Beyond growth? The significance of non-growth anabolism for microbial carbon-use efficiency in the light of mineralassociated carbon stabilization

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Bölscher, Tobias | Vogel, Cordula | Olagoke, Folasade, K | Meurer, Katharina H.E. | Herrmann, Anke, M | Colombi, Tino | Brunn, Melanie | Domeignoz-Horta, Luiz, A

Edité par CCSD -

International audience. The microbial origin of stabilised soil organic C has received increasing attention. Findings suggest that microbial-derived C may make up a quarter to more than half of total soil C with the majority stabilised in mineral-organic associations. The microbial metabolic performance is a key factor in soil C dynamics, because microbes determine the fate of C during decomposition. They partition C between anabolic biosynthesis of various new microbial metabolites and catabolic C emissions (i.e. respiration). This partitioning is commonly referred to as microbial carbon-use efficiency (CUE). The reuse of C during biosynthesis provides a potential for the accumulation of microbial metabolic residues on mineral surfaces.Current concepts of microbial CUE neglect microbial non-growth metabolites which can make up considerable parts of the microbially processed C. Commonly, CUE is quantified from C incorporated into biomass or used for growth and C released as CO2. Extracellular metabolites, such as polymeric substances (EPS), exoenzymes or nutrient mobilizing compounds, as well as intracellular maintenance metabolites, such as storage compounds or endoenzymes, are ignored although they experience similar affinities to mineral surfaces as growth residues.Based on theoretical considerations and two case studies, we highlight that disregarding non-growth anabolism can lead to severe underestimations of CUE and thus the potential for microbial-derived C stabilization in mineral-organic associations. The two case studies demonstrate that neglecting exoenzyme and EPS production can underestimate CUE by more than 100% and up to 30%, respectively. Our findings and considerations challenge the current ways how CUE is measured. We will therefore (I) scrutinise the idea that non-growth anabolism can be ignored for CUE investigations, (ii) suggest adjustments to common CUE approaches, (iii) demonstrate that current assessments of CUE measure only an ‘apparent’ CUE which could significantly underestimate ‘actual’ CUE, (iv) outline research needs and potential ways forward.

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