Abstract
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.
Highlights
Chlorella vulgaris is the first documented Chlorella green alga discovered by Beijerinck in 1890 [1]
On and from which lipid droplets form and microfibrils emanate into the extracytoplasmic space respectively (Figure 2(C), Figure 2(D)), indicating that the double-membraned cytoplasmic envelope serves the functions of endoplasmic reticulum and Golgi apparatus for synthesizing lipids and building materials of cell wall
The presence of typical chlorophyte pigments of chlorophyll a (Chl a), Chlorophyll b (Chl b) and lutein in TDX16-DE (Figure 4) indicates that TDX16-DE belongs to the Chlorophyta, and the highest similarity of 99.7% between the 18S rRNA gene sequences of TDX16-DE and C. vulgaris suggests that TDX16-DE is most similar to C. vulgaris
Summary
Chlorella vulgaris is the first documented Chlorella green alga discovered by Beijerinck in 1890 [1]. About 1000 orbicular-shaped Chlorella and Chlorella-like green algae have been isolated [2]. Xing coccoid chlorophytes is a difficult task, because their origins and evolutionary histories/relationships, the crucial information necessary for their accurate assignments, are unknown In this circumstance, the Chlorella and Chlorella-like green algae are classified traditionally by comparing the degrees of resemblance of their morphological, biochemical, physiological and ultrastructural features [3]-[10] and currently by inferring their origins and evolutionary histories/relationships via phylogenetic analyses of the molecular data, e.g., sequences of the SSU and ITS regions of nuclear-encoded ribosomal DNA [11]-[23]. The taxonomy of Chlorella and Chlorella-like green algae is problematic and unstable, as indicated by the frequent taxonomic revisions
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