Free-floating Ulva in the southwest Netherlands: species or morphotypes? Amorphological, molecular and ecological comparison

Free-floating Ulva L. biomass in the eutrophic brackish 'Veerse Meer' lagoo n (southwest Netherlands) consists of four morphologically identified speci es: U. curvata (Kutzing) De Toni, U. lactuca L., U. rigida C. Agardh and U. scandinavica Bliding. U.curvata could be recognized easily because of the c haracteristic central cavity in the holdfast of the attached plants, the ar rangement of cells in rows and the single pyrenoid in each cell. U. rigida was distinguished by the thick thallus and the large number of pyrenoids. T he Veerse Meer isolate, however, was slightly different from the isolate fr om the Oosterschelde estuary (the Netherlands). U. lactuca and U. scandinav ica showed a high degree of overlap in thallus thickness and cell size, but U. scandinavica usually possessed more pyrenoids. However, doubts have fre quently been expressed about the use of some morphological characters in Ul va taxonomy. To determine the Validity of such characters in the identifica tion of Ulva species, the morphological variation within and between morpho logical species was recorded and a molecular data set generated. To detect possible ecophysiological differences between species, optimum temperatures and salinities for growth were determined experimentally. The sequences of the nuclear ribosomal DNA internal transcribed spacer 2 (ITS2) and flankin g regions of U. lactuca, U, rigida and U.scandinavica from the Veerse Meer were all identical, but differed from that of U. rigida from the Oostersche lde estuary. Ulva species from the Veerse Meer were most closely related to U.armoricana and U. rigida from Brittany (2.9 % and 3.5 % divergence respe ctively); the difference between U. rigida from the Veerse Meer and from th e Oosterschelde estuary was 7.5 %. Rooted trees, based on a comparison of t hese sequences with sequences of other Ulva and Enteromorpha species obtain ed from the literature, using Monostroma arcticum as outgroup, suggested th at Ulva is paraphyletic with respect to Enteromorpha. The optimum temperatu re for growth of U. curvata was 25 degrees C; for all other species it was 10 degrees C. The optimum salinity for growth was 30 degrees C for all isol ates. It is concluded that U. lactuca, U rigida and U. scandinavica from th e Veerse Meer are all members of one highly polymorphic species.