Johansenicoccus eremophilus gen. et sp. nov., a novel evolutionary lineage in Chlorophyceae with unusual genomic features

Abstract

Background – Green algae are a diverse group of photosynthetic eukaryotes, yet are still vastly understudied compared to land plants. For many years, green algae were characterized based on their morphology and life cycles. More recently, phylogenetic and genomic analyses have been added to the phycological toolkit for a better understanding of algal biodiversity and evolutionary history. Material and methods – A desert strain of green algae was isolated from Joshua Tree National Park (JTNP) in southern California as part of a larger biodiversity survey. The alga’s nuclear rRNA genes as well as the chloroplast genome were sequenced, annotated, and analysed in addition to a morphological assessment. Results – Morphologically this strain is especially similar to Pseudomuriella and Rotundella, and its lipid profile resembles that of other soil algae, but phylogenomic analyses demonstrate that it is a distinct evolutionary lineage in Chlorophyceae. The alga exhibits several unusual genomic features, the most remarkable being its highly derived yet apparently functional nuclear rRNA genes, 18S and 28S. Both genes are GC-rich and bear many compensatory base changes to maintain a similar secondary structure to that of other green algae. The chloroplast genome has a distinct gene order and repeat arrangement from other published green algal plastomes, but contains the expected genes and also provides phylogenetically informative data. Conclusion – We conclude that the strain be placed into a new species and genus in the class Chlorophyceae, and propose the name Johansenicoccus eremophilus for this new taxon. Johansenicoccus eremophilus exemplifies science’s insufficient understanding of the range of genomic variations among inconspicuous soil algae.