Geographic distribution and ecophysiological adaptations of desmids (Zygnematophyceae, Streptophyta) in relation to PAR, UV radiation and temperature: a review
Although conjugating algae are considered to have a cosmopolitan freshwater distribution, numerous ecological and taxonomic investigations revealed that many desmid taxa (at the level of genus, species and variety) are capable of occupying specific geographic zones, characterized by particular climatic attributes. Earlier studies have dealt with influences of temperature and irradiation (photosynthetically active radiation and ultraviolet radiation) on the physiology and ultrastructure of desmids. Yet, recent investigations demonstrated a clear relationship between these climatic factors and the distributional potential of conjugating algae, taking into account their photosynthetic, physiological and ultrastructural adaptations which had been revealed during and after certain temperature and irradiation treatments. Despite the fact that desmids can be considered as high-light-adapted algae, various species- and strain-specific characteristics and adaptations appeared in accordance with the light intensities predominating at their source localities, as estimated by their photosynthetic performance (obtained from PAM fluorometry and oxygen evolution measurements), pigment composition and morpho-anatomical characteristics. Interestingly, the high-light adaptation of photosynthesis as well as the relatively high growth temperature optima for majority of the desmid species investigated may provide some support for Coesel’s hypothesis on the origin of desmids in the tropical zone.