Sulfate-reducing capability of nitrate-dependent anaerobic gaseous alkanes degrader

Microbial oxidation of short-chain gaseous alkanes (SCGAs, including ethane, propane and butane) are important sinks to mitigate the emission of SCGAs to the atmosphere. 'Candidatus Alkanivorans nitratireducens' has been discovered to be capable of utilizing nitrate as an electron acceptor to anaerobically oxidize these SCGAs. However, little is known about its metabolic diversity in sulfate reduction, despite sulfate being widely present in both marine and freshwater ecosystems. Here, we show that sulfate can be reduced by 'Ca. A. nitratireducens' as an alternative electron acceptor. Genomic analysis confirmed that the genome of 'Ca. A. nitratireducens' harbours genes involved in sulfate reduction. Short-term incubation of the enriched 'Ca. A. nitratireducens' showed immediate consumption of propane and sulfate, suggesting the capability of 'Ca. A. nitratireducens' to utilize sulfate as an electron acceptor. Long-term incubation further confirmed its ability to utilize sulfate. However, propane oxidation rates and sulfate reduction rates gradually decreased during the long-term incubation, accompanied by a deline in the relative abundance of 'Ca. A. nitratireducens'. After the long-term adaptation with sulfate as the sole acceptor, both anaerobic propane oxidation and nitrate reduction capability of 'Ca. A. nitratireducens' can be partly recovered by switching the electron acceptor back from sulfate to nitrate. Overall, this study indicates sulfate can be utilized but is not the preferred electron acceptor for 'Ca. A. nitratireducens'. The findings deepen our understanding on the metabolic flexibility of 'Ca. A. nitratireducens'.