4 P52220RA 9. This elegant, multistep process is a pervasive feature of life as we know it, but energetic challenges are ever-present. If the electrical potentials of electron donor and acceptor are too closely aligned, for example, it won’t be possible to squeeze much energy from their coupling. The concentrations of the reactants and the speed at which enzymes can mobilize them are also key factors. These two components—the magnitude of energy available from a particular pairing and the rate of such reactions—determine how much energy a cell can produce. 10. The other half of the equation—the cost of living, as it were—is often harder to evaluate. Cataloging the biochemical parts list of a particular cell is one challenge. Individual biosynthetic pathways—the production of lipids from glycerol derivatives, for example—are relatively well characterized under “standard” conditions, but a cell’s ever-changing chemical environment can render baseline calculations inaccurate. Scaled over millions of such reactions, the margin of error may be a substantial proportion of the available energy. And this is just considering the biosynthesis of new cellular material. In most environments, microbes must always be vigilant against biochemical breakdown resulting from environmental stresses, calling on energy reserves to restore old enzymes or patch holes in cell walls. Competition among residents may also demand additional energy expenditure, such as powering flagella to swim around in search of food or producing antibiotic molecules to keep predatory neighbors at bay. 11. If, however, we are able to estimate how much energy is required for survival, and compare that to how much energy is available to be extracted from the environment, we can begin to consider “extreme” organisms in a more objective fashion. Some of the most “exotic” environments actually offer luxurious energetic balances; it’s the microbes with low net energy availability that are the real extremophiles, whether they live an expensive existence in a high-energy environment, or an ascetic life in an energetic desert. Easy living High energy availability, low energy requirements 12. The hot springs of Yellowstone National Park are uniquely beautiful palettes: concentric rings transition from blue in the pools’ centers to green, then yellow, orange, and red at the water’s periphery. The mesmerizing visuals contrast sharply with the damp, sulfurous odors wafting across your nostrils and the stern warnings from signs and rangers to keep your distance. Against this otherworldly backdrop, the 1966 discovery of viable cells living in the ultrahot waters came as a surprise that forced a reconsideration of microbial limits. After all, water temperatures frequently topped out well above the tolerance range of most known organisms. Nearly all of E. coli’s enzymes, for example, unfold and become ineffective at 60 °C. 13. Hot-springs microbes have traditionally been labeled “extremophiles,” yet their energetic bank account is typically well in the black. Like their moderate-temperature relatives that ply the planet’s oceans, thermophilic cyanobacteria gain energy from light-driven reactions that mobilize electrons from water. Along the outer edges of thermal springs, energy-generating light is abundant, and cyanobacteria flourish. Indeed, the vibrant colors we see are the plentiful microbial pigments that coat the limestone surfaces.