Potential N2 fixation by sulfate-reducing bacteria in a marine intertidal microbial mat

The first report that sulfate-reducing bacteria (SRB) can fix N-2 was published a half century ago. Over the last 50 yr, it has slowly emerged that N-2 fixation is widespread among Gram-negative, mesophilic SRB. However, the ecological role of SRB N-2 fixation is not well understood. In some marine systems, SRB may contribute significantly to observed N-2 fixation (acetylene reduction), To date, evidence of SRB N-2 fixation has relied on inferring the results from the use of metabolic inhibitors and most probable number counts, This study attempted to assess more directly the role of SRB N-2 fixation in a temperate marine microbial mat system within the Rachel Carson National Estuarine Research Reserve (RCNERR; Beaufort, North Carolina, USA). The SRB inhibitor sodium molybdate (a structural analog of sulfate) was employed to characterize potential SRB N-2 fixation at night. Sodium molybdate inhibited nighttime nitrogenase activity (NA) by as much as 64%. Sodium molybdate had no effect on daytime NA, Reverse transcription-polymerase chain reaction (RT-PCR) was employed to characterize organisms expressing the dinitrogenase reductase gene (nifH), an essential gene for N-2 fixation, Several nifH sequences obtained from RT-PCR were highly similar to the nifH sequences of anaerobic organisms, including several SRB. Estimates of ATP production, based on sulfate reduction rates, imply sulfate reduction is capable of supporting molybdate-inhibited NA. The evidence suggests that SRB may contribute to N-2 fixation in the RCNERR mats.