Planctomycetes are a phylum of aquatic bacteria and are found in samples of brackish, and marine and fresh water. They reproduce by budding. In structure, the organisms of this group are ovoid and have a holdfast, at the tip of a thin cylindrical extension from the cell body called the stalk, at the nonreproductive end that helps them to attach to each other during budding.
Cavalier-Smith has postulated that the Planctomycetes are within the clade Planctobacteria in the larger clade Gracilicutes, but this is not generally accepted.
Structure
The bacteria belonging to this group lack peptidoglycan, (also called murein) in their cell walls, which is an important heteropolymer present in most bacterial cell walls that serves as a protective component. Instead their walls are made up of glycoprotein which is rich in glutamate.
Planctomycetes have a distinctive morphology with the appearance of membrane-bound internal compartments, often referred to as the paryphoplasm (ribosome-free space), pirellulosome (ribosome-containing space) and nucleoid (condensed nucleic acid region, in these species surrounded by a double membrane). Until the discovery of the Poribacteria, planctomycetes were the only bacteria known with these apparent internal compartments. Three-dimensional electron tomography reconstruction of a representative species, Gemmata obscuriglobus, has yielded varying interpretations of this observation. One 2013 study found the appearance of internal compartments to be due to a densely invaginated but continuous single membrane, concluding that only the two compartments typical of Gram-negative bacteria - the cytoplasm and periplasm - are present. However, the excess membrane triples the surface area of the cell relative to its volume, which may be related to Gemmata's sterol biosynthesis abilities. A 2014 study using similar methods reported confirmation of the earlier enclosed compartment hypothesis.
It has recently been shown that Gemmata obscuriglobus is able to take in large molecules via a process which in some ways resembles endocytosis, the process used by eukaryotic cells to engulf external items.
Genome
RNA sequencing shows that the planctomycetes are related to the Verrucomicrobia and possibly the Chlamydiae. A number of essential pathways are not organised as operons, which is unusual for bacteria. A number of genes have been found (through sequence comparisons) that are similar to genes found in eukaryotes. One such example is a gene sequence (in Gemmata obscuriglobus) that was found to have significant homology to the integrin alpha-V, a protein that is important in transmembrane signal transduction in eukaryotes.
Life cycle
The life cycle of many planctomycetes involves alternation between sessile cells and flagellated swarmer cells. The sessile cells bud to form the flagellated swarmer cells which swim for a while before settling down to attach and begin reproduction.
Phylogeny
The currently accepted taxonomy is based on the List of Prokaryotic names with Standing in Nomenclature (LSPN) and the phylogeny is based on 16S rRNA-based LTP release 111 by The All-Species Living Tree Project
Notes:
â™ Strains found at the National Center for Biotechnology Information (NCBI) but not listed in the List of Prokaryotic names with Standing in Nomenclature (LSPN)
♪ Prokaryotes where no pure (axenic) cultures are isolated or available, i. e. not cultivated or can not be sustained in culture for more than a few serial passages
References
External links
- Beyond Prokaryotes and Eukaryotes : Planctomycetes and Cell Organization.
- Gemmata at Microbewiki
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