The complex relationship between microbial growth rate and yield and its implications for ecosystem processes

Growth rate and efficiency are fundamental traits of microbes that significantly influence how communities and ecosystems function. However the microbiological literature shows an apparent contradiction in the relationship between growth rate and yield (defined as the portion of consumed substrate that is converted into biomass or ATP). Pirt (1965) defined maintenance energy in growing bacterial cultures and predicted that slow growth rates would be associated with inefficient growth. This idea is consistent with studies of bacterial growth efficiency (BGE) in marine and aquatic ecosystems. However, a separate body of literature supports a negative relationship between growth rate and yield, and finds that this rate-yield tradeoff is central in evolution and the coexistence of species. The concept of yield arises in the terrestrial biogeochemical literature, but under the term, carbon use efficiency (CUE), where researchers are concerned with the rates of carbon dioxide (CO2) lost from the ecosystem through microbial respiration, and how the partitioning of plant litter into soil carbon (C) and CO2 is affected by climate change. Here the rate-yield tradeoff also has important implications. I believe the seemingly contradictory points of view expressed in the literature can be reconciled by considering the growth conditions, ecological strategies and level of organization treated by these various studies. This opinion article is not intended as a comprehensive review. Excellent reviews and analyses of various aspects of this literature exist (Russell and Cook, 1995; Del Giorgio and Cole, 1998; Ferenci, 1999; Carlson et al., 2007; Van Bodegom, 2007; Wang and Post, 2012; Sinsabaugh et al., 2013). These works have resolved much of the perceived ambiguity in the field, but to my knowledge no one has directly addressed the apparent paradox mentioned above.