A Hutton Club seminar by Andy Bray
First things first – the title does have the word rock in it and I feel that I must explain myself - the talk focused on fungi that destroyed rocks, hence why I thought it would be appropriate to talk about.
What was the seminar about and why is it important?
Weathering of rocks has resulted in the formation of the soils which has allowed all the life in the biosphere to proliferate in the multitude of ways that we see today. Weathering of rocks can, over time, have an impact on the carbon cycle. The seminar looked at how biological weathering of rocks by plants and their symbiotic fungal counterparts can affect the carbon draw-down from the atmosphere. The seminar started by discussing how plants have contributed over time to cause major draw-downs of carbon.
In the beginning
There was the lifeless planet called Earth. Fast forward a billion years and you find earth is not so lifeless anymore and that life is starting to shape and affect the planet:
· 2.7 billion years ago cyanobacteria started to produce Oxygen
· 1.2 billion years ago there were red and brown algae
· 0.45 billion years ago the first land plants arrived e.g. liverwort
· Present day: humans are here and producing a lot of carbon
Enter the plants
Plants evolve further from simple gymnosperms like liverwort in the Ordovician. They start of as very simple, with no vascular system or roots and would have lived on the soil surface. By the Carboniferous, plants have become taller, there are vast forests of gymnosperms with a vascular system. By the Cretaceous period the first angiosperms have evolved.
As plants continue to evolve throughout earth’s history, they are also changing and shaping the planet:
· They alter the atmosphere by releasing Oxygen and fixing carbon via photosynthesis
· They affect the soil as roots move through it, drawing up water and nutrients
· They promote weathering as plant roots can break up soil and other surfaces (physical weathering) while also breaking up rocks via chemical release from roots (chemical weathering)
· Plants have a direct link to solar energy, as light from the sun allows plants to photosynthesise
As can be concluded from the above, plants are important on earth. They have fixed oxygen to allow us to breathe and actively uptake carbon from the atmosphere. Plants have also formed mutualistic relationships with other organisms.
Enter the fungi
More than 80% of plant species have formed mutualistic relationships with fungi. Fungi are focused on the root tips of the plant and have access to nutrients in the soil. In fact, 90% of root tips in Boreal forests are covered by fungi. What are the benefits of this mutualism? The fungi get 1/3 of the carbon that the plant fixes through photosynthesis. The plant gains nutrients from the fungi.
The fungal hyphae on the plant roots can spread through the rhizosphere and cover huge distances if they were spread end to end. In 1m³ of soil from a biome, there will be:
· 200km of hyphae in a boreal forest
· 750km of hyphae in a grassland soil
· 400km of hyphae in the average agricultural soil (this figure changes based on agricultural practices carried out on soils e.g. fertilisers, fungicide and pesticide use)
What techniques were used?
Follow this link to find out more directly from the source: https://www.sciencedirect.com/science/article/pii/S0016703711005072
The seminar was about growth experiments between plant seedlings of Scots pine (to simulate a boreal forest) and fungal hyphae. The plants seedlings would be grown with different mineral sources to see if the fungi would utilise the mineral source. The main mineral that was used was biotite. Using a powerful electron microscope and pH dyes on the root tips of the growing hyphae the team were able to look closely at how much the hyphae were growing and how much of the biotite they had used.
Vertical scanning interferometry was used to observe the fungi on the surface of metals and see how much was dissolved and absorbed by the fungi. The technique looks at topographic differences on the surface of a mineral. Selected area electron diffraction (SAED) was used to see the deformation of the biotite up close.
Main results
Fungi was affecting the chemistry of the rhizosphere due to the fungus respiring and secreting organic acids as a bi-product. The acid lowers the pH of the soil allowing for mineral dissolution to occur. The organic acids change the soil pH at the soil/hyphae interface. A pH of between 4 and 6 is expected in a typical boreal forest due to respiration in the soil. A lower pH allows more dissolution of nutrients in the soil which are then easier to be absorbed by the fungi. The plant host then takes up the nutrients.
Main results:
· When biotite was used a mineral source a pH of 4.5 was recorded from the root tip
· High concentrations of organic acids are needed in the soil to show any large increases in mineral dissolution in the soil
· Symbiotic fungi can increase weathering by an order of 1 magnitude.
· The fungi can expose fresh surfaces of biotite to more weathering
· Organic acid production by fungal hyphae did contribute to bio-weathering rates, albeit on a very small scale
How did the seminar advance our knowledge?
The whole seminar was about how everything, from plants to humans, affect the earth. Humans have been burning fossil fuels and releasing carbon into the atmosphere for several hundred years. Currently global warming is looking to be a major threat to humans because of the way we have altered the planet. We are now looking for ways to reduce the amount of carbon in the atmosphere. This seminar was looking to find a link between weathering of rocks by fungal hyphae and draw-down of carbon from the atmosphere.
Production of minerals from the break-up of rocks by fungal hyphae in the soil can produce ions of calcium and magnesium. These ions will then be transported to the ocean. Long term carbon cycles can then be driven by these minerals and this can affect the draw-down of carbon from the atmosphere. In the past there have been major CO₂ draw-downs due to the expansion of vascular plants.
Could this draw-down have been in part due to symbiosis with fungi?
For the moment there are only tenuous links between hyphae and carbon draw-down as it occurs on such a small scale there is no current way to utilise hyphae to majorly reduce our carbon draw-down. The seminar pointed out that there could be work done on this in the future and how prevalent any carbon capture or reduction studies are to reduce the impact of global warming. With time hopefully we can utilise the world around us, in particular hyphae and plants, to make a change for the better for the planet. As plants have caused large reductions in atmospheric carbon in the past, our best hopes may lie with further study of bio-weathering of rocks by fungi.
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