2 - Photobionts

Summary

Major differences in cyanobacteria versus algae

Nearly 40 genera of algae and cyanobacteria have been reported as photobionts in lichens (Tschermak-Woess 1988; Büdel 1992). Three genera, Trebouxia, Trentepohlia, and Nostoc, are the most frequent photobionts. The genera Trebouxia and Trentepohlia are of eukaryotic nature and belong to the green algae; the genus Nostoc belongs to the oxygenic photosynthetic bacteria (cyanobacteria). Eukaryotic photobionts are also referred to as “phycobionts” while cyanobacterial photobionts are sometimes called “cyanobionts.” The vast majority of eukaryotic photobionts belongs to the green algae (phylum Chlorophyta) which share many cytological features and their pigmentation, e.g. the presence of chlorophylls a and b, with the land plants (Bold and Wynne 1985; van den Hoek et al. 1993). Only two genera of eukaryotic photobionts containing chlorophylls a and c (phylum Heterokontophyta sensu van den Hoek et al. 1993) have thus far been reported: Heterococcus, Xanthophyceae, and Petroderma, Phaeophyceae (Tschermak-Woess 1988; Gärtner 1992).

Cyanobacteria are of prokaryotic nature and lack chloroplasts, mitochondria, and a nucleus, all of which are found in eukaryotic algae. In cyanobacteria, thylakoids lie free in the cytoplasm, often more or less restricted to the periphery. The circular DNA is not associated with histones and is concentrated in areas of the cytoplasm free of thylakoids which sometimes are called “nucleoplasm.”

Metabolite transfer from the autotrophic photobiont to the heterotrophic mycobiont depends on the type of photobiont involved.