Biological Energy Supply
All life uses adenosine triphosphate (ATP) for storing energy and driving cellular processes that require energy (such as flagellar rotation or protein synthesis). Therefore sufficient free energy must be biologically available (e.g., in the form of redox couples) and exceed a finite minimum in order to drive the formation of ATP from adenosine diphosphate (ADP). The presence of free energy in an ecosystem that equals or exceeds this finite minimum is necessary to sustain life and conserve ecosystem functions (Hoehler 2004; von Stockar et al. 2005). From a thermodynamic standpoint it is not possible to couple with complete efficiency all energy released during metabolism to ATP synthesis. The thermodynamic efficiency of the ecosystem must therefore account for the proportion of free energy of metabolism that can be captured and stored along with that proportion lost as heat (Hoehler 2004; von Stockar et al. 2005). Thus, the minimum free energy that must be available for biological processes in any given environment is the combination of the ATP synthesis energy, stoichiometry of ion release during ATP synthesis, and thermodynamic efficiency (Schink and Stams 2002). This quantity can be referred to as the biological energy quantum (BEQ) and provides a predictive measure of the favorability of biological activity analogous to the use of Gibbs free energy change to determine the likelihood of a chemical reaction proceeding (Hoehler 2004).
Coupled to the presence of an energy quantum that must be generated in a chemical reaction inside a biological system is the concept of maintenance energy (ME), which suggests that organisms require a certain minimum rate of energy intake to maintain molecular and cellular order and function (Hoehler 2004). Three levels of increasing ME requirement can be described (Morita 1997): (1) cell survival, (2) cell maintenance, and (3) active cell growth. Both BEQ and ME can be affected by a variety of environmental factors such as temperature (BEQ and ME requirements should decrease with temperature). BEQ and ME requirements thus define the minimum substrate concentration and substrate generation rate that must be sustained by a given environment in order to support life (Hoehler 2004). For some time the figure of −20 kJ mol−1 was described in the literature as the minimum quantum of free energy that could be converted by a biological system (Schink 1997). More recently, a free energy value as low as −4.5 kJ mol−1 was reported (Jackson and McInerney 2002) to support growth of a co-culture of butyrate-degrading organisms with methanogens in which one species lives off the products of another. Although the ability to measure thermodynamic minima such as BEQ and ME is still very much a work in progress, this result implies that microbial communities could be supported even in extreme subglacial aquatic environments despite ultra-oligotrophic substrate concentrations and the absence of sunlight.
Rothschild and Mancinelli 2001), brine channels in sea ice (Thomas and Dieckmann 2002), glaciers and ice shelves (Christner et al. 2003; Mueller et al. 2005), the subterranean deep biosphere (Pedersen 1993), cold deep-sea environments (Yayanos 1995; Bartlett 2002), deserts (Drees et al. 2006), and pH and salinity extremes (Vreeland et al. 2000; Fütterer 2004).
Subglacial aquatic systems are extreme environments for microbial life with low temperatures, elevated pressures, and no direct contact with the atmosphere or sunlight. Indirect evidence suggests that they may also have a high gas content (including oxygen) and low inorganic and organic nutrient concentrations. Most temperate latitude species are unlikely to proliferate and are also unlikely to compete effectively with species