DENITRIFICATION ASSOCIATED WITH GROUP-I AND GROUP-II METHANOTROPHS INA GRADIENT ENRICHMENT SYSTEM

The occurrence and distribution of Groups I and II methanotrophs and t heir potential impact on denitrification were studied in a diffusion c olumn model system simulating CH4 and O-2 sources and delivery in the environment. We used NO3--or NH4+-containing mineral salts media and t hree different inoculum sources: a swamp soil, a lake sediment and a 4 cultivated humisol. The methylotrophic community structure which deve loped in the diffusion columns was characterised using oligodeoxynucle otide probes specific for ribulose monophosphate pathway (Group I; 10 gamma probe) and serine pathway (Group II; 9 alpha probe) methylotroph s. Methanotrophs that grew near the top of the columns in zones of low CH4 and high O-2 concentration were generally from Group I; those gro wing at the bottom of the columns in zones of high CH4 and low O-2 con centration were from Group II. Only in the humisol were both Group I a nd II detected at the top of the column. Concomitant production of N2O with CH4 consumption, observed in the diffusion columns, was confirme d in enrichment cultures. At least three denitrifiers associated with methanotrophic growth and activity were isolated. Methanotrophs that g rew under high CH4 and low O-2 conditions were associated with a hypho microbium-like bacterium capable of denitrifying with methanol. Methan otrophic activity supported denitrification by (i) reducing the O-2 te nsion, and (ii) supplying organic compounds to the denitrifiers. Becau se this model system mimics many of the natural environments of methan otrophs, it is likely that the observed segregation of physiological t ypes of methanotrophs and their interaction with denitrifiers also occ ur in nature.