The relationship between inorganic nitrogen oxidation and organic carbon production in batch and chemostat cultures of marine nitrifying bacteria

Abstract

Rates of nitrification and organic C production were determined in batch and chemostat cultures of marine nitrifying bacteria; two NH 4 + -oxidizing species and one NO 2 − -oxidizing spezies. With increasing age in batch cultures and with decreasing flow rates in chemostats, cellular organic C and N concentrations declined while the intracellular ratio of C:N remained constant. With decreasing flow rates in chemostats, there was a reduction in (a) carboxylating enzyme activity per unit of cellular organic C (the potential for chemoautotrophic CO2 fixation), and (b) the yield of organic C. For both NH 4 + and NO 2 − oxidizers, rates of nitrification and C yield were lowest at very slow chemostat growth rates, when compared with optimal growth rates in batch cultures. For both NH 4 + and NO 2 − -oxidizing species, the stoichiometric relationship between nitrification and organic C production did not remain constant and appeared to be dependent on the availability of the inorganic N substrate. The organic C yield from NH 4 + oxidation and hence the free energy efficiency declined with increasing age in batch cultures and with decreasing flow rates in chemostats. The C yield from NO 2 − oxidation and the free energy efficiency at slow chemostat growth rates was also lower than that at the optimal growth rate in batch culture.