8.10B: Planctomycetes
- Describe the characteristics associated with Planctomycetes
Planctomycetes are a phylum of aquatic bacteria. They are found in samples of brackish, marine, and fresh water. They reproduce by budding. In structure, the organisms of this group are ovoid and have a holdfast, called the stalk, at the non-reproductive end that helps them to attach to each other during budding.
The organisms belonging to this group lack murein in their cell wall. Murein is an important heteropolymer present in most bacterial cell walls that serves as a protective component in the cell wall skeleton. Instead, their walls are made up of glycoprotein rich in glutamate. Planctomycetes have internal structures that are more complex than typically expected in prokaryotes. While they do not have a nucleus in the eukaryotic sense, the nuclear material can sometimes be enclosed in a double membrane. In addition to this nucleoid, there are two other membrane-separated compartments; the pirellulosome or riboplasm, which contains the ribosome and related proteins, and the ribosome-free paryphoplasm.
Cavalier-Smith has postulated that the Planctomycetes are within the clade Planctobacteria in the larger clade Gracilicutes. RNA sequencing shows that the planctomycetes are related to the Verrucomicrobia and possibly the Chlamydiae. A number of essential pathways are not organized 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. 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.
Key Points
- In structure, the organisms of this group are ovoid and have a holdfast, called the stalk, at the non-reproductive end that helps them to attach to each other during budding.
- The organisms belonging to this group have a glycoprotein rich in glutamate instead of murein in their cell wall.
- The nuclear material in planctomycetes can sometimes be enclosed in a double membrane.
Key Terms
- nucleoid : The irregularly-shaped region within a prokaryote cell where the genetic material is localized.
- operon : A unit of genetic material that functions in a coordinated manner by means of an operator, a promoter, and structural genes that are transcribed together.
- planctomycetes : A phylum of aquatic bacteria that are found in samples of brackish, and marine and fresh water.
- budding : a form of asexual reproduction in which a new organism develops from an outgrowth or bud on another one