Arnoldiella elbakyaniae sp. nov. is described based on epilithic material collected in a cascade of a stream in Northeastern Brazil, Piauí State. The stream belongs to the Parnaíba river basin, which encompasses one of the most poorly sampled regions in the country. We also propose here the combination of Basicladia emedii C.K. Peres & C.C.Z. Branco into Arnoldiella emedii comb. nov., following the new arrangement proposed for the group. Arnoldiella elbakyaniae can be recognized by the heterotrichous thallus with short erect, unbranched or sparsely branched filaments, and by the short and enlarged basal cells of the erect system. The new species differs from A. emedii by its shorter basal cells, the narrower and cylindrical axis cells, and the zoosporangia in terminal chains.
Arnoldiella emedii; Caatinga; green algae; lotic; tropical