Microbial life under extreme energy limitation

The discovery of abundant microbial life in the deep subsurface, where energy fluxes can be orders of magnitude lower than in laboratory cultures, challenges many of our assumptions about the requirements to sustain life. Here, Tori Hoehler and Bo Barker Jørgensen review our understanding of life in these extremely low-energy environments. A great number of the bacteria and archaea on Earth are found in subsurface environments in a physiological state that is poorly represented or explained by laboratory cultures. Microbial cells in these very stable and oligotrophic settings catabolize 104- to 106-fold more slowly than model organisms in nutrient-rich cultures, turn over biomass on timescales of centuries to millennia rather than hours to days, and subsist with energy fluxes that are 1,000-fold lower than the typical culture-based estimates of maintenance requirements. To reconcile this disparate state of being with our knowledge of microbial physiology will require a revised understanding of microbial energy requirements, including identifying the factors that comprise true basal maintenance and the adaptations that might serve to minimize these factors.