Along with bacteria and viruses, fungi make up the fascinating myco-biological world. Here to tell us more is Ali Ashby, from Cambridge University.
Chris - So Ali, what exactly is a fungus, and how is it different from bacteria and viruses?
Ali - Fungi are fascinating organisms. They are incredibly diverse and range in size from a few microns, as expected for a bacterium, right up to being one of the largest organisms on this planet.They're incredibly diverse. They diverged from bacteria very early on. They are not plants and they are not animal, they have their own kingdom.
Chris - Are they more specialised than bacteria? Are they more like us than bacterial cells?
Ali - They have more in common with animal cells so yes they are more like us, and we clearly shared a common ancestor early on in the evolutionary time.
Chris - So what is the biggest role of the fungi in the world around us?
Ali - The biggest role fungi play is that they are one of the primary decomposers. They break down woody lignin, and without them we would have lots of trees but virtually no soil, so plants wouldn't be able to survive. One of the other things that they do is that just about every plant in your garden has a fungal association. It could be an association with the roots, mycorrhizal associations, where both the fungus and the plant benefit from that interaction. Or they could be endophytic fungi which tend to grow in the leaves of plants and don't cause any visible signs of infection, yet they are beneficial in that they prevent animal from eating the leaves and also offer some resistance to other microbes.
Chris - So what are they doing in the roots? You mentioned a clever relationship between the fungus and a plants roots, so what's happening there?
Ali - The root structure for plants can only extend so far into the soil. There are different types of mycorrhizal fungi and these are the endomycorrhizal fungi and the ectomycorrhizal fungi. The endomycorrhizal fungi associate very closely with the roots of a plant, they actually get inside the outer surface of the plant and extent their hyphae much further than the roots of the plant.
Chris - So it's like a system of roots on top of roots?
Ali - That's right and so they get the nutrients that the plant is finding hard to get hold of like phosphorus and zinc. In return the plant gives the fungi the sugars that result form photosynthesis, so it's a mutualistic interaction.
Chris - So if I was to zoom in on a fungus, what would it actually look like?
Ali - Well if you could imagine the London Underground it's a little like that. It's a vast cotton wool network of hyphae, hollow tunnels like the underground that extend out and spread into the substratum. Fungi are like animals, and not like plants that can make their own food from external sources of energy but, like us, fungi actually have to utilise pre-made organic material. So they send out their hyphae and produce special enzymes that are released into the food source. They then digest the food and absorb the nutrients back into the cotton wool hyphal strands.
Chris - You mentioned that they are one of the largest organisms on Earth, so how big is big?
Ali - The latest measurement on Armillaria galica in Oregon, USA, was 3.6 miles, and that's just one organism! The mycelium is spread an incredible distance, and so it could arguably be the largest organism on this planet.
Chris - We now see fungi as a useful food source not just for breaking down wood but also for us to eat, to turn into marmite and things like that.
Ali - That's right, you'd be absolutely amazed if you look at the food industry. Next time you go to the supermarket just look at what you have in your shopping basket. Fungi help in the production of bread, wine, beer, Marmite, soy sauce, cheeses and even chocolate! Fungi play an incredible role both in the way the chocolate plant grows and also fungal fermentation makes the chocolate taste good.
Chris - So it's not just making the plant grow well?
Ali - No, it's actually a different fungi. The endomycorrhizal fungi help the root structure of the tree and allow it to grow, and then once the flowers develop, the coco pods develop. In those pods there are about 50 seeds and it's those seeds that you need for making chocolate. The seeds are surrounded by a mucilage and the fungi break down that mucilage and produce ethyl alcohol. It is at these early stages where the lactic acid is broken down to ethyl alcohol, that you get the flavour of the chocolate coming out.