Oxygen is vital to life, and the account of how it came to make up such a large proportion of the atmosphere, from its first appearance three billion years ago to the stabilisation of an oxygen-rich atmosphere in the late Palaeozoic around 500 million years ago, is also the tale of how life as we know it came to be.
New research, based on a rocky outcrop in far-flung north-eastern Scotland, now shows that fungi may have played a vital role in this story.
The Rhynie Chert is an incredibly well-preserved fossil bed near the town of Rhynie in Aberdeenshire. It contains, amongst other things, copious fossils of the earliest plants that began to colonise the land in the Lower Devonian period, about 410 million years ago. So well fossilised are these specimens that evidence of symbiosis between plants and fungi can be clearly seen, a relationship that continues to this day in the form of mycorrhiza, the fungus embedded in the roots of many modern plants.
These early plants had no roots, however, nor the vascular systems that allow modern plants to retain and circulate water. Importantly, the roots of modern plants enable them to take up vital nutrients, such as phosphorus, necessary for photosynthesis, from the soil. Early rootless land plants, obviously, didn’t have this ability.
And this is where the fungi come in.
Katie Field, Sarah Batterman and Benjamin Mills from the University of Leeds, UK, have shown that the evolutionary ancestors of the types of fungus involved in modern symbiosis with plants played an important role in “mining” phosphorus from rocks.
This is achieved through “biological weathering”, where fungi produce an acid that breaks down silicate rocks. These breakdown products are then absorbed by rain, which is slightly acidic due to dissolved carbon dioxide. This in turn, through a series of chemical reactions, leads to the deposition of limestone, calcium carbonate, a process that continually removes carbon dioxide from the atmosphere.
So how did this change the earth’s atmosphere?
Photosynthesis, which requires phosphorus, produces atmospheric oxygen, while the biological weathering generated in the process of acquiring phosphorus, pulls carbon dioxide from the atmosphere. Together, this led to the current oxygen-rich atmosphere that stabilised about 400 to 500 million years ago, according to the researchers’ experiments.
Mills, Batterman and Field grew symbiotic plants and fungi under atmospheric conditions resembling the early Palaeozoic and integrated their results into a biogeochemical computer model called Carbon Oxygen Phosphorus Sulphur Evolution, or COPSE.
The model showed that different kinds of fungi could have had wildly different and dramatic impacts on the Palaeozoic atmosphere. “Our critical finding,” says Batterman, “was that the nature of the relationship between fungi and plants could have transformed the atmospheric carbon dioxide, oxygen and ultimately global climate in very different ways, depending on the type of fungi present.”
What is certain, however, is that their work, “clearly shows the importance of fungi in the creation of an oxygenated atmosphere”.