5 P52220RA Turn over 14. This is not to say that life at high temperatures is easy. On the contrary, only through a range of sophisticated molecular adaptations, encoded by subtle edits in thermophiles’ genomes, has this forbidding niche become habitable. Protein stability is perhaps the main challenge for life at high temperature. Higher thermal energy causes hyperactive atoms to vibrate with more kinetic energy, threatening the structural integrity of the molecules that perform biochemical reactions. If sulfur-containing cysteine amino acids are positioned strategically within protein structures, disulfide bridges can form interatomic support beams that resist unfolding. Some thermophilic enzymes also have larger hydrophobic cores, away from the proteins’ exposed active sites, which act as additional glue to fight thermal destabilization. Other adaptations, such as simpler protein folds or fewer bound metal ions, further guard against molecular destabilization in the face of thermal stress. 15. Evolving the capability to handle high temperatures may not have been straightforward, and biosynthetic construction costs might have presented some hurdles, but the payoff does seem to have been worth it. By constructing heat-stable enzymes, the cyanobacteria inhabiting hot springs are able to photosynthesize in relative peace, away from the feeding frenzy of predatory microbes or larger creatures in habitats such as the ocean’s surfaces. Thus, while Yellowstone’s hot springs may seem like an extreme environment, not all of the microbes that inhabit them are struggling to survive. An expensive lifestyle High energy availability, high energy requirements 16. One needn’t travel to the bottom of the ocean or into a scalding hot spring to find microbes living at the edge of energetic feasibility. Sometimes, the most remarkable habitats are in your own backyard, beneath well-manicured Kentucky bluegrass or a haphazard array of lawn furniture. “Generic” temperate soils are among the richest microbial milieus on the planet, with each pinch of dirt hosting up to a billion cells, and down there, it’s all-out biochemical warfare. 17. Among the more prominent denizens of this dense microbial metropolis are representatives of the bacterial genus Streptomyces: stringy, rod-shaped organisms that develop centimeter-scale networks branching through the soil. Streptomyces gain energy through heterotrophy, the consumption of organic molecules such as sugars, amino acids, or aromatic compounds. These are energetically juicy molecules, and they’re abundant given the high density of plants in the vicinity, but it’s far from a free lunch. NOAA Okeanos Explorer Program, 2013 ROV Shakedown and Field Trials Methane munchers Off the coast of Virginia, methane bubbles flowing out of the seafloor sediment support a variety of life, including some truly extreme microbes.