A brief introduction to the morphological species concept of Spirogyra and emanating problems

The zygnematalean genus Spirogyra is a common freshwater alga and easily recognized by its spirally coiled chloroplasts. The traditional species delimitation within Spirogyra is based upon morphology, considering both asexual and sexual stages of its life cycle. In this study, we identified 16 Spirogyra strains originating from 14 sampling sites of Central Europe down to the species level and compared our observations to species descriptions provided in monographs. This case study demonstrates the uncertainties underlying the exclusive application of the morphological species concept, which indicates that many Spirogyra species might have been described too narrowly. We therefore suggest a polyphasic approach including molecular, morphological and ecophysiological traits. We discuss recent findings on Spirogyra phylogeny based on molecular tools and provide a possible solution to handle the current problems of Spirogyra taxonomy.