Reviews and syntheses: Heterotrophic fixation of inorganic carbon - Significant but invisible flux in environmental carbon cycling

Heterotrophic CO2 fixation is a significant yet underappreciated CO2 flux in environmental carbon cycling. 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 availability and quality of growth substrates, and drivers such as the CO2 partial pressure, heterotrophic CO2 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 47-85PgC, we roughly estimate that up to 5PgC might be derived from heterotrophic CO2 fixation, and up to 12PgCyr-1 originating from heterotrophic CO2 fixation is funneled into the global annual heterotrophic production of 34-245PgCyr-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.