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artifacts of environmental selection pressure: Dubinina et al. (1993a, p. 435), reported that “the pattern of bacterial growth changes drastically when the redox potential of the medium is brought down by addition of 500 mg/1 of sodium sulfide.” The differential growth of the two tightly associated partners in the consortium imitates the purported Thiodendron bacterial developmental patterns. The syntrophy is maintained by lowering the level of oxygen enough for spirochete growth. The processes of sulfur oxidation-reduction and oxygen removal from oxygen-sensitive enzymes, we suggest, were internalized by the chimera and retained by their protist descendants as developmental cues.

Metabolic interaction, in particular syntrophy under anoxia, retained the integrated prokaryotes as emphasized by Martin and Müller (1998). However, we reject their concept, for which no evidence exists, that the archaebacterial partner was a methanogen. Our sulfur syntrophy idea, by contrast, is bolstered by observations that hydrogen sulfide is still generated in amitochondriate, anucleate eukaryotic cells (mammalian erythrocytes) (Searcy and Lee, 1998).

T. acidophilum in pure culture attach to suspended elemental sulfur. When sulfur is available, they generate hydrogen sulfide (Searcy and Hixon, 1994). Although severely hindered by ambient oxygen, they are microaerophilic in the presence of small quantities (<5%) of oxygen. The Thermoplasma partner thus would be expected to produce sulfide and scrub small quantities of oxygen to maintain low redox potential in the spirochete association. The syntrophic predecessors to the chimera is metabolically analogous to Thiodendron where Desulfobacter reduces sulfur and sulfate producing sulfide at levels that permit the spirochetes to grow. We simply suggest the replacement of the marine sulfidogen with Thermoplasma. In both the theoretical and actual case, the spirochetes would supply oxidized sulfur as terminal electron acceptor to the sulfidogen.

The DNA of the Thermoplasma-like archaebacterium permanently recombined with that of the eubacterial swimmer. A precedent exists for our suggestion that membrane hypertrophies around DNA to form a stable vesicle in some prokaryotes: the membrane-bounded nucleoid in the eubacterium Gemmata obscuriglobus (Fuerst and Webb, 1991). The joint Thermoplasma-like archaebacterial DNA package that began as the consortium nucleoid became the chimera's nucleus.

The two unlike prokaryotes together produced a persistent protein exudate package. This step in the origin of the nucleus—the genetic integration of the two-membered consortium to form the chimera—is traceable by its morphological legacy: the karyomastigont. The attached swimmer partner, precursor to mitotic microtubule system, belonged to genera like the nearly ubiquitous consortium-former Spirochaeta or the cytoplasmic tubule-maker Hollandina (Margulis, 1993). The swimmer's attachment



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