Carbon cooperation

Another very interesting seminar today. This one, by David Beerling from Sheffield, dealt with the geological cycle of carbon (weathering; CaCO₃ burial; vulcanism) and the role that terrestrial vegetation plays on top of it.

Essentially, for a couple of decades it's been known that there's a neat feedback loop between CO₂ and climate, which is mediated through rock weathering on land. Vegetation on land is known to be associated with enhanced weathering, and the evolution of land plants (first in getting onto the land; later the advent of the angiosperms) is believed to be responsible for two significant decreases in atmospheric CO₂ in the Phanerozoic.

This was the background to the talk, but it went off in quite a different direction. Beerling then introduced the arbuscular mycorrhiza (AM) and the ectomycorrhiza (EM), two groups of fungi with strong mutual symbioses with land plants. While I knew that these played an important role in nutrient acquisition for some plants, I'd no idea quite how big a role this was. Apparently around 12% of photosynthate produced by a plant can be channelled to its symbiont.

The seminar then delved into some experimental work in which the strength of this relationship was examined (largely through ¹⁴C-labelling of plant photosynthate). This was augmented by some very elegant and clever growth experiments in which the potting medium was differentially composed of soils of different mineral value. These trials showed not only the strong link between the two species, but also how tenacious fungi are at locating and "mining" minerals for their plant partners. Some of the photographs showing the fungal growth were really impressive on this point.

Finally, the talk returned to the geological cycle of carbon, and armed with this new quantitative knowledge about the plant-fungus relationship, suggested instead that the underlying factor in the downward "jumps" in past atmospheric CO₂ was the evolution of the two groups of fungi. Fossil evidence points (as much as it can given large uncertainties) to the appearance of each group around the time that CO₂ drops.

Anyway, it was all quite a nice challenge to the "interesting-but-not-significant" view I'd formed of AM and EM during my biology degree. And intriguing to see cooperation, which I also typically view as "interesting-but-not-significant", put pole position in such key biogeochemical processes.