Photosynthesis in Lichen: Light Reactions and Protective Mechanisms

Abstract

Lichens are symbiotic associations (holobionts) established between fungi (mycobionts) and certain groups of cyanobacteria or unicellular green algae (photobionts). This symbiotic association has been essential in establishing the colonization of terrestrial and consequently dry habitats. About 44 genera of algae and cyanobacteria have been reported as lichen photobionts. Due to the uncertain taxonomy of many of these photobionts, these numbers were considered as approximations only. Ahmadjian (1993) estimates that only 25 genera were typical lichen photobionts. The most common cyanobionts are Nostoc, Scytonema, Stigonema, Gloeocapsa, and Calothrix, in order of frequency (Budel, 1992). Green algal photobionts include Asterochloris, Trebouxia, Trentepohlia, Coccomyxa, and Dictyochloropsis (Gartner, 1992). These authors assessed that more than 50% of all lichen species are associated with Trebouxia and Asterochloris species. However, this is just estimation since in only 2% of the described lichen species the photobiont genus is reported (Tschermak-Woess, 1989), mostly by the difficulties to isolate and then characterize the algae from the lichen thalli. Lichens are well known for their slow growth and longevity. Their radial growth is measured in millimetres per year (Hale, 1973), while individual lichens live for hundreds or even thousands of years. It is assumed that in lichens the photobiont population is under mycobiont control. Lichenologists have proposed some control mechanisms such as, cell division inhibitors (Honegger, 1987), phytohormones (Backor & Hudak, 1999) or nutrients competition (Crittenden et al., 1994; Schofield et al., 2003). Similar to plants, all lichens photosynthesise. They need light to provide energy to make their own matter. More specifically, the algae in the lichen produce carbohydrates and the fungi take those carbohydrates to grow and reproduce. The amount of light intensity needed for optimal lichen growth varies widely among species. The optimum light intensity range of most algal photobionts in axenic cultures is very low, between 16-27 μmol m-2 s-1. If the response of cultured photobionts to light is similar to that of the natural forms (lichen), then there must be additional mechanisms protecting the algae in the lichen that are not developed under culture conditions. Pigments and crystal of secondary metabolites in the