Chroococcidiorella tianjinensis, gen. et sp. nov. (Trebouxiophyceae, Chlorophyta), a green alga arises from the cyanobacterium TDX16

All algae documented so far are of unknown origin. Here, we provide a taxonomic description of the first origin-known alga TDX16-DE that arises from the Chroococcidiopsis -like endosymbiotic cyanobacterium TDX16 by de novo organelle biogenesis after acquiring its green algal host Haematococcus pluvialis ’s DNA. TDX16-DE is spherical or oval, with a diameter of 2.0-3.6 µm, containing typical chlorophyte pigments of chlorophyll a, chlorophyll b and lutein and reproducing by autosporulation, whose 18S rRNA gene sequence shows the highest similarity of 99.7% to that of Chlorella vulgaris . However, TDX16-DE is only about half the size of C. vulgaris and structurally similar to C. vulgaris only in having a chloroplast-localized pyrenoid, but differs from C. vulgaris in that (1) it possesses a double-membraned cytoplasmic envelope but lacks endoplasmic reticulum and Golgi apparatus; and (2) its nucleus is enclosed by two sets of envelopes (four unit membranes). Therefore, based on these characters and the cyanobacterial origin, we describe TDX16-DE as a new genus and species, Chroococcidiorella tianjinensis gen. et sp. nov., which sets the basis for multidisciplinary research.