For experiments run at 350 ppm CO2, the subtropical lichen Coenogonium does not exhibit net photosynthetic depression at water contents above those at which initial maximal values are reached. This is in contrast to most lichens for which such a depression occurs. We infer that the results reflect the unusual morphology of Coenogomum, in which the alga is the dominant symbiont, occurring as filaments at the lichen surface. This arrangement should minimize CO2 diffusion resistance to the algal surface. With light saturation occurring by 70 μmol photons m−2 s−1, Coenogoniium has one of the lowest light saturation values reported among lichens.