Epulopiscium fishelsoni

Epulopiscium fishelsoni ( Epulopiscium: guest at the banquet of a fish, fishelsoni after the discoverer ) is an unusually large gram-positive bacterium. It lives symbiotically in the gut of surgeonfish, where it helps in the breakdown of food, which has included the fish. The species was discovered in 1985, with lengths of up to 0.6 millimeters to the discovery of Thiomargarita namibiensis in 1999 as the largest known bacterium. However, there are in addition also cells which are only 30 microns in size, so that it comes to 2000 times the volume differences between the large and small representatives of the species.

Meanwhile some similar cells have been discovered in other surgeonfish. The species boundaries are not clear at this stage of research. Therefore, it is in part from Epulopiscium spp. or morphotypes spoken.

Physiology

Epulopiscium fishelsoni has a unique anatomy, the specifics of which are mostly used to overcome traditional limitations of the size of a bacterium. The reasons for the enormous size are unknown. One possible cause is the protection against bacteria -eating protists.

The cell wall is often folded so as to maximize the surface. It has a cortex with tubules, vesicles and other structures that are normally found only in eukaryotes. Perhaps these structures serve the intracellular transport. This Epulopiscium would fishelsoni an unusual example of convergent evolution in dimensions of individual cells.

Epulopiscium fishelsoni Type B lives in the intestinal tract of the nose doctor fish Naso tonganus and is 200-300 microns long and 50-60 microns wide. For this type has been reported that he is highly polyploid and carries up to 200,000 copies of its genome in itself, depending on the cell size. There was here a ratio of one copy of the genome to 1.9 μm3 cytoplasm. In comparison with Bacillus subtilis, a normal-sized bacterium with a genome of 0.7 μm3 cytoplasm so the genome - cytoplasm ratio is of a similar magnitude. The DNA of Epulopiscium fishelsoni type B was found near the cell surface. The study authors speculate that this rapid response to äußerere, local stimuli is possible, as if the genomes lie in the center of the cell. Even the giant bacterium Thiomargarita namibiensis the DNA found at the edge of the cell. Here the metabolic active cytoplasm is limited by a large central vacuole, which occupies 98 % of the cell volume, on a narrow margin, while in the central cytoplasm Epulopiscium fishelsoni apparently is active.

Through the cell size and the large number of genome copies to Epulopiscium has appropriated some advantages of eukaryotic cells.

Propagation

One of the most unusual aspects of Epulopiscium fishelsoni is its reproductive behavior. Unlike most bacteria, which reproduce asexually by cell division to Epulopiscium reproduced in a manner which is believed to have originated from the sporulation: usually two, in one morphotype but also up to twelve daughter cells grow in the mother cell until it dissolves and the new individuals are free.

Although sporulation among bacteria is not uncommon (see Bacillus subtilis ), it is usually more a means in poor or stressful situations than the usual way of propagation. The daughter cells are usually dormant stages, whereas the released cells from Epulopiscium are already active. Similar Reproduction ways to go other symbiotic intestinal bacteria such as Metabacterium polyspora that are phylogenetically close Epulopiscium. Maybe sporulation allows protection of individuals daughter from the harsh conditions of the digestive system.

In E. fishelsoni -like endosymbionts also proliferation was found by division or by mixed forms.

Ecology

From the bowels of the most surgeonfish species, different strains were isolated from Epulopiscium, a pure culture did not succeed so far though.

The relationship of surgeon fish and bacteria to each other is classified as mutualism. The exact nature of the relationship is still unknown, but is believed to be safe, that Epulopiscium fishelsoni the fish helps to obtain nutrients from eaten algae and detritus from.

The daily cycle of Epulopiscium fishelsoni runs synchronized with the daily activities of its host. During the day, picks up while the doctor fish algae, hiking the rounded, compact nucleoids at the ends of the cell and begin to stretch. If they have expanded in the late afternoon / early evening to around 50 to 75 % of the length of the cell, sporulation begins.

History and systematics

In 1985, the discovery of a large, cigar-shaped, initially unnamed protozoan was published by Israeli scientists Lev Fish Elson of the University of Tel Aviv and colleagues, whom they found in the intestines of Gold Polka Dot surgeonfish ( Acanthurus nigrofuscus ) from the Red Sea. He could reach a size of up to 0.5 mm, about three times the length of a paramecium, over 85 % of the cells, however, were less than 0.2 mm. They stopped him because of his size for a protists, but they could be assigned to any known Protistengruppe. Other reasons for the classification were as protist hair -like structures at the cell surface, and a structure inside the cells, which was maintained for a cell nucleus. The single-celled organisms came with a density 20000-100000 cells per milliliter before the gut of this herbivorous fish doctor. The authors considered him a symbiotic with the fish organism. For this assumption saying that they could not find him in any of several hundred specimens of Acanthurus nigrofuscus. Also in Acanthurus sohal he was discovered, but not in several other related species.

The naming Epulopiscium fishelsoni was in a publication in 1988.

A rRNA analysis Pace 1993 showed that it is a bacterium. While there could be allocated in the following order of Clostridiales, but its exact systematic position is still unclear. The genus name refers to the symbiotic lifestyle and means as much as "guest at the banquet of a fish "; the specific epithet honors the discoverer Lev Fish Elson.

In a study of the intestinal flora of reef fish in the Pacific a number of similar single-celled organisms were found in surgeonfish. They were classified according to their size and their Reproduktonsverhaltens. Large cells similar Acanthurus lineatus were Epulopiscium fishelsoni for example again found in Gold Polka Dot surgeonfish A. nigrofuscus and blue striped surgeonfish. They are referred to as A- morphotype. In the meantime, are known over ten morphotypes. Since the assignment to species is still unclear, they were the alternative, as, Epulos referred in their entirety '.