water should be made up before use for a calibration range of 1-10 /rmol/L glycine. The relative fluorescence signals are directly proportional to concentrations. The fluorescence response is linear from 0.01-100/rmol/L glycine. The technique does not show any salt dependence. 

After suitable hydrolysis the techniques outlined above may be used for DCAA determination. As yields of vapour phase hydrolysis have been shown to be consistently higher than those of the classical 6mol/LHCl/110°C/24h method Keil and Kirchman, 1991) the first technique is described in the following (Tsugita et al., 1987). 

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Figure 8.5 Experimental fluxes of NELt-1-, NO3U dissolved organic nitrogen (DON), urea, dissolved free amino acids (DFAA), and dissolved combined amino acids (DCAA) across the sediment-water interface in cores collected in Hog Island Bay (USA). (Modified from Tyler et al., 2003.)...

The decay of proteins and amino acids in phytoplankton cultures, under oxic and anoxic conditions, indicated very little selectivity (Nguyen and Harvey, 1997). However, 15 to 95% of the total particulate amino acid pool consisted of polypeptides/proteins in anoxic treatments versus 8 to 65% in oxic conditions. The similarity of particulate amino acid composition during phytoplankton decay agrees with other work in the Delaware Bay estuary (USA), which showed similar composition in the dissolved combined amino acid (DCAA) pool (Keil and Kirchman, 1993). 

Allochthonous DON sources from terrestrial runoff, plant detritus leaching, soil leaching, sediments, and atmospheric deposition may also represent important inputs to estuaries (Berman and Bronk, 2003). DON typically represents about 60 to 69% of the TDN in rivers and estuaries (Berman and Bronk, 2003). The major components of DON include urea, dissolved combined amino acids (DCAA), DFAA, proteins, nucleic acids, amino sugars, and humic substances (Berman and Bronk, 2003). However, less than 20% of DON is chemically characterized. 

DFAA, dissolved free amino acids DCAA, dissolved combined amino acids TDCHO, total dissolved combined carbohydrates DPA, dissolved primary amines. 

Figure 9.35 Cycling of particulate and dissolved [both free and combined amino acids (DFAA and DCAA)] in porewaters which are considered to be intermediates of biotic and abiotic processing. (Modified from Burdige and Martens, 1988.)...

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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