Anammox and Its Rocket Chemistry
Bacteria are often seen as rudimentary forms of life. But one look at their molecular structure is enough to convince us otherwise. Bacteria are extremely sophisticated, fully equipped with many exquisite molecular machines. One very strange group of bacteria discovered in the early 1990s, called anammox, provides a great example of the high-tech characteristics of bacteria. According to Laura van Niftrik and Mike Jetten, anammox bacteria are found in a wide variety of environments, including low-oxygen marine zones, treatment plant wastewater, coastal sediments, and lakes. It turns out that these bacteria are crucial to life on Earth: It is estimated they contribute up to fifty percent of N2 production from marine environments, resulting in the removal of fixed nitrogen. When discovered, anammox bacteria caused a real scientific stir. They are major players in Earth’s biogeochemical nitrogen cycle, and scientists wondered how such simple bacteria could perform a reaction previously considered impossible. Anammox converts NH3 and NO2 - into N2 under anaerobic conditions, that is, in the absence of O2. That is where it got its name: ANaerobic AMMonium OXidation. “Anammox bacteria do not conform to the typical characteristics of bacteria but instead share features with all three domains of life, Bacteria, Archaea, and Eukarya, making them extremely interesting from an evolutionary perspective.” I would go further and say that the existence of these crucial and unusual bacteria is in fact extremely difficult to explain from an evolutionary perspective. How does an anammox bacterium fulfill its indispensable mission of replenishing nitrogen? It uses rocket science and some highly sophisticated organic synthesis skills.
The bacterium has an internal organelle covered by a double-layer membrane, not at all peculiar in prokaryotic cells. The greatest surprise was what was inside the organelle. Inside, scientists found hydrazine, which has a variety of uses, including for rocket fuel! Anammox somehow makes, stores, and uses a highly toxic, corrosive, and explosive liquid. Can you imagine a creature evolving one step at a time to store this stuff inside itself? Imagine trying to synthesize pure hydrazine by trial and error inside a bacterium. It wouldn’t take long to kill it! How would a bacterium evolve a hydrazine synthesis protocol without all the machinery to safely hold and use hydrazine? Is it plausible that a bacterium gained the ability to use pure, toxic, and explosive hydrazine by a step-by-step process that has no way to predict the future advantages of the poison? Why would a proto-anammox bacterium, which had previously not used hydrazine, and survived just fine without it, risk its life to evolve the ability to produce and store hydrazine, before it would do it any good? Another surprise is that anammox bacteria store hydrazine in internal compartments called anammoxosomes. Obviously, anammox bacteria must handle this explosive molecule with the greatest care. Chemical and microscopic analysis of the anammoxosome double-layer membrane, which encloses the hydrazine, revealed another surprise: The membrane consists of unique and bizarre lipids made from “ladderanes.” These are highly sophisticated chemical structures that many synthetic chemists would not even attempt to make. A typical ladderane is pentacycloanammoxic acid, which is composed of five fused rings of cyclobutane. It resembles a ladder and contains concatenated square ring structures formed by fused four-carbon rings. Concatenated four-membered rings are one of the hardest to make because kinetics and thermodynamics work against them. But anammox bacteria seem to have skipped organic synthesis classes and gone ahead and built them anyway. But why go to all the effort? It appears that anammox bacteria did it only to use hydrazine as an agent to convert NH3 and NO2 - into N2 in the absence of O2. So why would a bacterium synthesize N2, an almost inert gas that is practically useless for life as such? Anammox bacteria live all over the world. They are abundant in the oceans. They undertake this nearly impossible task simply to produce N2.
But because of this “charity effort,” they regulate the N2 cycle and maintain the O2/N2 ratio of the Earth’s atmosphere.11 This little nanomolecular machine keeps the N2 at the balance needed for all life forms on our planet to survive. In essence, this little microbe uses rocket science12 to make life on earth possible, and sustainable. And we’re only beginning to understand this extraordinary bacterium. The enzymatic mechanism that makes hydrazine must also be incredible. “The crystal structure implies a two-step mechanism for hydrazine synthesis: a three electron reduction of nitric oxide to hydroxylamine at the active site of the γ-subunit and its subsequent condensation with ammonia.” The authors of the Nature paper go on to note a striking parallel: “Interestingly, the proposed scheme is analogous to the Raschig process used in industrial hydrazine synthesis.” So, again we find that another of our carefully planned inventions is only following in nature’s footsteps. The N2 gas that pairs with O2 in our atmosphere and is essential for life on Earth is, as another article puts it, “a byproduct of an exquisitely designed, precision nanomachine that knows a lot about organic redox chemistry and safe handling of rocket fuel.” The world of microbes proves more sophisticated with every discovery, manifesting more and more “surprises”—that is, evidence of foresight. Recently, we discovered another microbial wonder: the enigmatic comammox, or “complete ammonia oxidizer.” This bacterium can be found almost everywhere and does an even more spectacular job than anammox. Comammox perform complete nitrification on their own, a milestone of microbiology. Two different classes of nitrifier microbes have long been known to cooperate in carrying out the nitrification process where NH3 is oxidized to NO2 -, which is subsequently oxidized to NO3 -. But the comammox doesn’t share labor in nitrification. It catalyzes both nitrification steps, doing complete ammonia oxidation and thus conserving energy. It is difficult to escape the implications of all this: The need to sustain an atmosphere suited to life had to be anticipated from the start. And an array of microbes, equipped with a sophisticated arsenal of chemicals and capacities, had to be provided to meet that need.