Physiological life history strategies of photobionts of lichen species from Antarctic and moderate European habitats in response to stressful conditions

The vegetation of many terrestrial habitats across Antarctica is dominated by poikilohydric symbiotic lichens. Terrestrial habitats generally are characterised by extended exposure to desiccation and high irradiation. Physiological adaptation mechanisms of the algal partner (photobiont) are key factors in the successful colonisation of lichens of locations under severe environmental conditions. This study focused on isolated photobionts of the genus Trebouxia, from the continental Antarctic lichens Buellia frigida, Pleopsidium chlorophanum, the maritime Antarctic lichen Umbilicaria antarctica, and the Swedish lichen Fulgensia bracteata from a moderate temperate ecosystem at sea level. The photosystems PS II and PS I and the ratio of linear to cyclic electron transport were studied to elucidate adaptation mechanisms in the physiology of the photobionts in response to desiccation and light stress. The photobionts of the Antarctic lichens demonstrated striking tolerance to the stress conditions studied. Although the photobionts of U. antarctica and P. chlorophanum were genetically identical based on non-coding internally transcribed spacer (ITS), their physiological responses were clearly different, possibly indicating ecotypic differentiation. The photobiont of F. bracteata showed clearly different responses to those of the Antarctic photobionts. The response differences of the photobionts studied point to fundamental differences in life history strategies.