The sequencing batch reactor as a powerful tool for the study of slowly growing anaerobic ammonium-oxidizing microorganisms

Currently available microbiological techniques are not designed to deal wit h very slowly growing microorganisms. The enrichment and study of such orga nisms demands a novel experimental approach. In the present investigation, the sequencing batch reactor (SBR) was applied and optimized for the enrich ment and quantitative study of a very slowly growing microbial community wh ich oxidizes ammonium anaerobically. The SBR was shown to be a powerful exp erimental set-up with the following strong points: (1) efficient biomass re tention, (2) a homogeneous distribution of substrates, products and biomass aggregates over the reactor, (3) reliable operation for more than 1 year, and (4) stable conditions under substrate-limiting conditions. Together, th ese points made possible for the first time the determination of several im portant physiological parameters such as the biomass yield (0.066 +/- 0.01 C-mol/mol ammonium), the maximum specific ammonium consumption rate (45 +/- 5 nmol/mg protein/min) and the maximum specific growth rate (0.0027 . h(-1 ), doubling time 11 days). In addition, the persisting stable and strongly selective conditions of the SBR led to a high degree of enrichment (74% of the desired microorganism). This study has demonstrated that the SBR is a p owerful tool compared to other techniques used in the past. We suggest that the SBR could be used for the enrichment and quantitative study of a large number of slowly growing microorganisms that are currently out of reach fo r microbiological research.