THE PRASIOLALES (CHLOROPHYTA) OF ATLANTIC EUROPE: AN ASSESSMENT BASED ON MORPHOLOGICAL,MOLECULAR, AND ECOLOGICAL DATA,INCLUDING THE CHARACTERIZATION OF ROSENVINGIELLA RADICANS (KÜTZING) COMB. NOV.1

Despite a simple morphology and intensive studies carried out for more than two centuries, the systematics of the Prasiolales still presents several unsolved problems. The taxonomic relationships of several common species of Prasiolales, mostly from northern Europe, were investigated by a combination of morphological observations, culture experiments, and molecular analyses based on rbcL sequences. The results indicate that Rosenvingiella and Prasiola are separate genera. The capacity for production of tridimensional pluriseriate gametangia and the presence of unicellular rhizoids are the morphological features that discriminate Rosenvingiella from filamentous forms of Prasiola. The molecular data indicate that uniseriate filaments can be produced in at least three different species of Prasiola. The genetic diversity of uniseriate filamentous Prasiolales is higher than their simple morphology would indicate, and the provisional retention of Schizogonium Kützing 1843 as independent genus is recommended. The rbcL phylogeny confirms that Prasiola calophylla, P. crispa, and P. stipitata are distinct species, whereas P. stipitata and P. meridionalis are probably conspecific. Rosenvingiella polyrhiza is a strictly marine alga, and most records of Rosenvingiella in Europe are referable to Rosenvingiella radicans, proposed here as a new combination based on Ulothrix radicans Kützing 1849 . This is a primarily terrestrial alga that can occur from upper intertidal rock to locations situated hundreds of kilometers inland. The great confusion that has arisen in Europe between these two species in the last century is mostly due to misidentifications of marine populations of R. radicans.