OOS 72-1
Unifying concepts for communicating and interpreting measures of microbial efficiency
Microbial efficiency, or the proportion of metabolized carbon that results in biomass synthesis, is a critical step in terrestrial carbon cycling that determines the dynamics of soil organic matter pools and ecosystem carbon dioxide efflux. Despite recent reviews and multiple modeling applications, microbial efficiency is a topic that remains heavily divided among research approaches (e.g., physiological, ecological, mathematical) and the discipline-specific terminologies/methodologies they employ. A synthetic framework is required that unites these lines of evidence into a cohesive, comprehensive whole. Here we discuss a conceptual model that enhances the communication and interpretation of microbial efficiency by structuring the definition as a hierarchy integrating increasingly broad temporal and spatial drivers of the efficiency of carbon metabolism.
Results/Conclusions
Our framework uses interdisciplinary language and emphasizes the dependence of microbial efficiency on numerous processes both internal (e.g., assimilatory/dissimilatory demand, exudate production) and external (e.g., substrate recycling) to the cell. These processes are nested among three levels to reflect the biological scales at which they inform: 1) physiological (biomass production) efficiency measured for cultured cell populations, 2) gross production efficiency of natural microbial communities, and 3) net growth efficiency of natural communities across broader scales of time. Existing techniques for measuring microbial efficiency are designed to capture the first two of these, but few attempts have been made to assess the long-term effects of processes like biomass turnover and substrate recycling on biomass production. This third scale is introduced as a way to integrate all internal and external constraints on efficiency and, as such, embodies the ecosystem-wide capacity for carbon metabolized by microorganisms to be sequestered as net growth. Future directions of research will also be discussed, including aspects of carbon metabolism that require further exploration and the value of incorporating an ecosystem perspective of microbial efficiency into global models of carbon dynamics.