| Comments |
"Morita (primary source) proposed two levels of metabolism for starving, nongrowing microbes. Maintenance energy is the energy required for osmotic regulation, maintenance of intracellular pH, futile cycles, turnover of macromolecules, motility, and energy dissipation by proton leak and ATP hydrolysis, exclusive of biomass production and survival energy is the energy required just for repair of macromolecular damage. Remarkably, in Fig. 1 the metabolic data for microbial communities do fall into three groupings: the rate for exponential growth (uppermost band) is orders of magnitude greater than most of the rates for maintenance without growth (blue letters), and the rates for communities trapped in glacial ice or deep subsurface sediments (red letters) are lower by another three orders of magnitude. The clear separation justifies using Morita's term, survival metabolism, to distinguish the extremely weak metabolism of immobile, probably dormant communities from maintenance metabolism of communities more accessible to nutrient and free to move but still at too low a level for growth. Maintenance metabolism depends, of course, on nutrient availability as well as on temperature. The data of Parkes et al. (3) provide an example of the dependence on nutrient availability: maintenance metabolic rate decreases with sample depth in marine sediment depth as shown by points E1 , E2 , and E3." |