In situ measurements of microbial activity were performed on sediments of theHaakon Mosby Mud Vulcano (HVVM) in the Barents Sea. A lander was deployedcontaining a benthic chamber to measure O2 exchange and a profiling unit for microsensors(O2, H2S, pH and temperature). The volcano area consists of a centralarea where mud is expelled, surrounded by large fields of the sulfide oxidizingbacteria Beggiatoa. The outer rim of the HMMV is dominated by the symbioticmethane-oxidizing Pogonophora tubeworms. At a reference site outside the volcanoarea methane oxidation was insignificant. With microsensors the microenvironmentsin the zones of the main metabolic processes, anaerobic methane oxidation (AOM) andsulfide oxidation, were characterized. From the microprofiles local activities and diffusiveinterfacial fluxes were determined. The benthic chambers allowed total (advective+ diffusive) oxygen uptake rate measurements. Profound differences were found betweenthe Beggiatoa and Pogonophora fields. The process rates in Beggiatoa fields arecontrolled by diffusion, in Pogonophora fields by advection driven by benthic fauna.AOM is particularly high under the Beggiatoa fields in a narrow zone at 2 cm depth,characterized by a steep sulfide peak. Effectively, all methane diffusing upwards isoxidized in a zone less than 5 mm thick. No clear pH effects by AOM were observed.At the HMMV, AOM was tightly coupled to aerobic sulfide oxidation by Beggiatoa.Oxygen uptake rates measured on retrieved cores were ca 5 times lower than thosemeasured with the lander, demonstrating the importance of in situ measurements.