Heterotrophic bacterial utilization of nitrogenous and nonnitrogenous substrates, determined from ammonia and oxygen fluxes

We describe a simple procedure to allow the broad nature of the organic sub strates used for planktonic bacterial growth to be determined. The method a nalyzes the coupled oxygen and ammonia fluxes in terms of the relative prop ortions of nitrogenous and nonnitrogenous substrates assimilated by the mic roheterotrophs. The model uses a stoichiometric equation that requires the knowledge of the respiratory quotient, substrate C/N ratio, cell C/N quota, and bacterial carbon growth yield and the assumption that nitrification wa s not occurring. We discuss the uncertainties associated with the attributi on of values for these constants and illustrate the use and limitations of the approach in the interpretation of field observations on bacterioplankto n metabolism in a temperate coastal ecosystem. With the exception of a sing le observation, we were able to interpret the data using the proposed model . Our findings are that during the spring bloom, nitrogenous substrates mad e up 40-80% of the total, failing to <20% in the postbloom period. Thus, wi th essentially routine methods for determining oxygen and ammonia fluxes, w e have been able to determine a fundamental aspect of the cycling of organi c material by the bacterioplankton.