Summary The anaerobic, mesophilic and moderately halophilic strain L21‐Spi‐D4 T was recently isolated from the suboxic zone of a hypersaline cyanobacterial mat using protein‐rich extracts of Arthrospira (formerly Spirulina ) platensis as substrate. Phylogenetic analyses based on 16S rRNA genes indicated an affiliation of the novel strain with the Bacteroidetes clade MgMjR‐022, which is widely distributed and abundant in hypers...