Biological Nitrogen Fixation

Most of Earth's fixed nitrogen comes from living organisms rather than from lightning or industry. Symbiotic nitrogen fixation happens through a close physical partnership between a nitrogen-fixing organism and a plant host. The best-known example is nodulation in legumes: the bacterium Rhizobium fixes atmospheric nitrogen while living inside root nodules it induces on a leguminous host, a relationship that benefits both partners. Nodule formation proceeds through five recognisable stages (Figure 12.6): the legume root first secretes phenolic compounds that attract Rhizobium; the bacterium reaches the rhizosphere, enters a root hair, and causes it to curl as infection proceeds; an infection thread then grows inward, keeping the infected tissue separate from normal tissue; the bacterium becomes enclosed within a host-derived membrane inside the nodule, at which stage it is called a bacteroid; and finally, cytokinin produced by the bacteria together with auxin from the host plant promotes the cell division that builds the visible nodule. Because the nitrogen-fixing enzyme nitrogenase only works under anaerobic conditions, the nodule synthesises its own oxygen-scavenging pigment, leghaemoglobin, to keep oxygen away from the bacteroids - the same pigment is what gives healthy, actively fixing root nodules their characteristic pink colour. Not every symbiotic nitrogen fixer is a legume: non-legume hosts such as Alnus and Casuarina harbour the bacterium Frankia in root nodules, and Psychotria harbours the bacterium Klebsiella. Symbiotic fixation can also happen without any nodule forming at all - cyanobacteria such as Anabaena and Nostoc fix nitrogen while living inside lichens, the hornwort Anthoceros, and the water fern Azolla (specifically the strain Anabaena azollae), while Anabaena and Nostoc together colonise the coralloid roots of the gymnosperm Cycas. Separately from all of this, non-symbiotic (free-living) organisms fix nitrogen entirely on the …