The algal genus Trebouxia comprises non-motile coccoid green algae with characteristic pyrenoids and a lobate chloroplasts. Strains of Trebouxia are present in approximately one half of all lichens. Therefore it has become one of the best studied phycobionts with respect to its diversity in lichen symbioses. Studies have focused on phylogenetic relationships of the lichenized strains and on the specificity of occurrence with mycobiont (= lichen) species, and on its ecological responses. Here we present a review of the body of literature gathered mainly during the past ten years on the genus Trebouxia, reporting the most recent results of molecular and morphological analyses. We also revise the most up to date methodological approaches to study the diversity and the morphological variability of Trebouxia photobionts in lichen thalli and in axenic culture.
