acids to produce CO2, hydrogen gas, and acetate. The hand-me-down energy economy in anaerobic bacteria continues. The acetate produced by the acetogens itself acts as a feedstock for another type of bacteria, the methanogens, so called because they produce methane gas (CH4) as an end product. One type of methanogen accepts acetate from the CO2-reducing acetogens and fermenters, while a second type accepts acetate, hydrogen gas, and CO2.

You now begin to see why oxygen gas constitutes a deadly threat to organisms such as these. These wonderfully diverse biochemistries operate on fairly small redox potential differences. If you introduce oxygen, with its very high redox potential, you change the balance of forces that move electrons through and between bacteria. Oxygen draws electrons away from the painstakingly crafted anaerobic pathways built for them, so they cannot do the work the biochemical pathway are intended to do. Before you know it, the organism that relies on this pathway is dead. Since all the organisms that existed prior to the evolution of photosynthesis relied on anaerobic biochemistries, you can now see why the evolution of photosynthesis was such an ecological catastrophe.

Faced with the aerobic onslaught that accompanied the evolution of photosynthesis, the bacteria of the anaerobic world could die, adapt, or retreat. The first was probably the most common, but that is evolutionarily uninteresting, of course. The second course was equivalent to yielding to the inevitable: since oxygen would get the electrons eventually anyway, why not try to make them do work along the way? The bacteria that adopted this strategy, in fact, found many new metabolic opportunities open to them. Remember, there is nothing magical about glucose as food—all that is needed to make a molecule serve as food is a sufficiently large redox potential difference between the food molecule (the electron donor) and the oxidant (the electron acceptor). Because oxygen is such a potent oxidant, microbes could now strip electrons away from many compounds that avidly held them in the presence of weaker electron acceptors. The result was a proliferation of all sorts of novel metabolic pathways among the bacteria, which exploited heretofore strange "food" molecules, such as ammonia, carbon monoxide, elemental sulfur and other sulfur compounds, hydrogen gas, and even iron and silica rocks (Table 6.2). One of these pathways, which we inherited from a class of bacteria that learned the trick of using the electrons to shuffle protons around and make

Complex organic molecules ("cast-offs")

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