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

Abstract. Heterotrophic CO₂ fixation is a significant, yet underappreciated CO₂ flux in the global carbon cycle. In contrast to photosynthesis and chemolithoautotrophy – the main recognized autotrophic CO₂ fixation pathways – the importance of heterotrophic CO₂ fixation remains enigmatic. All heterotrophs – from microorganisms to humans – take up CO₂ and incorporate it into their biomass. Depending on the available growth substrates, heterotrophic CO₂ fixation contributes at least 2–8 % and in the case of methanotrophs up to 50 % of the carbon building up their biomass. Assuming a standing stock of global heterotrophic biomass of 47–85 Pg C, we estimate that up to 7 Pg C have been derived from heterotrophic CO₂ fixation and up to 20 Pg C yr⁻¹ originating from heterotrophic CO₂ fixation are 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 carbon fixation pathway should be included in present and future global carbon budgets.