Phylum Planctomycetes (Planctomyces & relatives)

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  • Phylum Planctomycetes
    • Class Planctomycetacia
      • Family Planctomycetales
        • Genus Planctomyces
        • Genus Gemmata
        • Genus Isosphaera
        • Genus Pirellula
        • Genus Blastopirellula
        • Genus Rhodopirellula
        • Genus Pirella
        • Genus Singulisphaera
        • Genus Nostocoida
    • Incertae sedis
      • Genus Brocadia
      • Genus Kuenenia

About this phylum

Diversity

The diversity of the Planctomycetes is unclear; although they are morphologically conspicuous, because they divide by budding, are stalked and often form spectacular rosettes, they are rarely isolated in pure culture for study. The Phylum is distantly but specifically related to the Chlamydiae and Verrucomicrobia.

Metabolism

Cultivated Planctomycetes are all aerobic mesophilic oligotrophic heterotrophs, except Isosphaera pallida, which is moderately thermophilic (up to 55°C), and Brocadia anammoxidans and Kuenenia stuttgartiensis, which carry out the anaerobic ammonia oxidation. The heterotrophs are capable of growth on a wide range of sugars and sugar derivatives, including polysaccharides. All lack peptidoglycan, and are resistant to β-lactam antibiotics such as ampicillin; this trait can be useful in enrichment cultures and isolations.

Morphology

Typically these organisms are cocci or oval-shaped. Most have stalks, but these can be too thin or short to be apparent by light microscopy. These stacks are external fibrous structures, unlike the cytoplasmic extensions of appendaged Bacteria. Stalked forms often form planktonic rosettes. Fimbrae are common, and newly-released buds are often flagellated and highly motile. They lack the peptidoglycen cell wall, but have an external pitted wall of unknown composition. The nucleoids of Planctomycetes are quite distinct and condensed.

P.bekeffi P.bekefii
Planctomyces bekefii
, showing the external fibrous stalk, fimbriae, crateriform pits, etc. : from The Prokaryotes, pp3711, 3712

All of the members of the Planctomycetes contain internal membrane-defined compartmentalization. The cytoplasm is divided into the riboplasm, containing ribosomes and DNA, and the paryphoplasm, that contains RNA (of unknown type or function) but not ribosomes. These compartments are separated by a membrane; the internal compartment contains the riboplasm and nucleoid and is termed the pirellulosome. In Gemmata, the nucleoid is separated from most of the riboplasm by an additional double-membrane ‘nuclear’ envelope. This "nucleus" is very different than those of eukaryotes, however, in that it contains apparently functional ribosomes. In the Brocadia (and presumably Kuenena), there is an additional membrane separating the ‘anammoxisomes’ from the rest of the cell. This membrane has an unusual lipid composition specially designed to keep the toxic intermediates of this reaction contained.

Habitat

Most Planctomycetes are aquatic, and appear most commonly in eutrophic environments. However, Isosphaera is found in the phototrophic mats of hot springs (35°C-55°C), Brocadia and Kuenenia are found in anaerobic waste digesters, and ssu-rRNA sequences from this group are isolated from a wide range of environments.

Example : Blastopirellula marina

B.marina B.marina
Blastopirellula marina : Margaret R. Lindsay, et al Arch Microbiol (2001) 175:413–429 , & The Prokaryotes, pp3720

Blastopirellula (previously Pirellula) marina is a relatively common freshwater specie, oval with short stalks and forms small, flower-like rosettes (as compared with the spectacular ‘fireworks’ rosettes of Planctomyces species). Blastopirellula has the simplest form of compartmentalization of the Planctomycetes. The intracytoplasmic membrane (ICM) is a simple ovoid separating the riboplasm, containing both the ribosomes and the nucleoid, from the paryphoplasma. The paryphoplasma is primarily at one end of the cell, i.e. it is polar.

Example : Isosphaera pallida

Isosphaera Isosphaera
Isosphaera pallida : Margaret R. Lindsay, Arch Microbiol (2001) 175:413–42, & The Prokaryotes, pp3727

Isosphaera pallida is an unusual member of this group phenotypically; individual cells are cocci, but these form filamentous chains that contain gas vacuoles and are motile by gliding. Budding occurs along the axis of the chain, and so daughter cells are formed interstitially. Isolated from hot spring phototrophic mats, it was originally mistaken for a cyanobacterium. I. pallida has a cell structure very similar to that of Planctomyces, with a single membrane (ICM) separating the cytoplasm into paryphoplasm and riboplasm. The paryphoplasm is highly polar, excluded mostly to one end of the cell as a sort of vesicle, but it still forms a thin layer between the cytoplasmic membrane and the ICM all the way around the cell.

Example : Brocadia anammoxidans

Brocadia Brocadia
Brocadia anammoxidans : Margaret R. Lindsay, Arch Microbiol (2001) 175:413–429, & Marc Strous (email), of the Radboud University Nijmegen, Department of Microbiology, the Netherlands.

Brocadia anammoxidans is phenotypically very different than the other Planctomycetes; it is an anaerobic autotroph, gaining energy by the production of dinitrogen gas by the reaction of ammonia and nitrite; this is known as anaerobic ammonia oxidation, or the ‘anammox’ reaction. Carbon fixation is by the acetyl-CoA pathway. B. anammoxidans has a bit more internal complexity than most Planctomycetes, but is based on the same cell structure as the previous examples. The paryphoplasm is a relatively thin layer all around the cell (not polarized), and there is an additional membrane-bound structure, the anammoxisome. This is a critical aspect of the anammox reaction, which includes a very highly reactive intermediate, hydrazine (a.k.a. rocket fuel). Note that the anammoxisome membrane is not attached to or part of the ICM, nor is the ICM attached to or part of the cytoplasmic membrane (this is true of all of the planctomycetes).

Example : Gemmata obscuriglobus

Gemmata Gemmata

Gemmata obscuriglobus : Margaret R. Lindsay Arch Microbiol (2001) 175:413–429

Gemmata obscuriglobus spherical or ovoid non-stalked specie. Previously thought to have a large indentation in the surface of cell, seen in many electron microscopic images, this seems to be an artifact of dehydration in preparation for microscopy. Gemmata is the most complex Planctomycete in internal structure. As in Brocadia anammoxidans (above), the paryphoplasm forms a relatively thin layer all around the cell, between the CM and the ICM. In Gemmata, however, there is an additional double-layered membrane within the riboplasm surrounding the nucleoid. This ‘nuclear envelop’ is studded on both sides with ribosomes, and continuous with the cell membrane. Openings in this membrane allows movement of riboplasm contents between two compartments.

What is the difference between paryphoplasm and periplasm?

A substantial issue with these cellular structures in Planctomycetes is distinguishing the paryphoplasm from typical Gram-negative periplasm. These organisms have a cell wall outside the cell membrane, but it is not a peptioglycan (what it is, is not known). They do not have a membrane outside of the cell wall; they lack the hallmark Gram-negative outer membrane. Or do they? What what’s being called the ICM (internal cellular membrane) is really the cytoplasmic membrane, and what’s being called the CM (cytoplasmic membrane) is really the outer membrane? In this case, the riboplasm becomes traditional cytoplasm, and the paryphoplasm becomes periplasm (sometimes pretty substantial, as in Thermotoga). The difference, then, with Planctomycetes would be that they lost the peptidoglacan cell wall, and reinvented it outside of the outer membrane, much like the S-layer of many Bacteria and Archaea.

However, even if this is the case (and the authorities on Planctomycetes argue it is not) that doesn't change the most interesting observation, that Gemmata has a sort of nucleus with a nuclear envelop. Yes, it has ribosomes in it, but maybe eukaryotic nuclei do, too (involved in nonsense-mediated decay), and surely there is some sort of functional differentiation between riboplasm inside and outside the nuclear envelop. Just for example, the ribosomes translating outside of the "nucleus" cannot be translating mRNAs that are still being transcribed; this lack of linkage between transcription and translation is usually cited as an important distinction between ‘prokaryotes’ and eukaryotes.