Table 1.

Five-order taxonomy of the Zygnematophyceae

Order Serritaeniales

S.Hess & J.de Vries ord. nov. Diagnosis: comprises unicells and filaments with smooth sidewalls, cells with axial or parietal chloroplasts, and simple cell walls (no pores and ornamentations), phylogenetically closely related to the type species (Serritaenia testaceovaginata; rbcL MW159377). Type:Serritaeniaceae S.Hess & J.de Vries fam. nov. Family Serritaeniaceae S.Hess & J.de Vries fam. nov. Diagnosis: with characteristics of order Serritaeniales; unicells and filaments with smooth sidewalls, cells with axial or parietal chloroplasts, and simple cell walls (no pores and ornamentations), embedded or not in the mucilage. Type:Serritaenia A.Busch & S.Hess 2021. Comment: Currently the Serritaeniales includes a single family, the Serritaeniaceae with the genera Serritaenia and Mougeotiopsis.

Order Zygnematales

Bessey emend. S.Hess & J.de Vries Emended description: Comprises unicells and unbranched and uniseriate filaments with smooth side walls, cells with stellate, plate- or ribbon-like chloroplasts and simple cell walls (no pores and ornamentations), phylogenetically closely related to strains SAG 698-1a (Genbank transcriptome shotgun assembly GFYA00000000). Type:Zygnema C.Agardh, 1817, nom. et typ. cons. Comment: Currently the Zygnematales includes a single family Zygnemataceae with the genera Cylindrocystis, Mesotaenium (current assumption, pending discovery of type species), Mougeotia, Zygnema, and Zygnemopsis. No culture is available from the type species of Zygnema.

Order Desmidiales

Bessey emend. S.Hess & J.de Vries Emended description: Comprises unicells and chain-like filaments. Cell walls and morphologies of diverse complexity, including the "placoderm desmids" with cell wall pores, ornamentations and clear isthmus, and species with smooth cell walls and without isthmus. Phylogenetically closely related to strain Desmidium aptogonum (RNA-seq ERX2100155). Type:Desmidium C.Agardh ex Ralfs, 1848. Comment: Currently the Desmidiales includes a single family Desmidiaceae with the genera Bambusina, Closterium, Cosmarium, Cosmocladium, Desmidium, Euastrum, Micrasterias, Netrium, Nucleotaenium, Onychonema, Penium, Phymatodocis, Planotaenium, Pleurotaenium, Staurastrum, Staurodesmus, Xanthidium, and more. No culture is available from the type species of Desmidium.

Order Spirogyrales

Clements emend. S.Hess & J.de Vries Emended description: Comprises filaments with smooth side walls, cells with one or more helical chloroplast and smooth cell walls without pores or ornamentation. Phylogenetically closely related to strain Spirogyra pratensis strain MZCH10213 (RNA-seq data: NCBI BioProject PRJNA543475, TSA GICF00000000). Type:Spirogyra Link, 1820, nom. cons. Comment: Currently the Spirogyrales includes only the genus Spirogyra. The closely related genus Sirogonium Kützing may also belong to this order, but this needs to be confirmed by phylogenomic studies. No culture is available from the type species of Spirogyra. The order Spirogyrales was originally validated by Clements (1909: 12); his description specified “Typically one-celled or filamentous algae, without zoospores; sexual reproduction by the conjugation of similar gametes; two fungous families.” No fungi are currently included in this order.