Fungi important in freshwater ecosystems

The leaves that fall into the stream are quickly colonised by fungi that break them down and make the nutrients available to other organisms in the watercourse.

The leaves that fall into the stream are quickly colonised by fungi that break them down and make the nutrients available to other organisms in the watercourse.

Access to fresh water is absolutely vital to our survival. Without it, we would suffer from a lack of drinking water, fish to eat and the ability to irrigate fields. Lakes, rivers and ponds are also home to countless species. But how these sensitive ecosystems are structured is not fully known.

‘Freshwater environments are some of the most threatened environments on earth,’ says Jennifer Anderson, who conducts research at the Department of Organismal Biology. ‘About one in three freshwater organisms is threatened. If we look at the International Union for Conservation of Nature’s Red List, we know the status of many of the fish in Europe. But when I say that one in three species is threatened, it refers to the organisms that we have studied. It doesn’t include the great diversity of species that we haven’t even developed tools or methods to assess.’

Pollution and land use

The biggest threats to freshwater environments are directly linked to humans, such as pollution, changed land use, water abstraction, and the release of pharmaceutical residues from wastewater. But changing temperatures and drought, which are becoming more common with climate change, also impact freshwater environments.

‘We put a huge amount of pressure on this very small asset,’ says Anderson. ‘Freshwater comprises 2.5 per cent of all of the water on earth and most of this is bound in ice or exists underground, so very little of the water is surface water.’

Jennifer Anderson with models of a highly
magnified freshwater fungi spore and a fish.

Anderson is one of few researchers in the world studying freshwater fungi. These microscopic fungi perform a very important function by breaking down dead organic material and making nutrients available to other organisms.

‘Fungi are the dominant microbial decomposers in streams and rivers,’ explains Anderson, ‘and they are an extremely important link between the carbon and the nutrients that end up in the streams and the other food webs in the stream. So they are absolutely crucial. Organisms that live in a stream are much like us – they lack the enzymes to digest tough plant fibres. So that’s what fungi do.’

Symbiosis and decomposing

In terms of lifestyle, these fungi show great variation, to say the least. Within a single species, some fungi can live as decomposers in a stream and others live in plant roots in symbiosis with their plant host.

‘This is not unknown among fungi,’ says Anderson. ‘They can have these double lifestyles, but we don’t know much about it. There is another example of fungi that live in plants and at the same time cause diseases in insects. So they can be connected to an insect that they killed and transfer the nutrients from it to the plant.’

Microscopic spores of the freshwater fungus Tetra-
cladium setigerum
, which were found in the Fyris
river. Photo: Jennifer Anderson

The knowledge gaps regarding freshwater fungi are large because very little research has been done on them. Because of this, no one knows how common they are and to what extent they are threatened. None of them have been assessed for conservation status.

‘We can at least learn to understand the biodiversity we actually have,’ says Anderson, ‘and that doesn’t only comprise beautiful fish. We can begin to look at what actions improve or worsen the biodiversity in our fresh waters.’

Åsa Malmberg

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