An overview of the biodiversity and biogeography of terrestrial green algae

Microscopic green algae are among the most widespread microrganisms occurring in terrestrial environments. For more than two centuries, generalizations on the diversity and biogeography of these organisms have been based entirely on morphological species concepts. However, ultrastructural and molecular data produced in the last 30 years have revealed a scenario in substantial contrast with morphological classifications. It has become clear that these organisms have been affected by an extreme morphological convergence, which has restricted their morphology to a narrow range, not indicative of their great genetic diversity. Their habit is very simple and uniform, usually referable to a few types (unicellular, uniseriate filamentous, sarcinoid colony) and offers very few characters useful for taxonomic and systematic purposes. These factors make the identification of terrestrial green algae and a correct interpretation of their biogeography very difficult. “Flagship” taxa with easily recognizable habit are the ones for which the best generalizations are possible. Examples of such taxa include the order Trentepohliales (for the highest diversity occurs in humid tropical regions of central-south America and south-eastern Asia) and members of the order Prasiolales (which are typically associated with polar and cold-temperate regions). In consideration of the recent developments, it is clear that many basic concepts about the biogeography of terrestrial green algae will have to be reconsidered critically. A deep understanding of this topic will require considerable work on many aspects of the biology of these organisms (systematics, distribution, dispersal, physiology), in which species circumscriptions based on molecular data will be a mandatory requirement.