Abstract
Planothidium marganaiensis sp. nov. is described from periphytic samples collected at the karst spring San Giovanni at Domusnovas, Sardinia (Italy). Light and scanning electron microscope observations revealed that the taxon shares similarities with species within the Planothidium frequentissimum sensu lato, such as P. naradoense and P. victori. Planothidium marganaiensis seems to be morphologically closely related to P. frequentissimum but stands out for its smaller valves, lower striae density, subrostrate to slightly protracted apices, and areolation pattern of the striae in the rapheless valve (four to five areolae per stria). Planothidium marganaiensis was found in slightly alkaline waters presenting medium hardness and mineralization and phosphorus enrichment.