Becky Ripley and Emily Knight dig deep into the underground web of plant roots and mycorrhizal fungi networks. Here lies a 400 million year old market economy, founded on the trading of resources. Nutrients are traded for carbon. Carbon is traded for nutrients. And the exchange rate between the two is constantly in flux, to level supply with demand. This highly-evolved symbiosis between plant and fungi is crucial to the survival of over 80% of all terrestrial plants. And it also acts as a colossal carbon store. A recent study found that 13 billion tons of CO2 are passed from plants to mycorrhizal fungi each year. It's one of the most effective and important market trading relationships in the world. And it turns out, these belowground trade deals are not so different to the aboveground deals that play out within our own market trading economy. Both move and shake to the very same economic principles of supply and demand. Can our economic theories be applied back to the fungi-plant deals in order for the fungi to capture more carbon in the face of climate change? Or, flip-reverse it, can we apply some of their age-old trading strategies to our own economic models? They may not have a brain, but they have 400 million years of evolution under their belt, so their trade strategies may well be more streamlined and more symbiotic compared to ours. Featuring Dr Bethan Manley, fungal geneticist and data scientist at The Society for the Protection of Underground Networks, and Dr Ted Loch-Temzelides, Professor of Sustainable Development at the Department of Economics at Rice University. Produced and presented by Emily Knight and Becky Ripley.