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Apart from sharing the basic cellular features of otherBacteria, the cyanobacteria possess unique and diagnostic characteristics that will be briefly described here, but less extensively than in the 1989 edition ofBergey's Manual of Systematic Bacteriology. The cell wall in cyanobacteria is of a Gram‐negative type. Cell division in most unicellular and colonial cyanobacteria and some filamentous forms is by binary fission. Regarding cell exterior and motility, fimbriae (or pili) occur abundantly with diverse patterns in many cyanobacteria. Although not all thylakoids in cyanobacteria appear to be invaginations of the cytoplasmic membrane, there are orderly “attachment points” or “thylakoid centers” associated with the periphery of the cytoplasm or the cytoplasmic membrane. In the cytoplasm of cyanobacteria there are many other components and “inclusions”, most of which can be visualized readily using various preparative techniques for transmission electron microscopy. They include glycogen granules, cyanophycin granules, carboxysomes (polyhedral bodies), polyphosphate (volutin) granules, and gas vacuoles. Heterocysts, akinetes, and hormogonia are some of the specialized cells and differentiation in Cyanobacteria. The chief physiological/biochemical characteristic of cyanobacteria, distinguishing them from all other procaryotes, is the dual photosystem that allows the use of H2O as photoreductant with the consequent liberation of O2. Two courses for the delimitation of genera have been followed by cyanobacterial specialists: (a) to retain “small” genera or (b) to gather many species into fewer genera.Cyanobacteria / General Characteristics of the Cyanobacteria

Abstract Cla.vi.chla.my'di.a. L. n. clava a knotty branch, rough stick, cudgel, club; N.L. fem. n. Chlamydia taxonomic name of a bacterial genus; N.L. fem. n. Clavichlamydia a club Chlamydia (the morphology of the bacteria includes the characteristic head‐and‐tail cells that have been described earlier from salmonid fish suffering from epitheliocystis). Pleomorphic or elongated , nonmotile , obligate intracellular bacteria , up to 2 μ m in length . Cells show a developmental cycle with morphologically distinct stages and thrive inside a host vacuole . Organisms were found within gill lesions of fish and are differentiated from all other chlamydiae by the morphology of the proposed elementary bodies , which show a characteristic head‐and‐tail form (Figure 1). Members have not yet been obtained in cell culture. Type species : “ Candidatus Clavichlamydia salmonicola ” corrig. Karlsen, Nylund, Watanabe, Helvik, Nylund and Plarre 2008. Taxonomic and Nomenclature Notes According to the List of Prokaryotic names with Standing in Nomenclature (LPSN), the taxonomic status of the genus Candidatus Clavichlamydia is: preferred name (not correct name) (last update, February 2025) * . LPSN classification: Bacteria / Pseudomonadati / Chlamydiota / Chlamydiia / Chlamydiales / Chlamydiaceae / Candidatus Clavichlamydia The genus Candidatus Clavichlamydia can also be recovered in the Genome Taxonomy Database (GTDB) as g__Clavichlamydia (version v220) ** . GTDB classification: d__Bacteria / p__Chlamydiota / c__Chlamydiia / o__Chlamydiales / f__Chlamydiaceae / g__Clavichlamydia * Meier‐Kolthoff et al. ( 2022 ). Nucleic Acids Res , 50 , D801 – D807 ; DOI: 10.1093/nar/gkab902 ** Parks et al. ( 2022 ). Nucleic Acids Res , 50 , D785 – D794 ; DOI: 10.1093/nar/gkab776

Abstract Xi.phi.ne.ma.to.bac'ter. Gr. neut. n. Xiphinema, ‐atos the genus name of the host organism; N.L. masc. n. bacter the equivalent of Gr. neut. n. baktron a rod; N.L. masc. n. Xiphinematobacter the rod‐shaped microbe associated with Xiphinema . Full‐grown cells are rod‐shaped with rounded ends, 0.7–1.0 × 2.1–3.2 μm ; however, cells in the J 1 (first juvenile) stage of nematode development have a wrinkled, pleomorphic shape. The longer entities usually consist of a mother cell from which a daughter cell is budding, giving rise to serial pairs typical of this bacterial genus. In thin sections cells have two or three membranes consisting of, from inside to outside, a cytoplasmic membrane, an electron‐dense outer membrane, and, in many individuals, a vacuolar membrane which is probably derived from the host cell membrane and which often shows discontinuities. No peptidoglycan layer is evident ; however, a periplasmic hexagonally arrayed monolayer of 10 nm protein units is sometimes present. Gram‐stain‐negative, nonmotile, and nonsporulating. DNA is often condensed at cell poles . Bacteria live as obligate cytoplasmic symbionts with maternal transmission in nematodes of the Xiphinema americanum group ( Nematoda, Longidoridae ), in which they are presumed to induce thelytokous ( mother‐to‐daughter ) parthenogenesis . DNA G + C content ( mol %): not determined. Type species : “ Candidatus Xiphinematobacter brevicolli ” Vandekerckhove, Willems, Gillis and Coomans 2000, 2203. Taxonomic and Nomenclature Notes According to the List of Prokaryotic names with Standing in Nomenclature (LPSN), the taxonomic status of the genus Candidatus Xiphinematobacter is: preferred name (not correct name) (last update, February 2025) * . LPSN classification: Bacteria / Pseudomonadati / Verrucomicrobiota / Terrimicrobiia / Terrimicrobiales / Chthoniobacteraceae / Candidatus Xiphinematobacter Candidatus Xiphinematobacter could not be recovered in GTDB ** . * Meier‐Kolthoff et al. ( 2022 ). Nucleic Acids Res , 50 , D801 – D807 ; DOI: 10.1093/nar/gkab902 ** Parks et al. ( 2022 ). Nucleic Acids Res , 50 , D785 – D794 ; DOI: 10.1093/nar/gkab776

Abstract Fri'tsche.a. N.L. fem. n. Fritschea named after Thomas R. Fritsche, an American physician and parasitologist, in honor of his contributions to our current knowledge on chlamydial diversity. Coccoid to rod‐shaped, nonmotile, obligately intracellular bacteria, up to 2.5 μm in length . Cells show a developmental cycle with morphologically distinct stages similar to those of the Chlamydiaceae . Organisms were found in whitefly and scale insects, but have not yet been obtained in cell culture. The genus is based on the description of “ Candidatus Fritschea bemisiae” and “ Candidatus Fritschea eriococci” (Everett et al., 2005; Thao et al., 2003) (Figure 1). Type species : “ Candidatus Fritschea bemisiae ” Everett, Thao, Horn, Dyszynski and Baumann 2005, 1585. Taxonomic and Nomenclature Notes According to the List of Prokaryotic names with Standing in Nomenclature (LPSN), the taxonomic status of the genus Candidatus Fritschea is: preferred name (not correct name) (last update, February 2025) * . LPSN classification: Bacteria / Pseudomonadati / Chlamydiota / Chlamydiia / Chlamydiales / Simkaniaceae / Candidatus Fritschea Candidatus Fritschea could not be recovered in GTDB ** . * Meier‐Kolthoff et al. ( 2022 ). Nucleic Acids Res , 50 , D801 – D807 ; DOI: 10.1093/nar/gkab902 ** Parks et al. ( 2022 ). Nucleic Acids Res , 50 , D785 – D794 ; DOI: 10.1093/nar/gkab776

Abstract Me.tha.no.bre.vi.bac'ter. M.L. neut. n. methanum methane; L. masc. adj. brevis short; M.L. masc. n. bacter equivalent of Gr. neut. n. bakterion rod, staff; M.L. masc. n. Methanobrevibacter short methane (‐producing) rod. Oval rods or cocci to short rods , usually occurring in pairs or chains; about 0.5–0.7 µm in width and 0.8–1.4 µm in length. Rarely, filaments are formed. Nonsporing, Gram‐positive. Cell walls are composed of pseudomurein . Nonmotile. Strict anaerobes. Optimum temperature, 37–40°C; maximum, ~45°C; minimum, ~30°C. The mol % G + C of the DNA is : 27.5–31.6. Type species : Methanobrevibacter ruminantium (Smith and Hungate 1958) Balch and Wolfe 1981, 216 (Effective publication: Balch and Wolfe in Balch, Fox, Magrum, Woese and Wolfe, 1979, 284) ( Methanobacterium ruminantium Smith and Hungate 1958, 717). Taxonomic and Nomenclature Notes According to the List of Prokaryotic names with Standing in Nomenclature (LPSN), the taxonomic status of the genus Methanobrevibacter is: correct name (last update, February 2025) * . LPSN classification: Archaea / Methanobacteriati / Methanobacteriota / Methanobacteria / Methanobacteriales / Methanobacteriaceae / Methanobrevibacter The genus Methanobrevibacter can also be recovered in the Genome Taxonomy Database (GTDB) as g__Methanobrevibacter (version v220) ** . GTDB classification: d__Archaea / p__Methanobacteriota / c__Methanobacteria / o__Methanobacteriales / f__Methanobacteriaceae / g__Methanobrevibacter * Meier‐Kolthoff et al. ( 2022 ). Nucleic Acids Res , 50 , D801 – D807 ; DOI: 10.1093/nar/gkab902 ** Parks et al. ( 2022 ). Nucleic Acids Res , 50 , D785 – D794 ; DOI: 10.1093/nar/gkab776

Abstract Mag.ne'to.bac.te'ri.um. Gr. n. magnes magnet, comb. form magneto‐; Gr. n. bakterion a small rod; M.L. neut. n. Magnetobacterium a magnetic rod. Rod‐shaped, large, magnetic cells occurring in freshwater sediments. The original description is based on bacteria enriched from the littoral sediment of a freshwater lake in Southern Germany (Chiemsee); similar types of bacteria were also observed in sediments of other freshwater lakes and ponds in Southern Germany and Brazil. Counts of active bacteria in different vertical layers of Chiemsee sediment indicated that “ Candidatus M. bavaricum” is a typical gradient organism, particularly adapted to zones characterized by low levels of oxygen, where it reaches the highest abundance (Spring et al., 1993). A minor fraction of these bacteria was also found in the anoxic zone, whereas high concentrations of oxygen can apparently not be tolerated by this bacterium over longer periods of time. Type species : “ Candidatus Magnetobacterium bavaricum ” Spring, Amann, Ludwig, Schleifer, van Gemerden and Petersen 1993, 2398. Taxonomic and Nomenclature Notes According to the List of Prokaryotic names with Standing in Nomenclature (LPSN), the taxonomic status of the genus Candidatus Magnetobacterium is: synonym (and no standing) (last update, February 2025) * . LPSN classification: Bacteria / Pseudomonadati / Nitrospirota / Nitrospiria / Nitrospirales / Nitrospiraceae / Candidatus Magnetobacterium The genus Candidatus Magnetobacterium can also be recovered in the Genome Taxonomy Database (GTDB) as g__Magnetobacterium (version v220) ** . GTDB classification: d__Bacteria / p__Nitrospirota / c__Thermodesulfovibrionia / o__Thermodesulfovibrionales / f__Magnetobacteriaceae / g__Magnetobacterium * Meier‐Kolthoff et al. ( 2022 ). Nucleic Acids Res , 50 , D801 – D807 ; DOI: 10.1093/nar/gkab902 ** Parks et al. ( 2022 ). Nucleic Acids Res , 50 , D785 – D794 ; DOI: 10.1093/nar/gkab776