Rhizobium

Rhizobium bacteria form nodules on legume roots that perform a feat of chemistry no plant can manage alone: converting atmospheric nitrogen into ammonia usable by plants. This biological nitrogen fixation is one of the most important ecological processes on Earth, making nitrogen available to ecosystems that would otherwise be nitrogen-limited. The bacteria get a protected home and plant sugars; the plant gets access to atmospheric nitrogen.

The partnership is tightly regulated on both sides. Plants only support nodules on roots where nitrogen is scarce - if soil nitrogen is abundant, forming nodules wastes energy. Bacteria invest heavily in nitrogenase enzymes only when they're in a stable nodule environment. Each partner commits resources only when the other holds up their end. Cheaters - bacteria that accept sugars without fixing nitrogen, or plants that form nodules but don't provide carbon - are selected against.

The fixed nitrogen doesn't stay private. When legume roots and nodules decompose, fixed nitrogen enters the soil, becoming available to neighboring plants. This is why traditional agriculture rotated legumes with grain crops - the nitrogen fixed by peas or beans fertilized the wheat that followed. The mutualism's benefits leak into the broader community.

The business parallel is joint ventures that create industry-wide benefits. When companies cooperate to solve problems neither could solve alone - developing standards, creating infrastructure, building markets - the benefits often spread beyond the partnership. Rhizobium teaches that some mutualisms are most valuable when their outputs become public goods.