Neocallimastigomycota: Characteristics, Taxonomy, Nutrition

Neocallimastigomycotas  is a division of obligate endosymbiotic fungi in the digestive tract of ruminant and non-ruminant herbivorous mammals, as well as herbivorous reptiles. They can be unicellular or multicellular and have flagellate spores (zoospores).

Until recently they were considered an order within the phylum Chytridiomicota, but in 2007 the group was elevated to the phylum category. It is currently divided into 8 genera and about 20 species have been described.

Neocallimastigomycotas species develop under anaerobic conditions for which they possess specialized organelles called hydrogensomes. These organelles fulfill functions similar to mitochondria in organisms that live under aerobic conditions.

During their life cycle they form zoospores that adhere to plant material. Later these encyst and germinate. As they develop, they form sporangia that will give rise to new zoospores.

This group of fungi plays an important role in the complex ecology of the digestive system of herbivores. In addition, they produce potentially useful enzymes in biotechnology that have been used as digestives in food formulas for animals.

General characteristics

Neocallimastigomycotas are obligate endosymbiotic organisms, that is, they are not found in free life, but always associated with the digestive tract of herbivorous animals. They are uni to multicellular fungi, with a cell wall.

They produce vegetative thalli that develop sporangia, from which zoospores with one or more flagella originate. These zoospores located in the rumen of herbivores were initially classified as protozoa.

Zoospores are considered uniflagellate when 90% of the spores have a single flagella, and the remaining 10% have two to four flagella. The multiflagellate groups present zoospores with more than four flagella, and in some species up to 17 flagella have been observed.

Known predators of Neocallimastigomycotas, such as protozoa, attack zoospores and produce enzymes that degrade the cell walls of the fungus.

Metabolism and biological impact

Some adaptations of interest that these fungi present is that they develop in an anaerobic environment. They do not present mitochondria, cytochromes and some biochemical characteristics typical of the oxidative phosphorylation cycle.

Instead, they have specialized organelles similar to mitochondria called hydrogensomes, which produce cellular energy from glucose metabolism without the need for oxygen.

The hydrogenase contained in hydrogensomes produces hydrogen, CO2, formate and acetate, as metabolic waste. These compounds, together with lactate and ethanol, are the main end products of fermentation .

They are produced from the degradation and anaerobic fungal fermentation of polysaccharides of the plant cell wall.

Biotechnological applications

The ability of Neocallimastigomycota to degrade plant fibers assigns them a relevant biological role in the nutrition of many herbivores, mainly ruminants.

In this sense, it has been experimented to add anaerobic mushroom supplements to the diet, with very good results.

For non-ruminant herbivores, such as chickens, the supply of the fungus is not effective. This possibly due to its inability to survive in the digestive tract of these types of animals.

However, adding directly the enzymes produced by Neocallimastigomycotas in their food supplements has been successful.

The biochemical capabilities of Neocallimastigomycotas also make them potentially useful in biotechnology for the conversion of lignocellulose into bioenergy products.

Phylogeny and taxonomy

Neocallimastigomycotas were originally classified as Chytridiomicotas. Subsequently, taking into account morphological, ecological and ultrastructural characters, they were given the rank of edge.

Around 8 genera and 20 species of Neocallimastigomycotas are known, although numerous isolates have not yet been classified.

Genders

Anaeromyces , Neocallimastix , Orpinomyces and Piromyces , have a fibrous branched rhizoidal thallus with sporangia. In Anaeromyces the thallus is polycentric (numerous sporangia) with uniflagellate zoospores.

Neocallimastix is monocentric (a single sporangium) with multiflagellate zoospores. Orpinomyces has polycentric thallus and multiflagellate zoospores. Piromyces has a monocentric thallus with uniflagellate zoospores.

Two genera present thalluses composed of bulbous vegetative cells (bulbous mycelium) and sporangia: Caecomyces and Cyllamyces .

They differ because Caecomyces have one or very few zoosporangia, growing directly on the bulbous cells or at the end of a simple sporangiophore. Cyllamyces produce multiple sporangia on branched sporangiophores.

Two new genera are proposed ( Oontomyces and Buwchfawromyces ) based on molecular information.

Nutrition

These fungi degrade cellulose and hemicellulose present in plant cells that are consumed by the herbivores that host them.

They play an important role in the digestive process of these species due to the production of cellulolytic, hemicellulolytic, glycolytic and proteolytic enzymes, which break down plant tissues.

Habitat

Neocallimastigomycota are not free-living. They are located in the anaerobic environment of the rumen, hindgut and faeces of ruminant or non-ruminant herbivores.

Host species

They can be found mainly in ruminant mammals, both domesticated (sheep, goats, cows and horses) and wild (yak, zebras, giraffes, gazelles, monkeys, elephants, rhinos, hippos, bison, llamas, kangaroos among others). They also inhabit herbivorous reptiles, such as the green iguana.

Reproduction

Colonization of plant material

The plant material that enters the rumen is colonized by flagellated zoospores that are released from sporangia.

Zoospores bind to plant material in chemotactically oriented directions (following the chemical signals of sugars released by plant debris). Then they shed their flagella and encyst.

Germination and tissue penetration

Germination occurs through the emission of a germ tube from the opposite end to where the flagellum was.

Cyst development is different in monocentric and polycentric species. In monocentric species the nuclei remain in the cyst and enucleated rhizoids are generated (without nuclei). The cyst grows and forms a single sporangium (endogenous development).

In polycentric species, nucleated rhizoids are produced that generate several sporangia (exogenous development).

The cysts produce rhizomycelia that grow, and penetrate deep into plant tissues. These secrete enzymes that digest plant tissue and obtain the nutrients required to produce sporangia.

When the sporangium matures, it releases from one to 80 zoospores. Fungi mainly colonize vascular tissues of plants and the more fibrous the diet, the larger the fungal population.

It is considered that Neocallimastigomycotas infect host animals via the air, through resistance structures.

References

  1. Bauchop T. (1981). The anaerobic fungi in rumen fiber digestion. Agriculture and Environment 6 (2-3): 339–348.
  2. Griffith GW, S Baker, K Fliegerova, A Liggenstoffer, M van der Giezen, K Voigt and G Beakes. (2010). Anaerobic fungi: Neocallimastigomycota. IMA Fungus 1 (2): 181–185.
  3. Gruninger, RJ., AK Puniya, TM Callaghan, JE Edwards, N Youssef, SS Dagar, K Fliegerova, GW Griffith, R Forster, A Tsang, T McAllister, and MS Elshahed. (2014) Anaerobic fungi (phylum Neocallimastigomycota): advances in understanding their taxonomy, life cycle, ecology, role and biotechnological potential. FEMS Microbiol Ecol 90: 1–17.
  4. Liggenstoffer AS, NH Youssef, MB Couger and MS Elshahed. (2010). Phylogenetic diversity and community structure of anaerobic gut fungi (phylum Neocallimastigomycota) in ruminant and non-ruminant herbivores. The ISME Journal 1–11. 
  5. Powell MJ and PM Letcher. (2014). Chytridiomycota, Monoblepharidomycota, and Neocallimastigomycota. Chapter 6: 141-175. In : DJ McLaughlin and JW Spatafora (Eds . ) Systematics and Evolution, 2 nd Edition The Mycota VII Part A. Springer-Verlag Berlin Heidelberg.429 p.
  6. Wang X, X Liu, and JZ Groenewald. (2016). Phylogeny of anaerobic fungi (phylum Neocallimastigomycota), with contributions from yak in China. Antonie van Leeuwenhoek 110 (1): 87–103.

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