Overexpression of native carbonic anhydrases increases carbon conversion efficiency in the methanotrophic biocatalyst Methylococcus capsulatus Bath

Abstract

ABSTRACT Methanotrophic bacteria play a vital role in the biogeochemical carbon cycle due to their unique ability to use CH 4 as a carbon and energy source. Evidence suggests that some methanotrophs, including Methylococcus capsulatus , can also use CO 2 as a carbon source, making these bacteria promising candidates for developing biotechnologies targeting greenhouse gas capture and mitigation. However, a deeper understanding of the dual CH 4 and CO 2 metabolism is needed to guide methanotroph strain improvements and realize their industrial utility. In this study, we show that M. capsulatus expresses five carbonic anhydrase (CA) isoforms, one α-CA, one γ-CA, and three β-CAs, that play a role in its inorganic carbon metabolism and CO 2 -dependent growth. The CA isoforms are differentially expressed, and transcription of all isoform genes is induced in response to CO 2 limitation. CA null mutant strains exhibited markedly impaired growth compared to an isogenic wild-type control, suggesting that the CA isoforms have independent, non-redundant roles in M. capsulatus metabolism and physiology. Overexpression of some, but not all, CA isoforms improved bacterial growth kinetics and decreased CO 2 evolution from CH 4 -consuming cultures. Notably, we developed an engineered methanotrophic biocatalyst overexpressing the native α-CA and β-CA with a 2.5-fold improvement in the conversion of CH 4 to biomass. Given that product yield is a significant cost driver of methanotroph-based bioprocesses, the engineered strain developed here could improve the economics of CH 4 biocatalysis, including the production of single-cell protein from natural gas or anaerobic digestion-derived biogas. IMPORTANCE Methanotrophs transform CH 4 into CO 2 and multi-carbon compounds, so they play a critical role in the global carbon cycle and are of interest for biotechnology applications. Some methanotrophs, including Methylococcus capsulatus , can also use CO 2 as a carbon source, but this dual one-carbon metabolism is incompletely understood. In this study, we show that M. capsulatus carbonic anhydrases are critical for this bacterium to optimally utilize CO 2 . We developed an engineered strain with improved CO 2 utilization capacity that increased the overall carbon conversion to cell biomass. The improvements to methanotroph-based product yields observed here are expected to reduce costs associated with CH 4 conversion bioprocesses.