The genus Tribonema (Xanthophyceae) in the Netherlands An integrated field and culture study
Lokhorst, Gijsbert M.
Comparative studies of field collections and uni-algal cultures have given an improved classification of the genus Tribonema (Xanthophyceae) in the Netherlands. In total 10 Tribonema species are reported including Tribonema hormidioides (Vischer) comb. nov. (Basionym: Heterothrix hormidioides Vischer). Despite considerable overlapping variation, diagnostic value is attached to the cell dimensions (cell width as well as cell length/width ratio), cell shape, number of chloroplasts per cell, developmental pattern of terminal cells in juveniles and reproductive behaviour of the species in culture. In some cases, the chloroplast size in relation to cell volume is also a helpful character. The distribution pattern of the Tribonema species in the field is briefly described. Owing to the finding of Xanthonema characteristics in Tribonema species, the generic boundaries between these closely related genera are evaluated. Transmission electron microscopy shows that the cell wall of both genera is composed of overlapping H-shaped segments, but that the typical end(s) of broken, healthy filaments ("zweispitzig" in Tribonema versus non-"zweispitzig'in Xanthonema) is seemingly caused by differently structured cross walls.