Regulation of gross growth efficiency and ammonium regeneration in bacteria by substrate C: N ratio1

Abstract

Natural assemblages of marine bacteria were cultured on combinations of C and N sources (amino acids, glucose, and NH 4 + ) 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 NH 4 + regeneration during exponential growth. The efficiency of this regeneration ( R N ) 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 : N S ) decreased relative to the C : N ratio of the bacterial biomass (C : N B ). 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 : N S was 1.5 : 1. Hence, R n varied from 0% when C: N S was 10 : 1 up to 86% when C : N S was 1.5 : 1. Inorganic sources of both N and P were taken up only in stoichiometric quantities during this phase of growth. Regeneration of NH 4 + during the stationary phase as well as of PO 4 3− occurred, most likely due to endogenous metabolism or cell death, but the magnitude of this regeneration seemed to increase greatly only when C : N S was > 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: N S of available substrates in marine waters is > 10 : 1. Hence, actively growing bacteria may be inefficient remineralizers of N.