The rise of plant-fungal partnerships corresponds with a 90% reduction in atmospheric CO2 levels. Today, between 80-90% of all plant species form a trade symbiosis with mycorrhizal fungi. Mycorrhizal fungi grow into large networks of tubular cells, known as mycelium (individual cells are called hyphae), that forage for nutrients in the soil and exchange them with their plant partners. A single gram of soil can contain up to 90 meters of mycelium. The total length of mycorrhizal mycelium in the top ten centimeters of soil is around 450 quadrillion kilometers: around half the width of our galaxy. Mycelium forms a sticky living mesh that provides soil with much of its structure. Mycorrhizal fungi do more than provide plants with nutrients. Mycorrhizal fungi are also important in pathogen protection, heavy metal tolerance, and water uptake.

Soils are some of the most complex ecosystems on Earth, and fungi must trade to survive. Animals rely on a central nervous system to make trade decisions, but fungal networks must evaluate trade environments without a brain. First, they must forage for nutrients in the soil (like phosphorus and nitrogen). Second, they must exchange these nutrients for carbon compounds (like sugars and fats). To do this, mycorrhizal fungi have evolved sophisticated trading strategies, and can discriminate between plant partners, exchanging more resources to plants that provide them with more carbon. Fungi can capitalize on value differences across complex trade networks by moving resources to where they gain a better price from plant ‘buyers’. In one study, when faced with an unequal supply of nutrients across their networks, mycorrhizal fungi moved phosphorus to areas of scarcity, where it was in higher demand and therefore fetched a higher ‘price’. By doing so, the fungus was able to receive larger quantities of carbon in return. Fungi can even hoard resources until they retain a higher ‘price’. Researchers are using new tools to tag nutrients inside fungal networks and track fungal trade decisions.

Underground, mycorrhizal fungi form networks of hyphae potentially connecting roots of diverse host plants. The function of these networks, called common mycorrhizal networks (CMNs), has been debated for decades. In some cases, they may facilitate the flow of nutrients and carbon, mediate both cooperative and competitive relationships between plants, and help protect plants from pests and pathogens. Once a plant has ‘plugged into’ a CMN, the network may serve as a physical conduit for the movement of nutrients and chemicals. The vast majority of these studies have been done in the laboratory, so more field work is needed to understand the function of CMNs under natural conditions.

How do fungi control the flows of nutrients across their large and complex networks? Fungal networks are bathed in a rich field of sensory information and must integrate a complex array of chemical, physical, and environmental stimuli. Fungal networks must constantly remodel themselves, sending carbon to growing tips to build new trade routes, and collect nutrients, like phosphorus and nitrogen, to send to plants roots. Scientists are studying the complex flow patterns inside fungal networks to find out how we can better harness the power of mycorrhizal associations to store carbon and support the health of ecosystems.