Effect of transient oxic conditions on the composition of the nitrate-reducing community from the rhizosphere of Typha angustifolia.

Within a nitrate-reducing bacterial community, a niche differentiation between denitrifying and nitrate ammonifying bacteria may be determinated by a complex of environmental parameters, such as the availability of carbon, nitrate, and oxygen. Hence, oxygen- and carbon-releasing aerenchymatous plants may affect the composition of the nitrate-reducing community in waterlogged sediment. The composition of the nitrate-reducing community in the rhizosphere of the aerenchymatous plant species Typha angustifolia was compared with the community in nonrhizospheric sediment. All three functional groups (NO2 (-) accumulators, N2O producers, and presumed NH4 (+) producers) were present at both sites with an ratio of 36:45:12 and 43:22:18 for nonrhizospheric and rhizospheric sediments, respectively. Most of the isolated were gram-negative, and approximately 50% of these strains demonstrated an obligatory oxidative metabolism.In the absence of nitrate, Enterobacteriaceae (belonging to the NO2 (-) accumulating group) became dominant during enrichment of bacteria from the rhizosphere of T. angustifolia in a chemostat with glycerol (20 mM) as substrate, both under strictly anoxic and transient oxic conditions. Addition of nitrate to the chemostats led to the predominance of denitrifying pseudomonads, irrespective of the presence or absence of oxygen. However, in the presence of nitrate under anoxic conditions, enterobacteria persisted in the medium together with pseudomonads.It was concluded that oxidative bacteria such as pseudomonads are the better competitors for limiting amounts of glycerol, provided oxygen or nitrate is present. In the absence of these electron acceptors, fermentative bacteria become dominant.