Reviews and syntheses: Heterotrophic fixation of inorganic carbon – significant but invisible flux in environmental carbon cycling

Heterotrophic CO 2 fixation is a significant yet underappreciated CO 2 flux in environmental carbon cycling. In contrast to photosynthesis and chemolithoautotrophy – the main recognized autotrophic CO 2 fixation pathways – the importance of heterotrophic CO 2 fixation remains enigmatic. All heterotrophs – from microorganisms to humans – take up CO 2 and incorporate it into their biomass. Depending on the availability and quality of growth substrates, and drivers such as the CO 2 partial pressure, heterotrophic CO 2 fixation contributes at least 1 %–5 % and in the case of methanotrophs up to 50 % of the carbon biomass. Assuming a standing stock of global heterotrophic biomass of 47–85 Pg C, we roughly estimate that up to 5 Pg C might be derived from heterotrophic CO 2 fixation, and up to 12 Pg C yr −1 originating from heterotrophic CO 2 fixation is funneled into the global annual heterotrophic production of 34–245 Pg C yr −1 . These first estimates on the importance of heterotrophic fixation of inorganic carbon indicate that this pathway should be incorporated in present and future carbon cycling budgets.