Enrichment of anaerobic methanotrophs in sulfate-reducing membrane bioreactors

Anaerobic oxidation of methane (AOM) in marine sediments is coupled to sulfate reduction (SR). AOM is mediated by distinct groups of archaea, called anaerobic methanotrophs (ANME). ANME co-exist with sulfate-reducing bacteria, which are also involved in AOM coupled SR. The microorganisms involved in AOM coupled to SR are extremely difficult to grow in vitro. Here, a novel well-mixed submerged-membrane bioreactor system is used to grow and enrich the microorganisms mediating AOM coupled to SR. Four reactors were inoculated with sediment sampled in the Eckernförde Bay (Baltic Sea) and operated at a methane and sulfate loading rate of 4.8 L L^-1 day^-1 (196 mmol L^-1 day^-1) and 3.0 mmol L^-1 day ^-1. Two bioreactors were controlled at 15°C and two at 30°C, one reactor at 30°C contained also anaerobic granular sludge. At 15°C, the volumetric AOM and SR rates doubled approximately every 3.8 months. After 884 days, an enrichment culture was obtained with an AOM and SR rate of 1.0 mmol g_{volatile suspended solids} ^-1 day^-1 (286 7mu;mol gdry weight^-1 day^-1). No increase in AOM and SR was observed in the two bioreactors operated at 30°C. The microbial community of one of the 15°C reactors was analyzed. ANME-2a became the dominant archaea. This study showed that sulfate reduction with methane as electron donor is possible in well-mixed bioreactors and that the submerged-membrane bioreactor system is an excellent system to enrich slow-growing microorganisms, like methanotrophic archaea.