TheEpsilonproteobacteria, including members of the genusSulfurovum, are regarded as important primary producers in hydrothermal systems. However, theirin situgene expression in this habitat has so far not been investigated. We report a metatranscriptomic analysis of aSulfurovum-dominated biofilm from one of the chimneys at the Loki's Castle hydrothermal system, located at the Arctic Mid Ocean Ridge. Transcripts involved in hydrogen oxidation, oxidation of sulfur species, aerobic respiration and denitrification were abundant and mostly assigned toSulfurovum, indicating that members of this genus utilize multiple chemical energy sources simultaneously for primary production.Sulfurovumalso seemed to have a diverse expression of transposases, potentially involved in horizontal gene transfer. Other transcripts were involved in CO2 fixation by the reverse TCA cycle, the CRISPR-Cas system, heavy metal resistance, and sensing and responding to changing environmental conditions. Through pyrosequencing of PCR amplified 16S rRNA genes, theSulfurovum-dominated biofilm was compared with another biofilm from the same chimney, revealing a large shift in the community structure ofEpsilonproteobacteria-dominated biofilms over a few metres.