UREASE ACTIVITY IN NEUTROPHILIC AUTOTROPHIC AMMONIA-OXIDIZING BACTERIA ISOLATED FROM ACID SOILS

Autotrophic ammonia-oxidizing bacteria belonging to the genera Nitrosomonas and Nitrosospira were isolated from acid soils from several sites in Scotland. All were neutrophilic, with maximum specific growth rates in the pH range 7.5-8.5 and with pH minima for growth on ammonia > 6.5. Most soil isolates, but not a laboratory strain of Nitrosomonas europaea, were capable of hydrolysing urea at a pH value of 8.0 and strains of Nitrosospira could grow with urea as the sole N source at a pH value of 5.5. There was therefore no evidence for acidophilic strains of autotrophic ammonia-oxidizers but urea hydrolysis may enable autotrophic nitrification in such acid soils.