The Chimera: Archaebacterium/Eubacterium Merger
Study of conserved protein sequences [a far larger data set than that used by Woese et al. (3)] led Gupta (10) to conclude “all eukaryotic cells, including amitochondriate and aplastidic cells received major genetic contributions to the nuclear genome from both an archaebacterium (very probably of the eocyte, i.e., thermoacidophil group and a Gram-negative bacterium … [t]he ancestral eukaryotic cell never directly descended from archaebacteria but instead was a chimera formed by fusion and integration of the genomes of an archaebacerium and a Gram-negative bacterium” (p. 1487). The eubacterium ancestor has yet to be identified; Gupta rejects our spirochete hypothesis. In answer to which microbe provided the eubacterial contribution, he claims: “the sequence data …. suggest that the archaebacteria are polyphyletic and are close relatives of the Gram-positive bacteria” (p. 1485). The archaebacterial sequences, we posit, following Searcy (14), come from a Thermoplasma acidophilum-like thermoacidophilic (eocyte) prokaryote. This archaebacterial ancestor lived in warm, acidic, and sporadically sulfurous waters, where it used either elemental sulfur (generating H2S) or less than 5% oxygen (generating H2O) as terminal electron acceptor. As does its extant descendant, the ancient archaebacterium survived acid-hydrolysis environmental conditions by nucleosome-style histone-like protein coating of its DNA (14) and actin-like stress-protein synthesis (15). The wall-less archaebacterium was remarkably pleiomorphic; it tended into tight physical association with globules of elemental sulfur by use of its rudimentary cytoskeletal system (16). The second member of the consortium, an obligate anaerobe, required for growth the highly reduced conditions provided by sulfur and sulfate reduction to hydrogen sulfide. Degradation of carbohydrate (e.g., starch, sugars such as cellobiose) and oxidation of the sulfide to elemental sulfur by the eubacterium generated carbon-rich fermentation products and electron acceptors for the archaebacterium. When swimming eubacteria attached to the archaebacterium, the likelihood that the consortium efficiently reached its carbon sources was enhanced. This hypothetical consortium, before the integration to form a chimera (Fig. 1), differs little from the widespread and geochemically important “Thiodendron” (17, 18).