hydrogenosomes are actually related to mitochondria—they share a common ancestor. This was difficult to prove as most hydrogenosomes have lost their entire genome, but it is now established with some certainty. 1 In other words, whatever bacteria entered into a symbioticrelationship in the first eukaryotic cell, its descendents numbered among them both mitochondria
and
hydrogenosomes. Presumably, said Martin—and this is the crux of the dilemma faced today—the original bacterial ancestor of the mitochondria
and
hydrogenosomes was able to carry out the metabolic functions of both. If so, then it must have been a versatile bacterium, capable of oxygen respiration as well as hydrogen production. We’ll return to this question in a moment. For now, lets simply note that the ‘hydrogen hypothesis’ of Martin and Müller argues that it was the hydrogen metabolism of this common ancestor, not its oxygen metabolism, which gave the first eukaryote its evolutionary edge.
Martin and Müller were struck by the fact that eukaryotes containing hydrogenosomes sometimes play host to a number of tiny methanogens, which have gained entry to the cell and live happily inside. The methanogens align themselves with the hydrogenosomes, almost as if feeding ( Figure 3 ). Martin and Müller realized that this was exactly what they were doing—the two entities live together in a kind of metabolic wedlock. Methanogens are unique in that they can generate all the organic compounds they need, as well as all their energy, from nothing more than carbon dioxide and hydrogen. They do this by attaching hydrogen atoms (H) onto carbon dioxide (CO 2 ) to produce the basic building blocks needed to make carbohydrates like glucose (C 6 H 12 O 6 ), and from these they can construct the entire repertoire of nucleic acids, proteins, and lipids. They also use hydrogen and carbon dioxide to generate energy, releasing methane in the process.
While methanogens are uniquely resourceful in their metabolic powers, they nonetheless face a serious obstacle, and we have already noted the reason in Chapter 1 . The trouble is that, while carbon dioxide is plentiful, hydrogen is hard to come by in any environment containing oxygen, as hydrogen and oxygen react together to form water. From the point of view of a methanogen, then, anything that provides a little hydrogen is a blessing. Hydrogenosomes are a double boon, because they release both hydrogen gas and carbon dioxide, the very substances that methanogens crave, in the process of generating their own energy. Even more importantly, they don’t need oxygen to do this—quite the contrary, they prefer to avoid oxygen—and so they function in the very low-oxygen conditions required by methanogens. No wonder the methanogens suckle up to hydrogenosomes like greedy piglets! The insight of Martin and Müller was to appreciate that this kind of intimate metabolic union might have been the basis of the original eukaryotic merger.
Bill Martin argues that the hydrogenosomes and the mitochondria stand at opposite ends of a little-known spectrum. Rather surprisingly, to anyone who is most familiar with textbook mitochondria, many simple single-celled eukaryotes have mitochondria that operate in the absence of oxygen. Instead of using oxygen to burn up food, these ‘anaerobic’ mitochondria use other simple compounds like nitrate or nitrite. In most other respects, they operate in a very similar fashion to our own mitochondria, and are unquestionably related. So the spectrum stretches from aerobic mitochondria like our own, which are dependent on oxygen, through ‘anaerobic’ mitochondria, which prefer to use other molecules like nitrates, to the hydrogenosomes, which work rather differently but are still related. The existence of such a spectrum focuses attention on the identity of the ancestor that eventually gave rise to the entire spectrum. What, asks Martin, might this common ancestor have looked like?
3 The
and
hydrogenosomes. Presumably, said Martin—and this is the crux of the dilemma faced today—the original bacterial ancestor of the mitochondria
and
hydrogenosomes was able to carry out the metabolic functions of both. If so, then it must have been a versatile bacterium, capable of oxygen respiration as well as hydrogen production. We’ll return to this question in a moment. For now, lets simply note that the ‘hydrogen hypothesis’ of Martin and Müller argues that it was the hydrogen metabolism of this common ancestor, not its oxygen metabolism, which gave the first eukaryote its evolutionary edge.
Martin and Müller were struck by the fact that eukaryotes containing hydrogenosomes sometimes play host to a number of tiny methanogens, which have gained entry to the cell and live happily inside. The methanogens align themselves with the hydrogenosomes, almost as if feeding ( Figure 3 ). Martin and Müller realized that this was exactly what they were doing—the two entities live together in a kind of metabolic wedlock. Methanogens are unique in that they can generate all the organic compounds they need, as well as all their energy, from nothing more than carbon dioxide and hydrogen. They do this by attaching hydrogen atoms (H) onto carbon dioxide (CO 2 ) to produce the basic building blocks needed to make carbohydrates like glucose (C 6 H 12 O 6 ), and from these they can construct the entire repertoire of nucleic acids, proteins, and lipids. They also use hydrogen and carbon dioxide to generate energy, releasing methane in the process.
While methanogens are uniquely resourceful in their metabolic powers, they nonetheless face a serious obstacle, and we have already noted the reason in Chapter 1 . The trouble is that, while carbon dioxide is plentiful, hydrogen is hard to come by in any environment containing oxygen, as hydrogen and oxygen react together to form water. From the point of view of a methanogen, then, anything that provides a little hydrogen is a blessing. Hydrogenosomes are a double boon, because they release both hydrogen gas and carbon dioxide, the very substances that methanogens crave, in the process of generating their own energy. Even more importantly, they don’t need oxygen to do this—quite the contrary, they prefer to avoid oxygen—and so they function in the very low-oxygen conditions required by methanogens. No wonder the methanogens suckle up to hydrogenosomes like greedy piglets! The insight of Martin and Müller was to appreciate that this kind of intimate metabolic union might have been the basis of the original eukaryotic merger.
Bill Martin argues that the hydrogenosomes and the mitochondria stand at opposite ends of a little-known spectrum. Rather surprisingly, to anyone who is most familiar with textbook mitochondria, many simple single-celled eukaryotes have mitochondria that operate in the absence of oxygen. Instead of using oxygen to burn up food, these ‘anaerobic’ mitochondria use other simple compounds like nitrate or nitrite. In most other respects, they operate in a very similar fashion to our own mitochondria, and are unquestionably related. So the spectrum stretches from aerobic mitochondria like our own, which are dependent on oxygen, through ‘anaerobic’ mitochondria, which prefer to use other molecules like nitrates, to the hydrogenosomes, which work rather differently but are still related. The existence of such a spectrum focuses attention on the identity of the ancestor that eventually gave rise to the entire spectrum. What, asks Martin, might this common ancestor have looked like?
3 The
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