The “Thiodendron” stage refers to an extant bacterial consortium that models our idea of an archaebacteria-eubacteria sulfur syntrophic motility symbiosis. The partners in our view merged to become the chimeric predecessor to archaeprotists. The membrane-bounded nucleus, by hypothesis, is the morphological manifestation of the chimera genetic system that evolved from a Thiodendron-type consortium. Each phenomenon we suggest, from free-living bacteria to integrated association, enjoys extant natural analogues.
Study of marine microbial mats revealed relevant bacterial consortia in more than six geographically separate locations. Isolations from Staraya Russa mineral spring 8, mineral spring Serebryani, Lake Nizhnee, mud-baths; littoral zone at the White Sea strait near Veliky Island, Gulf of Nilma; Pacific Ocean hydrothermal habitats at the Kurile Islands and Kraternaya Bay; Matupi Harbor Bay, Papua New Guinea, etc. (17) all yielded “Thiodendron latens” or very similar bacteria. Samples were taken from just below oxygen-sulfide interface in anoxic waters (17, 18). Laboratory work showed it necessary to abolish the genus Thiodendron because it is a sulfur syntrophy. A stable ectosymbiotic association of two bacterial types grows as an anaerobic consortium between 4 and 32°C at marine pH values and salinities. Starch, cellobiose, and other carbohydrates (not cellulose, amino acids, organic acids, or alcohol) supplemented by heterotrophic CO2 fixation provide it carbon. Thiodendron appears as bluish-white spherical gelatinous colonies, concentric in structure within a slimy matrix produced by the consortium bacteria. The dominant partner invariably is a distinctive strain of pleiomorphic spirochetes: they vary from the typical walled Spirochaeta 1:2:1 morphology to large membranous spheres, sulfur-studded threads, gliding or nonmotile cells of variable width (0.09–0.45 μm) and lengths to millimeters. The other partner, a small, morphologically stable vibrioid, Desulfobacter sp., requires organic carbon, primarily acetate, from spirochetal carbohydrate degradation. The spirochetal Escherichia coli-like formic acid fermentation generates energy and food. Desulfobacter sp. cells that reduce both sulfate and sulfur to sulfide are always present in the natural consortium but in far less abundance than the spirochetes. We envision the Thiodendron consortium of “free-living spirochetes in geochemical sulfur cycle” (ref. 18, p. 456) and spirochete motility symbioses (19) as preadaptations for chimera evolution. Thiodendron differs from the archaebacterium-eubacterium association we hypothesize; the marine Desulfobacter would have been replaced with a pleiomorphic wall-less, sulfuric-acid tolerant soil Thermoplasma-like archaebacterium. New thermoplasmas are under study. We predict strains that participate in spirochete consortia in less saline, more acidic, and higher temperature sulfurous habitats than Thiodendron will be found.
When “pure cultures” that survived low oxygen were first described [by B. V. Perfil'ev in 1969, in Russian (see refs. 17 and 18] a complex life history of vibrioids, spheroids, threads and helices was attributed to “Thiodendron latens”. We now know these morphologies are artifacts of environmental selection pressure: Dubinina et al. (ref. 17, 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/l 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 (20). 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) (21).
T. acidophilum in pure culture attach to suspended elemental sulfur. When sulfur is available, they generate hydrogen sulfide (16). Although severely hindered by ambient oxygen, they are microaerophilic in the presence of small quantities (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 (22). 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 (19). The swimmer's attachment structures hypertrophied as typically they do in extant motility symbioses (19). The archaebacterium-eubacterium swimmer attachment system became the karyomastigont. The proteinaceous karyomastigont that united partner DNA in a membrane-bounded, jointly produced package, assured stability to the chimera. All of the DNA of the former prokaryotes recombined inside the membrane to become nuclear DNA while the protein-based motility system of the eubacterium, from the moment of fusion until the present, segregated the chimeric DNA. During the lower Proterozoic eon (2,500–1,800 million years ago), many interactions inside the chimera generated protists in which mitosis and eventually meiotic sexuality evolved. The key concept here is that the karyomastigont, retained by amitochondriate protists and later by their mitochondriate descendants, is the morphological manifestation of the original archaebacterial-eubacterial fused genetic system. Free (unattached) nuclei evolved many times by disassociation from the rest of the karyomastigont. The karyomastigont, therefore, was the first microtubule-organizing center.