The Order Planctomycetales, Including the Genera Planctomyces, Pirellula, Gemmata and Isosphaera and the Candidatus Gene
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The Order Planctomycetales, Including the Genera Planctomyces, Pirellula, Gemmata and Isosphaera and the Candidatus Genera Brocadia, Kuenenia and Scalindua NAOMI WARD, JAMES T. STALEY, JOHN A. FUERST, STEPHEN GIOVANNONI, HEINZ SCHLESNER AND EIKO STACKEBRANDT
Introduction The order Planctomycetales comprises a remarkable group of budding bacteria. They and their nearest relatives, the chlamydiae (Weisburg et al., 1986; see The Genus Chlamydia–Medical in this Volume), are the only known cell-wall containing bacteria that lack peptidoglycan. Furthermore, the planctomycetes are morphologically distinctive because of their budding division, their spherical to ovoid cells with crateriform pits (Figs. 1 and 2), and the nonprosthecate appendages (stalks) produced by some members of the group (Fig. 3). Multicellular aggregates or rosettes are formed by some species that produce polar holdfasts (Fig. 3). One genus, Isosphaera, is a multicellular filamentous bacterium that moves by gliding. Other motile members of the group produce flagella. A relatively recent addition to the morphological oddity of the planctomycetes is the discovery of cellular compartmentalization, posing a challenge to the traditionally held view of the prokaryote:eukaryote dichotomy (Fuerst and Webb, 1991; Lindsay et al., 1997; Lindsay et al., 2001). Knowledge of the order is limited owing to the relatively few species that have been obtained in pure culture and characterized. However, through the application of molecular microbial ecology techniques over the last 10 years, it has become apparent that planctomycetes are ubiquitous in a wide range of terrestrial and aquatic environments; the physiological diversity underlying this geographic ubiquity has not yet been fully explored. The availability of genome sequence data should provide a valuable resource for the future investigation of planctomycete biology and promises to reveal previously unknown aspects of these unique organisms.
General Planctomycete Phylogeny The planctomycetes have a typical and unique ribosomal RNA composition. Early analysis of
their 16S rRNA by oligonucleotide cataloging and sequence analyses originally placed this order as a deep branch within the Bacteria (Stackebrandt et al., 1984) and/or as a group that is undergoing rapid evolution (Woese, 1987; Liesack et al., 1992b). Thus, the planctomycetes differed markedly from the heterotrophic budding and prosthecate bacteria that fall within the alpha-proteobacteria. This was supported by studies of their 5S rRNA (Bomar et al., 1988), which indicated that its length is significantly shorter than that of most bacteria, ranging from 109 to 111 nucleotides rather than the “minimal” length of 118 bases of other bacteria and archaea (Erdmann and Wolters, 1986). In addition, position 66 lacks an insertion, and numerous t
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