One green algal specimen from China was identified as Gongrosira fluminensis Fritsch, due to its unique morphology that pseudoparenchymal basal stratum with rounded or polygonal cells grew into dense little-branched upright threads that reached approximately the same height with specific akinete formation and germination and formed strong cushions without calcification. Examination of the ultrastructural characteristics of plasmodesmata and pyrenoid confirmed that Gongrosira fluminensis Fritsch should be excluded from the order Chaetophorales. The phylogenetic evidence based on DNA sequence data from the nucleus (18S rDNA, ITS rDNA) and chloroplast (tufA) sequences clearly revealed that the Gongrosira fluminensis Fritsch should be classified in the Ulvales (Ulvophyceae) as the new combination species of the genus Lithotrichon Darienko et Pröschold, instead of the Chaetophorales (Chlorophyceae). More specimens in conjunction with natural morphological investigation and molecular analyses are required to reevaluate the microfilamentous genus Gongrosira Kützing and reveal hidden diversities among the Ulvophyceae.

No abstract