Anaerobic fungi and their biotechnological potential
Anaerobic fungi were first identified by Colin Orpin in 1975 and are the earliest diverging lineage of fungi which retain some ancestral features lost in higher evolved fungi. They show an obligate anaerobic life-style, i.e. are unable to live in presence of oxygen, lack mitochondria but possess hydrogenosomes, and were first discovered as symbionts of large herbivorous mammals such as cattle and sheep, where they live along with bacteria, protozoa and archaea in complex consortia, perfectly adapted to degrade plant biomass. More recently, anaerobic fungi have also been discovered in anaerobic habitats beyond the digestive tract of larger herbivores including landfill sites, deep ocean- and lake sediments.
The best characterized habitat of these fungi remains, however, the herbivore gut. Since large herbivores are unable to digest plant material themselves, they rely on diverse gut microbiota to conduct the degradation of plant organic matter and anaerobic fungi play a crucial role in this microbial population. While catabolism of lignin can only occur in the presence of oxygen, the anoxic rumen habitat has evolved to efficiently degrade lignocellulose by physical disruption, mediated via the cud chewing of the animal, the penetrative growth of fungal rhizoids as well as the depolymerisation of the cellulose- and hemicellulose components by potent exo-enzymes produced by the microbes living in their rumen.
As lignocellulosic biomass is the largest source of raw material that can be used for the production of renewable biofuels, a proper degradation and utilization of this material within anaerobic bioreactors is crucial to allow for an effective biotechnological exploitation of this substrate but is currently not achieved. Thus, a proper implementation of anaerobic fungi into such systems bears a great biotechnological potential.
Anaerobic fungi exhibit a complex life cycle, involving the propagation via motile zoospores using chemotaxis to detect ingested plant material for colonization. The zoospores germinate on the plant cell surface and form a rhizoidal network that physically breaks apart the plant tissue and ultimately forms one or more sporangia. The release of the next generation of zoospores is triggered by nutrients emitted from freshly ingested forage. In this way the highly motile AF zoospores act as primary colonisers of plant material.
Furthermore, it has been shown that interactions with methanogens, bacteria and protozoa can influence the growth and the activity of anaerobic fungi and it is known that they often grow in co-culture with methanogenic archaea, highly abundant in the digestive tracts of herbivores. It is thought that methanogens use the acetate, formate, CO2 and hydrogen gas, produced by AF and convert these into methane. It was further reported that some anaerobic fungi are obligately depending on these partners as known from syntrophic interactions.
Both, the complex life cycle and the possible syntrophic needs of anaerobic fungi hamper an easy cultivation of this microbial group.
To address these points and solve some problems impeding their successful implementation into biotechnology, the project HiPoAF addresses a broad range of aims and objectives covered within distinct work packages, combining the knowledge of different experts in their field.
Summary of the complex life cycle of anaerobic frungi, from Gruninger et al. 2015 (DOI: 10.1111/1574-6941.12383).
Taxonomy of Anaerobic Fungi
The phylum Neocallimastigomycota contains only one order (Neocallimastigales) and within that order, only one family has been described so far (Neocallimastigaceae). The genera and species within that family are, however, surprisingly diverse in their morphology, making rough discrimination by microscopic analysis possible.
And if you think, Neocallimastigomycota is the most complex name scientists could have come up with, think again: of the around 20 genera described so far, at least two were given a welsh name. Want to know how to pronounce Buwchfawromyces and Cyllamyces correctly? Listen to Dr. Joan Edwards' explanations on the taxonomy of anaerobic fungi without knotting her tongue irreversibly.
Fun Fact: everytime a non-welsh person pronounces their name correctly, somewhere a Buwchfawromyces smiles.
Watch a confused balloon-horse shaped zoospore moving on an object slide!