Colonisation of aggregates by ammonia oxidising bacteria within a model system
Aggregates provide a distinct niche within the water column with elevated levels of nutrients compared to the surrounding water. Differences have been observed between aggregate and planktonic communities of ammonia oxidising bacteria within the environment, but such differences have not been explained. Ammonia oxidising bacteria are important due to their role in the first rate-limiting step of nitrification in which nitrogen is converted from its most reduced to its most oxidised form. The aim of this study was to determine mechanisms responsible for aggregate colonisation using a model system. To achieve this aim, a model aggregate system was developed in which colonisation of aggregates by ammonia oxidising bacteria under different conditions could be determined. Candidate ammonia oxidiser strains, Nitrosomonas europaea and Nitrosospira tenuis, were used to demonstrate the effect of the sampling method on observed distribution of AOB within the model system. The degree of bias of the b-AMO and CTO PCR primer sets was assessed and indicated that the predominance of Nitrosospira species in many environmental surveys was not due to primer bias. The composition of particulate communities of AOB determined the extent of colonisation by other species of AOB. The immediate colonisation of aggregates by N. tenuis suggested that differences in particulate and planktonic AOB communities were not solely due to differences in substrate availability and activity of differing AOB under such conditions, although the ability of attached communities of AOB to respond more quickly when substrate becomes available was again illustrated. This study provides evidence of the differences between particle-attached and free-living AOB and indicates how such differences can be related to the ecology of these organisms in natural environments.