Methane-oxidizing archaea, aerobic methanotrophs and nitrifiers coexist with methane as the sole carbon source

R.B.Costa^a, D.Y.Okada^b, T.P.Delforno^c, E.Foresti^a

Biological Processes Laboratory, Department of Hydraulics and Sanitation, São Carlos School of Engineering (EESC), University of Sao Paulo (USP), Engenharia Ambiental - Bloco 4-F, Av João Dagnone, 1100, Santa Angelina, 13.563-120, São Carlos, SP, Brazil.

Abstract

Methane oxidation plays a key role in carbon and nutrient cycling and has the potential to be applied in engineered bioprocesses, including wastewater and gas treatments. To provide insights into the dynamics of the methanotrophic community under microoxic and anoxic conditions, two sequencing batch reactors under microoxic (MO2-SBR) and anoxic (Anox-SBR) conditions were operated. The methane oxidation rate was higher under microoxic conditions (5.3 ± 0.9 mmol.batch cycle^-1) than anoxic conditions (3.1 ± 0.8 mmol.batch cycle^-1). Higher methane oxidation led to higher nitrate reduction rates (9.4 ± 2.5 mgN.batch cycle^-1 and 4.0 ± 2.0 mg N.batch cycle^-1 for MO₂-SBR and Anox-SBR, respectively). 16S rDNA sequencing revealed reads corresponding to aerobic oxidizers (0.5% and 2.0% for Anox-SBR and MO2-SBR, respectively), to the Nitrosospira genus (26.6% and 28.3% for Anox-SBR and MO₂-SBR, respectively), and to anaerobic methane-oxidizing archaea (ANME) (4.0% and 3.5% for Anox-SBR and for MO2-SBR, respectively). Nitrifying organisms are capable of oxidizing methane due to the homology between the enzymes ammonia monooxygenase and methane monooxygenase. These findings seem to indicate that methane oxidation is carried out by versatile metabolic pathways and couples with other biological processes, such as denitrification.

Graphical abstract

Keywords: Methane oxidation, Anaerobic methanotrophic archaea, Nitrifiers, Syntrophy, Mixotrophy.