Planctomycetes – a phylum of emerging interest for microbial evolution and ecology John A. Fuerst Department of Microbiology and Parasitology, University of Queensland, Brisbane, Queensland 4072, Australia j.fuerst@mailbox.uq.edu.au Planctomycetes are a group of budding, peptidoglycan-less bacteria of increasing significance for microbial evolution, ecology, cell biology and genomics. Studies of both cultured isolates and clone library sequences from natural communities have enriched this significance. Their display of unusual distinctive features such as compartmentalized cell organization, ability of some species to grow anaerobically and autotrophically via oxidation of ammonium, and the possession of large genomes combined with their wide distribution in a variety of habitats reinforces an increasing interest in them. 1) Introduction to the Planctomycetes Planctomycetes are an example of one of several groups of prokaryotes the true significance of which for microbiology, and for ecology and biology as a whole, is becoming recognized due to insights from the application of molecular sequencing and phylogenetics in combination with microbial ecology, modern electron microscopy preparative methods and chemotaxonomy. Such groups also include the verrucomicrobia (Hedlund et al., 1997); (Janssen et al., 2002), the acidobacteria (Hugenholtz et al., 1998) (Liles et al., 2003) and TM7 phyla (Hugenholtz et al., 2001) among the Bacteria, and the Korarcheota and mesophilic crenarcheotes among the Archaea (DeLong, 1998a; DeLong, 1998b). However, the riches of knowledge awaiting the deep study of bacterial diversity are being exemplified nowhere better than by our increasing understanding of the potential importance of the planctomycetes. This is an unusual yet deceptively non-‘extreme’ group of bacteria, like actinomycetes initially mistaken for fungi (Starr & Schmidt, 1989), and resembling Archaea in their possessing protein cell walls, but brought back into the Bacterial fold with the application of electron microscopy, 16S rRNA phylogenetics and the determination of a bacteria-like reaction to diphtheria toxin (Stackebrandt et al., 1984; Starr & Schmidt, 1989). Many of the early observations and species designations were based on natural microbial communities or enrichments (Starr & Schmidt, 1989), and even now some of those species such as the rosette-forming Planctomyces bekefii (type species of the genus) remain uncultured, and enrichments such as bioreactor cultures remain an important contributor to our knowledge of new planctomycetes. The planctomycetes, organisms within the order Planctomycetales, are members of the distinct phylum Planctomycetes (also known as a ‘division’) of Domain Bacteria (Garrity et al., 2003), a phylum which represents a deep-branching group within the Bacteria on the basis of 16S rRNA sequence phylogenetics (Schlesner & Stackebrandt, 1986); (Fuerst, 1995; Van De Peer et al., 1994). A recent important phylogenetic study applying an alignment of only slowly evolving positions to tree generation suggests that this division may be the deepest branching among the Bacteria, rather than hyperthermophiles like the Aquificales (Brochier & Philippe, 2002) though there is controversy about this conclusion (Di Giulio, 2003). Planctomycetes are distinctive for their peptidoglycan-less cell walls and budding reproduction and other cell organization features of great evolutionary significance discussed below. These bacteria have been identified in diverse freshwater, marine and soil habitats and even invertebrate animals (Fuerst, 1995; Fuerst et al., WFCC (World Federation of Culture Collections) Newsletter 38 (January 2004), pp. 1-11