Aims: How benzene is metabolized by microbes under anoxic conditions is not fully understood. Here, we studied the degradation pathways in a benzene- mineralizing, nitrate- reducing enrichment culture.

Methods  and  results:  Benzene  mineralization  was  dependent  on  the  presence  of  nitrate and correlated to the enrichment of a Peptococcaceae phylotype only distantly related to known anaerobic benzene degraders of this family. Its relative abundance decreased after benzene mineralization had terminated, while other abundant taxa— Ignavibacteriaceae, Rhodanobacteraceae   and   Brocadiaceae— slightly    increased. Generally,  the  microbial  community  remained  diverse  despite  the  amendment  of  benzene  as  single  organic  carbon  source,  suggesting  complex  trophic  interactions  between  different  functional  groups.  A  subunit  of  the  putative  anaerobic  benzene  carboxylase previously detected in Peptococcaceae was identified by metaproteomic analysis  suggesting  that  benzene  was  activated  by  carboxylation.  Detection  of  pro-teins involved in anaerobic ammonium oxidation (anammox) indicates that benzene mineralization  was  accompanied  by  anammox,  facilitated  by  nitrite  accumulation  and the presence of ammonium in the growth medium.

Conclusions: The results suggest that benzene was activated by carboxylation and further assimilated by a novel Peptococcaceae phylotype.

Significance  and  impact  of  the  study:  The  results  confirm  the  hypothesis  that  Peptococcaceae are important anaerobic benzene degraders.