How fungi oxygenated the Earth

Now that’s an eye-catching headline, isn’t it? The thought of mushrooms and toadstools – that famously don’t produce oxygen, unlike green plants with their oxygenic photosynthesis – adding oxygen to the planet’s atmosphere. What? How? Tell me more…

Well, and as you probably suspect, it’s not direct activity of the fungi that generates the oxygen, but the effect that these mycological miracle workers had on photosynthetic plants during the planet’s geological past, via their role as partners in that ancient plant-fungus mutualism known as mycorrhiza. And this story has a vital contribution from phosphorus (P) – that essential plant macronutrient which is famously often in short supply in the environment and which therefore constraints plant growth and photosynthesis, and therefore the plant’s oxygenation contribution to the planet’s atmosphere.

Obtaining sufficient amounts of P by early land plants was constrained by two major factors. One was the low amounts of available P in the soil – the thickness of which vital layer was extremely thin in those ancient times until this important planet-hugging, life-sustaining, brown mantle had developed sufficiently due to physical, chemical and biological activity. Second was the rather rudimentary nature of the root-like structures – rhizoids – of those early plants that didn’t have the substrate-anchoring, ground-penetrating, water and nutrient-absorbing and -transporting properties of true roots (which evolved later on…). Thus, any association between primitive plants and organisms with an increased ‘soil’ penetrating capacity – such as fungi with their extensive hyphal network – would be a potential benefit to the plant in terms of greatly enhanced capacity to extract water and nutrients from the substrate.

Modern-day experimental work by Benjamin Mills et al. supports the view that enhanced mycorrhizal-facilitated acquisition of P by early – non-vascular* – land plants in the Palaeozoic Era (approx. 541–250 millions of years ago (Ma)) could have been responsible for enhanced plant growth and photosynthesis. And it’s this latter fungus-enhanced plant biochemistry which would have had the consequence of increasing atmospheric O₂ concentrations (ultimately, to values approximating those of the present day).

Any such mycorrhizal-promoted photosynthesis would presumably also have the benefit of increasing the amount of plant-fixed carbon compounds transferred to the heterotrophic fungal partner, promoting growth of the latter and increasing the volume of the nascent soil that could be exploited for more P, etc.

This co-operative behaviour is also inferred to have had a profound effect on P cycling and transfer of this important element – and all the other essential elements that constitute the body of plants – between ecosystems on the planet. This would also have had an impact upon climate as CO₂ was removed from the atmosphere and replaced by oxygen. Development of the land flora, was therefore an evolutionary event of major global consequence.

Mills et al’s study not only serves to underline the importance of phosphorus to plant biology (and, by extension, vegetating the planet and making Earth habitable), but also to emphasise the pivotal role potentially played by fungi in the development of that land flora. Furthermore, although primarily ‘backward-looking’, this palaeoecological insight has present and future relevance. If activity of modern-day mycorrhiza [which are numerous and considered to be present in 80-95% of all plant species – were to be reduced by any ‘factor’ (e.g. human action/inaction that affected soil or mycorrhizal fungi…), then global productivity and biogeochemical cycling might be affected, to the future detriment of all life on the planet. But, what are the chances of humans learning a lesson from the past to guide their future conduct..?

[Ed. – the Introduction to Mills et al‘s paper is a masterful example of how to integrate references into scientific writing. If only our undergraduates would believe us when we – repeatedly and continually… – stress and emphasise that evidence-based writing [i.e. citing – appropriate – references for statements made] is the bedrock and hallmark of scientific papers… They could all learn a lot from that scientific paper.]

* Since these plants don’t have true roots the fungal-plant association is strictly termed mycorrhizal-like – to distinguish it from true fungus-root mycorrhiza.