Effect of carbon:nitrogen ratio on kinetics of phenol biodegradation by Acinetobacter johnsonii in saturated sand

In polluted soil or ground water, inorganic nutrients such as nitrogen may be limiting, so that Monod kinetics for carbon limitation may not describe microbial growth and contaminant biodegradation rates. To test this hypothesis we measured 14CO2 evolved by a pure culture of Acinetobacter johnsonii degrading 120 micrograms 14C-phenol per ml in saturated sand with molar carbon:nitrogen (CN) ratios ranging from 1.5 to 560. We fit kinetics models to the data using non-linear least squares regression. Phenol disappearance and population growth were also measured at CN1.5 and CN560. After a 5- to 10-hour lag period, most of the 14CO2 evolution curves at all CN ratios displayed a sigmoidal shape, suggesting that the microbial populations grew. As CN ratio increased, the initial rate of 14CO2 evolution decreased. Cell growth and phenol consumption occurred at both CN1.5 and CN560, and showed the same trends as the 14CO2 data. A kinetics model assuming population growth limited by a single substrate best fit the 14CO2 evolution data for CN1.5. At intermediate to high CN ratios, the data were best fit by a model originally formulated to describe no-growth metabolism of one substrate coupled with microbial growth on a second substrate. We suggest that this dual-substrate model describes linear growth on phenol while nitrogen is available and first-order metabolism of phenol without growth after nitrogen is depleted.