DISSOLVED COMBINED AMINO-ACIDS IN MARINE WATERS AS DETERMINED BY A VAPOR-PHASE HYDROLYSIS METHOD

We examined a vapor-phase hydrolysis method for estimating dissolved combined amino acid (DCAA) concentrations in seawater. The DCAA concentrations determined by this hydrolysis method were 1.54 +/- 0.43 (n = 62, range 0.8-3.0) times higher than that determined by traditional acid hydrolysis in the liquid phase. Both methods completely hydrolyzed proteins added to seawater. The vapor-phase method gave higher DCAA estimates than traditional acid hydrolysis in the subarctic Pacific (the vapor:traditional ratio was 2.00 +/- 0.80, n = 5), the Delaware Estuary (1.48 +/- 0.44, n = 57) and in sediment samples from the Great Delaware Marsh (1.63 +/- 0.43, n = 8). Large molecular weight DCAA (> 30 000 Da) had higher ratios than small DCAA (< 30 000 Da). The two methods differ in acid composition, hydrolysis temperature and time (vapor-phase hydrolysis requires only 23 min), but we found that only vapor-phase hydrolysis resulted in significant increases in DCAA concentrations. These results suggest that DCAA concentrations in seawater may have been underestimated by as much as 300%.