Regulation of Gross Growth Efficiency and Ammonium Regeneration in Bacteria by Substrate C:N Ratio

Natural assemblages of marine bacteria were cultured on combinations of C and N sources (amino acids, glucose, and NH4 +) to span a range of substrate C:N ratios from 1.5: 1 to 10: 1. Catabolic metabolism of the N component of amino acid substrates led to NH4 +regeneration during exponential growth. The efficiency of this regeneration (RN) and also of the carbon gross growth efficiency (GGE) generally was independent of the sources of C and N, but increased as the C:N ratio of the substrate (C:Ns) decreased relative to the C:N ratio of the bacterial biomass (C:NB). The elemental chemical composition (C:N:P ratio) of the bacterial biomass was relatively invariant at about 45:9:1 and the gross growth efficiency varied from a threshold value of about 40-50% at$C:N_s > 6: 1$up to 94% when C:Nswas 1.5:1. Hence, RNvaried from 0% when C:Nswas 10:1 up to 86% when C:Nswas 1.5:1. Inorganic sources of both N and P were taken up only in stoichiometric quantities during this phase of growth. Regeneration of NH4 +during the stationary phase as well as of PO4 3- occurred most likely due to endogenous metabolism or cell death, but the magnitude of this regeneration seemed to increase greatly only when C:Nswas >6:1. Considering that amino acids frequently do not provide all of the N required and that carbohydrates often are the major C source for growth of marine bacteria, we speculate that C:Nsof available substrates in marine waters is > 10:1. Hence, actively growing bacteria may be inefficient remineralizers of N.