Filamentous fungi grow by hyphal branching and extension, creating networks which serve as highways for motile soil bacteria on the go. But the bacteria must pay a toll for traveling fungal roads.
Soil microbes coexist in cosmopolitan communities. Auxotrophic microbes cannot produce all the nutrients they need to grow, and so trade with others to compensate.
Fungi grow extensive networks to service plants: yielding minerals and other nutrients in return for sugar that plants can copiously produce. Plants are not the only trading partners that fungi have.
Soil bacteria are often motile, enabling them to forage and expand their settlements. Like people, the easiest way for bacteria to travel is on water. But bacteria can conveniently travel farther on fungal hyphal highways, where water is provided – at a price.
Aspergillus nidulans is a soil fungus that gets by fine without vagabond bacteria. Vegetative growth creates a network through which the fungus can itself communally exchange nutrients.
But bacteria do come by, enticed by the prospect of using the fungal network as a hyphal highway. Therein an opportunity lies for A. nidulans.
Bacillus subtilis is a sophisticated bacterium. A natural soil dweller, B. subtilis also cultivates relationships with ruminants and humans: comfortably living in animal guts in return for aiding host digestion.
When in the soil, B. subtilis is happy to find a network built by A. nidulans. Negotiations between the bacterium and fungus result in B. subtilis gaining access to the extensive roadways built by A. nidulans, along with all water the bacteria care to consume. In return, A. nidulans saves itself the metabolic bother of manufacturing its own thiamine, which B. subtilis agrees to provide. The fungus and bacteria make respective genetic adjustments that seal the contract between them: A. nidulans shutting down its own B1 production, while B. subtilis ups its thiamine output.
Gayan Abeysinghe et al, “Fungal mycelia and bacterial thiamine establish a mutualistic growth mechanism,” Life Science Alliance (22 September 2020).
Caroline Ash, “Hyphal toll roads through the soil,” Science (9 October 2020).