Adaptation to Temperature of Nitric Oxide-Producing Nitrate-Reducing Bacterial Populations in Soil

Abstract

The population size of nitrate-reducing bacteria in an Egyptian soil and in two German soils was determined by most probable number (MPN) counts using different incubation temperatures between 8 and 50 °C. The highest numbers were observed at 25–30 °C irrespectively from where the soil originated. Bacteria were randomly isolated from the MPN dilution tubes using the same temperature as in the MPN, and were then morphologically and physiologically characterized. At temperatures >30 °C practically all of the isolates were Gram+ and 75–100% of them formed spores. At all the temperatures and soils tested, most (about 75%) of the isolates were denitrifiers, but some of the isolates also belonged to the nitrate-respiring or nitrate-ammonifying bacteria. Up to 80% of the isolated nitrate reducers formed NO during growth. The following species were identified among 11 randomly isolated Gram- denitrifying bacteria: Pseudomonas corrugata (4 isolates), P. glathei (2 isolates), P. fluorescens subgroup B (2 isolates), P. fluorescens subgroup A (1 isolate), P. aurantiaca (1 isolate), and P. chlororaphis (1 isolate). Three bacterial strains which were isolated at 8, 25 and 40 °C., respectively, also preferred these temperatures for optimum denitrification, N2O formation and NO formation. Our results demonstrate that denitrifying populations that are adapted to a particular temperature (psychrophilic and mesophilic) can be isolated from various soils, although the bulk of the soil nitrate reducers grew best at temperatures of 25–30 °C.