The biogeochemistry of dissolved combined amino acids in marine waters.

摘要

The dissolved combined amino acid pool (DCAA) in marine waters was investigated. Initially, the relationship between dissolved organic nitrogen (DON) uptake and ammonium uptake or excretion was examined via a series of experiments using 0.8 {dollar}mu{dollar}m filtrates (the bacterial size fraction). Results from fourteen experiments indicated that dissolved free amino acid and ammonium utilization by heterotrophic bacteria accounted for approximately 90% of the nitrogen required for growth. Other sources of DON, such as DCAA, were not important in these experiments.; To facilitate investigations of the DCAA pool, a vapor-phase method for hydrolyzing combined amino acids was applied. DCAA concentrations determined by this hydrolysis method were 1.54 {dollar}pm{dollar} 0.43 times higher than those determined by traditional hydrolysis in the liquid phase. Large molecular weight DCAA ({dollar}>{dollar}30,000 daltons) had higher vapor:traditional ratios than did small DCAA ({dollar}<{dollar}30,000 d). These results suggest that previous estimates of DCAA may have been underestimated by as much as 300%.; Ribulose-1,5-bisphosphate carboxylase (RuBPcase) was used as a model protein to study bacterial utilization of protein. Kinetic uptake studies of unmodified RuBPcase and RuBPcase which had been abiotically glucosylated suggested that as much as 100% of the DCAA present in marine waters was utilized in a manner similar to that of the glucosylated protein. Less than 10% of the DCAA pool could be considered kinetically similar to unmodified RuBPcase. Independent estimates of protein concentrations indicated that 60-80% of the total DCAA was protein.; To determine whether DCAA could abiotically become resistant to bacterial degradation, abiotic modification rates were experimentally determined. Abiotic transformations proceeded at a rate of 0.023 d{dollar}sp{lcub}-1{rcub}{dollar}, a rate consistent with the observed amounts of modified protein detected in the Delaware Bay. From these data, I hypothesized that protein released as labile DON can be geochemically altered to produce less labile DCAA. Efficient utilization of "fresh" DCAA may be prevented by the concurrent release and preferential utilization of other labile DON, such as DFAA. This increases the residence time of DCAA and allows geochemical reactions, which would otherwise be repressed, to occur.