Biodiversity and biogeography of microalgae: progress and pitfalls

The role of microbial diversity in ecosystem functioning is becoming increasingly recognized. Microalgae are a large group of organisms that are extremely diverse and heterogeneous from evolutionary and ecological viewpoints. However, estimates of their diversity are nebulous and require substantial input from systematics, dispersal analyses, and biogeography. Our generally poor understanding of microalgal biogeography has generated several hypotheses. According to one view, microalgae are cosmopolitan. However, use of advanced morphometric and molecular techniques suggest a restricted distribution (i.e., endemism) for many microalgae that were previously considered as cosmopolitan. The quest to have an exclusive theory of microalgal biogeography depends upon efforts to solve the problems related with appropriate taxonomic unit, genotype–morphospecies relationship, extent of synonymy in known species, degree of coexistence of endemic and cosmopolitan species, reasons for apparent restricted distribution, ...