One step closer to eliminating the nomenclatural problems of minute coccoid green algae: Pseudochloris wilhelmii, gen. et sp. nov. (Trebouxiophyceae, Chlorophyta)

Abstract

‘Chlorella’ and ‘Nannochloris’ were traditional genera of minute coccoid green algae with numerous species described in the past century, including isolates used as experimental test organisms. In the last few years, the introduction of DNA-based phylogenetic analyses resulted in a large number of taxonomic revisions. We investigated and reclassified a taxonomically problematic group within the Trebouxiophyceae (comprising ‘Nannochloris eucaryotum’ UTEX 2502, ‘N. eucaryotum’ SAG 55.87 and ‘Chlorella minutissima’ SAG 1.80), distantly related to the recently described Chloroparva isolates (97.5–97.9 % 18S rRNA gene pairwise similarity). Cryopreserved material of SAG 55.87 was selected as holotype for a novel species – Pseudochloris wilhelmii Somogyi, Felföldi & Vörös – whose phylogenetic position confirmed the proposal of a new genus. Pseudochloris wilhelmii had spherical to oval cells with an average diameter of 2.6 × 2.8 µm and a simple ultrastructure characteristic of small green algae. Vegetative cells sometimes contained several lipid droplets occupying a large portion of the cells. The cell wall consisted of an outer trilaminar layer and an inner microfibrillar sheet. Cells divided by autosporulation, forming two or four daughter cells per autosporangium. The pigment composition was typical of green algae, with chlorophylls a and b, and lutein as the dominant carotenoid.

Key words:

• 18S rRNA gene analysis
• brackish
• Chlorella
• coccoid chlorophytes
• green algae
• marine
• Nannochloris
• picoeukaryote
• Pseudochloris wilhelmii
• taxonomic revision

Acknowledgements

The study was sponsored by the Hungarian Research Fund (OTKA K 73369). The authors are grateful to Balázs Németh and Éva Koltai for their technical assistance and to Csilla Jónás for skilful assistance in electron microscope sample preparation and ultrathin sectioning. The authors thank Professor C. Wilhelm for his help in networking for the pigment analysis. We are grateful to Károly Pálffy for the correction of English grammar, and to the anonymous reviewers and the Editors for their helpful suggestions. Boglárka Somogyi and Tamás Felföldi were supported by the Bolyai János Research Grant (Hungarian Academy of Sciences).

Supplementary information

The following supplementary material is available for this article, accessible via the Supplementary Content tab on the article’s online page at http://dx.doi.org/10.1080/09670262.2013.854411

Table S1. Main properties (habitat, morphological and phylogenetic attributes) of pico-sized (< 3 μm) coccoid chlorophytes within the Trebouxiophyceae.

Fig. S1. Transmission electron micrograph showing the Golgi apparatus (white arrowhead) in a vegetative cell of SAG 55.87. C, chloroplast; CW, cell wall; N, nucleus; PM, plasma membrane; T, thylakoid. Black arrowhead indicates the outer trilaminar layer of the cell wall. Asterisk indicates a starch grain. Scale bar represents 0.2 μm.

Supplementary file. Alignment of 18SrDNA sequences (fasta format).