Autotrophic Methanotrophy in Verrucomicrobia: Methylacidiphilum fumariolicumSolV Uses the Calvin-Benson-Bassham Cycle for Carbon Dioxide Fixation

Genome data of the extreme acidophilic verrucomicrobial methanotrophMethylacidiphilum fumariolicum strain SolV indicated the ability of autotrophic growth. This was further validated by transcriptome analysis, which showed that all genes required for a functional Calvin-Benson-Bassham (CBB) cycle were transcribed. Experiments with13 CH4 or13 CO2 in batch and chemostat cultures demonstrated that CO2 is the sole carbon source for growth of strain SolV. In the presence of CH4 , CO2 concentrations in the headspace below 1% (vol/vol) were growth limiting, and no growth was observed when CO2 concentrations were below 0.3% (vol/vol). The activity of the key enzyme of the CBB cycle, ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO), measured with a13 C stable-isotope method was about 70 nmol CO2 fixed · min−1 · mg of protein−1 . An immune reaction with antibody against the large subunit of RuBisCO on Western blots was found only in the supernatant fractions of cell extracts. The apparent native mass of the RuBisCO complex in strain SolV was about 482 kDa, probably consisting of 8 large (53-kDa) and 8 small (16-kDa) subunits. Based on phylogenetic analysis of the corresponding RuBisCO gene, we postulate that RuBisCO of the verrucomicrobial methanotrophs represents a new type of form I RuBisCO.