Salinity effect on an anaerobic methane- and ammonium-oxidising consortium: Shifts in activity, morphology, osmoregulation and syntrophic relationship

Nitrate-dependent anaerobic methane oxidation (AOM) is a microbial process of both ecological significance for global methane mitigation and application potential for wastewater treatment. It is mediated by organisms belonging to the archaeal family 'Candidatus Methanoperedenaceae', which have so far mainly been found in freshwater environments. Their potential distribution in saline environments and their physiological responses to salinity variation were still poorly understood. In this study, the responses of the freshwater 'Candidatus Methanoperedens nitroreducens'-dominated consortium to different salinities were investigated using short- and long-term setups. Short-term exposure to salt stress significantly affected nitrate reduction and methane oxidation activities over the tested concentration range of 15-200 parts per thousand NaCl, and 'Ca. M. nitroreducens' showed the higher tolerance to high salinity stress than its partner of anammox bacteria. At high salinity concentration, near marine conditions of 37 parts per thousand, the target organism 'Ca. M. nitroreducens' showed stabilized nitrate reduction activity of 208.5 mu mol day(-1) g(CDW)(-1) in long-term bioreactors over 300 days, in comparison to 362.9 and 334.3 mu mol day(-1) g(CDW)(-1) under low-salinity conditions (1.7 parts per thousand NaCl) and control conditions (similar to 15 parts per thousand NaCl). Different partners of 'Ca. M. nitroreducens' evolved in the consortia with three different salinity conditions, suggesting the different syntrophic mechanisms shaped by changes in salinity. A new syntrophic relationship between 'Ca. M. nitroreducens' and Fimicutes and/or Chloroflexi denitrifying populations was identified under the marine salinity condition. Metaproteomic analysis shows that the salinity changes lead to higher expression of response regulators and selective ion (Na+/H+) channeling proteins that can regulate the osmotic pressure between the cell and its environment. The reverse methanogenesis pathway was, however, not impacted. The finding of this study has important implications for the ecological distribution of the nitrate-dependent AOM process in marine environments and the potential of this biotechnological process for the treatment of high-salinity industrial wastewater.

Automated summary

Nitrate-dependent anaerobic methane oxidation is a microbial process that has ecological significance for global methane mitigation and application potential for wastewater treatment. This study investigated the physiological responses of a freshwater consortium dominated by 'Candidatus Methanoperedens nitroreducens' to different salinities. The results showed that the target organism had higher tolerance to high salinity stress and that the activities of nitrate reduction and methane oxidation were significantly affected by salt stress. The study also identified a new syntrophic relationship between 'Ca. M. nitroreducens' and denitrifying populations under marine salinity conditions. The findings have important implications for the ecological distribution of the nitrate-dependent AOM process in marine environments and the potential of this biotechnological process for the treatment of high-salinity industrial wastewater.