Novel findings on hopanoid occurrences among sulfate reducing bacteria: Is there a direct link to nitrogen fixation?

Hopanoids in multiple bacterial groups are among the most widespread organic compounds in sediments and rocks. The capability of hopanoid production was thought to be restricted to aerobic bacteria but several recent studies have revealed production by specific anaerobic bacteria, including dissimilatory sulfate reducers of the deltaproteobacterial genus Desulfovibrio. We reveal here two further marine Desulfovibrio species (D. salexigens and D. zosterae) as a rich source of hopanoids. Furthermore, a blast search for squalene–hopene cyclase, an enzyme essential for hopanoid biosynthesis, suggests that other sulfate reducing bacteria (SRB) are also hopanoid producers. In D. salexigens and D. zosterae bacteriohopanetetrol and 35-aminobacteriohopane-32,33,34-triol were prevalent. They co-occurred with minor amounts of 35-aminobacteriohopane-31,32,33,34-tetrol and 35-aminobacteriohopane-30,31,32,33,34-pentol. Notwithstanding the unresolved physiological function of hopanoids, nearly all bacterial hopanoid producers known are capable of N fixation. We therefore tested a potential relationship between hopanoid concentration and the status of N fixation in D. bastinii using 15N labelled N2. The results suggest that hopanoid production and distribution are not immediately coupled to the N fixation process in SRB. However, since all hopanoid-producing sulfate reducers are capable of N fixation, an unexplained relationship between hopanoids and N fixation cannot be excluded.