Bonjour et joyeuses fêtes à tous,
Je voudrais revenir sur cette assertion de Svenn :
En plus des commentaires que j'avais déjà écrit, je tiens à recopier un passage d'une publication de William Martin au sujet du rôle présumé de la capacité à utiliser l'oxygène comme accepteur d'électrons dans la démultiplications des capacités métaboliques (ce qui d'après Bill Martin est faux) :
Early evolution without a tree of life
Cordialement.The lack of true intermediates in the prokaryote to eukaryote transition has long been a puzzle, but now it is clear that it has a bioenergetic cause, because prokaryote genome complexity is constrained by membrane bioenergetics [147]. Mitochondria released that constraint and allowed a growth in genome size and complexity and in the number of proteins that a cell can express by four to five orders of magnitude; energy per gene is the key variable [147,148]. Prokaryotes can readily surpass many eukaryotes in terms of cell size [149], but they cannot hold a candle to eukaryotes in terms of true cell complexity. The reason is bioenergetic and boils down to mitochondria, which are the prerequisite to the origin of cell complexity – hence phagocytosis – in eukaryotes [147], not vice versa. On a good day, that would to put an end to a time-consuming debate regardig the nature of the host that acquired the mitochondrion: prokaryote or phagotroph, the recent history of which was readably summarized by O’Malley [150].
And what about oxygen in eukaryote evolution? Mitochondria afford eukaryotes 10,000 to 100,000 more energy per gene [147], while oxygen affords only a factor of 10-20 [147]. Nonetheless people still seem to think that the advent of oxygen was a decisive event in eukaryote evolution. We all know that O2 levels limited animal (but not plant) body size during evolution [151,152]. Under the view of oxygen in Earth history that was current in the 1980s, the rise of atmospheric O2 some 2.3 Ga ago was thought to coincide with, and to have provided causal impetus to, the origin of eukaryotes and mitochondria [153]. But since the mid-1990’s, a fundamentally different view of Proterozoic ocean chemistry has emerged from isotope geochemistry. In the new and current model of Proterozoic ocean chemistry, the O2 that started accumulating in the atmosphere ~2.3 billion years ago began oxidizing continental sulfide deposits via weathering [154], carrying very substantial amounts of sulfate into the oceans, and providing the substrate required for sulfate reducing prokaryotes, hence fueling marine biological sulfate reduction (BSR). Marine BSR became a globally significant process, as evidenced by the sedimentary sulfur isotope record [155]. Marine BSR produces marine sulfide – H2S – and lots of it. The presence of that sulfide means that the oceans were not only sulfidic during that time, they were also anoxic, both for chemical reasons and because sulfate reducers are strict anaerobes. Although the photic zone (the upper 200 meters or so) was producing oxygen during that time, in the lower photic zone and below, the oceans were anoxic and sulfidic [156]. Importantly, that anoxic and sulfidic condition persisted until ~580 MY ago, the same time when the first animal macrofossils appear in the geological record, with oxygenation of the oceans allowing preexisting and diversified animal lineages to increase in size [157,158].
As such, eukaryotes, which arose some 1.5 Ga ago [159], diversified into their major lineages during anoxic and sulfidic times. It should therefore hardly be surprising (one would think) that diverse eukaryotic lineages have retained oxygen-independent forms of mitochondrial ATP synthesis from their facultatively anaerobic common ancestor [127,128,129,160,161,162] or that mitochondrial sulfide utilization is widespread among eukaryotic lineages [163,164].
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