9 September 2011, by Tom Marshall
Researchers have described a major hidden group of fungi for the first time. The fungi live in soils all over the planet, but were revealed only recently; scientists think we may only have discovered around 10 per cent of the vast fungal kingdom.
The hidden fungal class had been detected before through environmental genetic sequencing. But until scientists managed to grow a member of the class in the lab, they couldn't formally describe it.
That hurdle's now been cleared, after an international team of researchers led by Swedish biologist Anna Rosling managed to cultivate Archaeorhizomyces finlayi in controlled conditions and then carried out detailed genetic analysis to place it and its group of relations within the wider fungal tree of life.
'It's a really important discovery,' says Dr Gwen-Aelle Grelet of Aberdeen University, one of the authors of the paper, which appears in Science. 'It's a very significant advance in understanding the diversity of soil fungi - in some studies, 20 per cent of the species that were sampled belonged to this group.' The group is thought to be among the most abundant of the enigmatic fungal lineages, with an estimated 250 species.
The archaeorhizomycetes are ancient - the class diverged from the rest of the fungal world a very long time ago, and has been diversifying in the soil ever since. The wider group that the researchers think it belongs to, the ascomycetes, is a varied and economically important group that includes baker's yeast, penicillin and various parasitic fungi that live on plant hosts.
Species drawn from the new class have turned up in environmental genomic sampling in North America, Scandinavia, Scotland, Tasmania and Queensland. Grelet notes that these are some of the most heavily-sampled areas, and that more research in other areas with similar climates could well turn up further family members.
Many that we know about so far live underground in tundra and other high-latitude environments, many around the roots of pine and birch trees. But their exact function in these ecosystems isn't yet fully known. Their activity can be strongly seasonal - some grow quickly in the warm months and then shut down over winter - so the scientists initially wondered if they lived in a mutually-beneficial symbiotic relationship with some of the plants in their environment.
A well-known type of fungi, known as mycorrhizal fungi, forms these mutually-beneficial relationships with plants. They form an extended network of filaments around and sometime in the roots of a host plant, enabling it to absorb more water and nutrients, and in exchange draw energy from those roots. This mutually-beneficial relationship means the fungi are more active in summer when the plants are growing.
But it turns out this isn't what the archaeorhizomycetes are up to. They don't form the characteristic mycorrhizal structures, and they neither improve nor harm the growth of the plants around whose roots they are living. Yet each species only lives in specific ecosystems, around specific kinds of plants.
Grelet says lots more research will be needed before we can answer these questions. 'They could be playing a very important role in these ecosystems - for example they could be vital in cycling carbon from the atmosphere to the soil,' she explains. 'We just don't know yet.'
One clue comes from the kinds of food they can eat by producing specialized enzymes. The archaeorhizomycetes seem able to cope both with being fed glucose - the substance that mycorrhizal fungi get from their host plant roots - and with a diet of cellulose, the more complex carbon compounds that other fungi get from decaying plant material. 'They seem equally happy feeding on glucose or on cellulose, which is extremely unusual,' Grelet comments.
One possibility is that they have a relationship not with the plants themselves, but with the mycorrhizal fungi that live on their roots. This would explain their seasonal activity. But even if it's true, we don't know what that relationship might be. They might be helping their fellow fungi just as they in turn help their plant hosts; or they might be parasites, exploiting their fungal partners and ultimately harming both them and the plants.
In either case the archaeorhizomycetes can't be exclusively dependent on these host organisms - the team has already shown they can grow in laboratory conditions without them. It's possible that when host organisms are available, the archaeorhizomycetes form a relationship with them, but that at other times they live on decaying plant matter.
The next step, Grelet says, will be to test how archaeorhizomycetes function with or without host organisms and with different foodstuffs, to find out more about the conditions they favour in the wild, and what role they may fulfil in their natural environment.
Some of the genetic sequencing of Archaeorhizomycetes for the paper was done by Filipa Cox of Imperial College London as part of her PhD project, co-funded by NERC and Forest Research.