Fungi: Rock to Heavy Metal

A new area of research being driven by Dundee life scientists is revealing remarkable abilities of fungi to interact with minerals and metals, providing environmentally-friendly mechanisms for treatment of pollution, land reclamation and improvement of plant growth.

Led by Professor Geoffrey Gadd in the College of Life Sciences, the research explores the unique taste that fungi seems to have for rock and heavy metal.

Metals are components of the Earth, essential for all life and the integrity of our planet. From aluminium to zinc, they are found everywhere - in the air, in the soil, in our food and in our bodies. And we need many metals to survive - copper, iron, zinc, magnesium and calcium are just some of the metals the body needs to use food, make energy and build cells, tissues and bones.

However, practically all metals can be poisonous in excess, and can cause a variety of damaging effects in people who accidentally consume them. Some radioactive forms of metals (radionuclides) can also be accumulated, such as radiocaesium. Pollutant metals and radionuclides can make their way into the food chain causing serious health concerns. Unlike organic pollutants, metals cannot be degraded but their chemical form can be altered by the power of microorganisms, and this can result in detoxification.

Fungi are an interesting and unique group of organisms with their own kingdom, just like animals and plants. In evolutionary terms they are our closest relative which is why many fungi are used as research models in molecular and cell biology and genetics. Yeasts, moulds and mushrooms are all fungi and there are an estimated 1.5 million different species in the biosphere. By breaking down dead organic material, they continue the cycle of nutrients through ecosystems, and most plants could not grow without the symbiotic fungi that inhabit their roots and supply essential nutrients. Other fungi provide drugs such as penicillin, foods like mushrooms, truffles and morels, and the bubbles in bread, champagne, and beer.

Fungi will also live almost anywhere. They have been found growing in the harshest of environments, in the desert and polar regions, in the sea and on rocks. Their favourite home is the soil where their biomass can often outweigh all other soil organisms. In fact the largest living organism on Earth is a tree-pathogenic fungus (Armillaria ostoyae) which extends over 10 square kilometres and is estimated to weigh over 100 tonnes. And they can "eat" or attack almost everything in their environment, including heavy metals.

"The fact that fungi interact with heavy metals has potentially important consequences for human activity. Fungi also play a significant, if often overlooked, role in the degradation of rocks and stone - including building materials," Professor Gadd said.

"Despite this, their role as agents of environmental change has not been fully appreciated. Our research is providing new insights into the part that fungi can play in shaping the world around us and discovering ways that we can manipulate fungal growth and feeding habits for our own benefit."

Rocks are composed of minerals, the vast majority of which contain metals. They might be considered to be an inhospitable habitat for life to flourish yet fungi can thrive even in the harshest of environments. In fact lichens, which are a special kind of fungal growth form, are among the first colonizers of barren rocks.

The ability of fungi to grow on a range of rocks and mineral-based surfaces, including concrete and other building materials is significant, with positive and negative implications. Fungi can produce acidic by-products which help them use nutrients in the minerals but this begins to decompose the rock in a form of biological weathering (bioweathering). This can result in the return of essential nutrients and metals like calcium, phosphate and potassium back into the soil where they can nourish plants and microbes. In other cases, the released elements can form other minerals. A recent discovery was fungal ability to make a kind of limestone.

Perhaps more significant is the degradation of the rock or stone, structure or building where the fungi grows, coupled with their ability to grow in the harshest of environments.

"This ability of fungi to attack and degrade concrete and other materials has implications not only for the weathering and corrosion of buildings but is also relevant to nuclear decommissioning, for example," Professor Gadd said.

"Clay minerals as well as concrete are used in the containment of nuclear waste, while contaminated surfaces are a problem in decommissioning. The consequences of microbial degradation may be serious and one example why improving our understanding of the roles of fungi in shaping our environment is so important"

Fungal ability to transform toxic metals is an area where Professor Gadd and his team have been making real progress.

"While toxic metals have always existed in soil and rocks, human activities have drastically altered the balance. All industrial and agricultural activities affect distribution of metals in the environment, and many sites can be heavily polluted. Some of the processes by which fungi interact with metals have potential for treating this contaminated land," he said.

Professor Gadd's team already works with special kinds of fungi with great potential for cleaning up unwanted contaminants in the soil and preventing uptake by plants. "Mycorrhizal fungi" have a special relationship with plants, living in association with the roots. "Over 90% of plant species have mycorrhizas and among these are the toadstools you see in woods and forests in the Autumn," he said. The plants provide nutrients to the fungus and in return the plant receives mineral nutrients and protection from the fungus.

"It is not surprising that mycorrhizal plants can often flourish on harsh and polluted soils - heather is a good example," he said. Such plants can provide an inexpensive, environmentally friendly way of reclaiming contaminated land.

Next stages of research are exploring the ways that fungi can transform metals and minerals and how these activities affect the environment and human health. As part of a project sponsored by the Natural Environment Research Council, Professor Gadd's team have been looking at fungal influence on depleted uranium and have already discovered a range of novel uranium biominerals, while projects sponsored by the nuclear industry are looking at a range of processes that can be used for clean-up.

"The role and potentials of fungi, as well as other microbes, in a host of environmental systems is an area of research that is only now beginning to gain momentum and one I believe will become increasingly important as we uncover more of their unique abilities," Professor Gadd said.