A new Patagonian Cosmarium species motivating distinction of a separate biogeographical desmid region

Cosmarium valerioënse is newly described from Laguna Valerio, situated on the Patagonian highland plateau Somuncurá. It is marked by a highly inflated semicell centre resulting in a remarkably quadrilobed apical cell view. In this feature it corresponds to some other Cosmarium species recently described from this climatically harsh part of the world. One of those species, viz. Cosmarium chapuense, was also encountered in Laguna Valerio showing a rather high degree of morphological variability. The exclusive phenomenon of such 'quadrilobate' Cosmarium species may give rise to distinguish a separate biogeographical region within the South American continent.