DISSOLVED COMBINED AMINO-ACIDS - CHEMICAL FORM AND UTILIZATION BY MARINE-BACTERIA

The chemical form and bacterial utilization of dissolved combined amino acids (DCAA) were studied in the Delaware estuary and coastal waters. Concentrations of DCAA estimated after vapor-phase hydrolysis were 1. 1-2.5-fold higher than those estimated after liquid-phase hydrolysis; concentrations ranged from 40 to 1,490 mug liter-1. Protein concentrations estimated by the bicinchoninic acid method were within 13% of DCAA measured by liquid-phase hydrolysis, suggesting that liquid-phase hydrolysis recovers primarily protein. Vapor-phase hydrolysis apparently recovers protein and an unknown component of DCAA because protein concentrations were only 30-80% of vapor-phase DCAA. Kinetic assays were used to estimate the amount of protein susceptible to bacterial degradation. Ribulose 1,5-bisphosphate carboxylase (RuBPcase) was used as a model protein because its uptake into biomass was inhibited by adding other proteins, including that extracted from the dominant phytoplankton in the estuary. Bacteria recognized only 1-10% of the DCAA as RuBPcaselike protein. Nearly half of the DCAA was kinetically similar to protein modified by abiotic glucosylation; the remaining DCAA was kinetically undefined. Competition studies showed that bacteria were capable of discriminating between protein and protein that had been glucosylated. Turnover times of protein and glucosylated protein averaged 4+/-8 (SD) and 288+/-344 h. We suggest that the DCAA pool is comprised of at least three pools: ''fresh'' protein that is assimilated rapidly; ''modified'' protein kinetically similar to glucosylated protein and utilized more slowly than fresh protein; and combined amino acids that are not in protein and may be present as amino acids sorbed to clays or associated with humic materials.