Carbon and nitrogen use efficiency of heterotrophic microbial communities: Concepts and emerging techniques
Terrestrial decomposer communities thrive on a wide range of substrates, which rarely ever meet their elemental demands. One of the most important mechanisms by which microbes are able to maintain elemental homeostasis is the release of the elements in excess by regulation of their element use efficiencies. Microbial carbon use efficiency (CUE; also growth yield) is usually defined as the allocation of consumed organic carbon to growth and is thus a synthetic representation of microbial metabolism. Similarly, microbial nitrogen use efficiency (NUE) can be defined as the partitioning of organic N taken up between growth and the release of inorganic N to the environment. Microbial element use efficiencies are thus fundamental for understanding organic matter decomposition and ecosystem carbon and nutrient storage. There are, however, several conceptional and methodological challenges and questions associated with microbial use efficiencies that will be addressed here. For example (i) is inorganic N utilization for microbial growth consistent with the current definition of NUE, (ii) are CUE and NUE overestimated by storage of C and N compounds that are not allocated to growth, (iii) is CUE underestimated by not accounting for microbial death during the measurement period and (iv) does the common approach to measure CUE by 13C-labelled substrates actually inflate CUE estimates?
NUE is determined by measuring gross organic N uptake and estimating the proportion of N allocated to growth by subtracting gross N mineralization from gross uptake. In contrast, CUE is usually estimated by following the partitioning of 13C-labelled substrates between biomass incorporation and respiration. This approach is thought to inflate CUE estimates and we will here provide CUE estimates by a novel technique that is based on the incorporation of 18O from labelled water into microbial DNA to determine growth. CUE estimates by this technique are considerably lower than estimates from the 13C approach and in the range predicted by thermodynamics. We will further demonstrate, for the first time, that heterotrophic microbial communities independently regulate both, their CUE and NUE to cope with elemental imbalances and that CUE and NUE are differentially affected by environmental factors such as temperature and drought. In an expansion of the stoichiometric concepts, we will present modelling results that suggest that the relative turnover rates of organic carbon and nitrogen during decomposition are regulated by microbial community dynamics, possibly allowing microbial decomposers to overcome large stoichiometric imbalances without the need to adapt CUE or NUE.