The common planktonic green algal genus Nephrocytium was traditionally assumed to have a close relationship with the genus Oocystis and accordingly has been included in the family Oocystaceae. Although the position of Nephrocytium within the Oocystaceae has differed according to some authors over the years, its inclusion in the family has not been questioned. Following molecular studies of Oocystis, the Oocystaceae (including Nephrocytium) was removed from the class Chlorophyceae and placed in the Trebouxiophyceae. However, recent molecular studies of some of the former Oocystaceae members have returned them to the Chlorophyceae. These studies suggested the placement of Nephrocytium in the Sphaeropleales, but no taxonomic positioning within the order has been determined for the genus. The relocation of Nephrocytium agrees with a strong morphological traitit lacks the oocystaceaean multilayered ultrastructure of the cell wall. Based on molecular markers (18S rDNA and tufA), and optical and electron microscopy, the present study aimed to establish the relationships of the genus within the Sphaeropleales. The results support the recognition of a new family, the Nephrocytiaceae, to accommodate Nephrocytium. Furthermore, we have carried out a review of previously described Nephrocytium species and based on morphological and molecular data we propose the description of Nephrocytium vieirae sp. nov.

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