For more than 100 years it was believed that bacteria were the only group responsible for the oxidation of ammonia. However, recently, a new strain of archaea bearing a putative ammonia monooxygenase subunit A (amoA) gene and able to oxidize ammonia was isolated from a marine aquarium tank. Ammonia-oxidizing archaea (AOA) were subsequently discovered in many ecosystems of varied characteristics and even found as the predominant causal organisms in some environments. Here, we summarize the current knowledge on the environmental conditions related to the presence of AOA and discuss the possible site-related properties. Considering these data, we deduct the possible niches of AOA based on pH, sulfide and phosphate levels. It is proposed that the AOA might be important actors within the nitrogen cycle in low-nutrient, low-pH, and sulfide-containing environments.

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A nitrifying sequential batch reactor operated under 2-day cyclic aerobic and anoxic conditions was pulse dosed with incremental sulfide concentrations during anoxic conditions. The nitrite-oxidizing bacteria were found to be more sensitive to sulfide than the ammonia oxidizers. A maximum of nitrite-N to (nitrite-N + nitrate-N) accumulation ratio of 0.75 was obtained at an initial pulse sulfide-S concentration of 45 mg/L under pH control at 7.5 +/- 0.2 and fully mixing conditions. Total ammonium nitrogen was removed almost 100% at a removal rate of 0.73 +/- 0.05 g/L x day, achieved during the aerobic days of the cycles. Denaturing gradientgel electrophoresis (DGGE) and fluorescence in situ hybridization (FISH) analyses indicated the shift in the ammonia- and nitrite-oxidizing populations triggered by sulfide addition, partial nitrification, and subsequent recovery to complete nitrification. Interestingly, archaeal amoA genes were retrieved under the conditions of sulfide addition. These results indicate that the pulse sulfide application can be used as a tool to accumulate nitrite, which is of importance for the subsequent anaerobic ammonium oxidation (anammox) process in the achievement of complete nitrogen removal.

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