Differences in photosynthetic performance between cyanobacterial and green algal components of lichen photosymbiodemes measured in the field.

Photosymbiodemes are lichens which contain a single mycobiont but have cyanobacteria and green algae as primary photobionts in different parts of the thallus. Members of a photosymbiodeme can be found as separate, free-living lichen species. The photosynthesis of the components of two Pseudocyphellaria photosymbiodemes were studied in the natural forest environment in New Zealand. It was found that the green algal component had a large photosynthetic advantage when thallus water contents were low and the thalli were in equilibrium with atmospheric humidity. The cyanobacterial components were at an advantage when thallus water contents were very high. The environment in which photosymbiodemes are found, forest margins in high humidity areas, seems to provide the correct combination of thallus water contents so that neither component has a major advantage. The photosymbiodemes appear to be models, preserved by the special habitat and showing a possible early stage in the evolution before the definite separation of ecologically specialised green algal and cyanobacterial species.