The N2(C2H2) fixing activity was surveyed in lichens of diverse structure and habitat, all containing a blue-green phycobiont; twenty-six species from fourteen genera are reported for the first time to fix nitrogen. Rates of N2(C2H2) fixation showed great variability between different lichen samples.
The proportion of heterocysts, as the assumed site of N2 fixation, was estimated in all the above lichens together with fourteen additional species. In lichens with one phycobiont, heterocysts form about 4% of the total algal cell population; in those with two phycobionts the blue-green member is found in special structures and has a very high proportion of heterocysts, typically 20–30%; the N2(C2H2) activity appears to reflect this. The high proportion may be associated with the proximity of the green phycobiont.