Insights into algae evolution for adapting to blue-green light garnered from the Isochrysis galbana genome
Isochrysis galbana is an important producer in the aquatic ecosystem because of its rich fucoxanthin content and fast growth. However, little is known about its evolutionary adaptation to live in a specific, complex and harsh environment. We report a high-quality genome sequence of I. galbana LG007, which has a 92.73 Mb genome size, a contig N50 of 6.99 Mb and 14,900 protein-coding genes. Phylogenomic inferences confirmed the monophyly of Haptophyta, showing I. galbana is a sister to E. huxleyi and C. tobinii. Evolutionary analysis revealed an estimated divergence of I. galbana from its close relative E. huxleyi ~133 million years ago, and I. galbana underwent one round of whole-genome duplication. Genes related to environmental adaptation and metabolic regulation in I. galbana were relatively conserved, but the basic transcriptional regulation toolkit for terrestrial plant development has been contracted or was not detected. The domain identification of one novel fucoxanthin biosynthesis gene that encodes diadinoxanthin-fucoxanthin hydroxylase (DFH) was investigated. Comparative genomic analysis revealed that I. galbana acquired several transcription factors specific to the control of pigment accumulation and production of LHCX2 to harvest blue-green light, which facilitates adaptation to the underwater environment. These findings provide new insights into the genomic characteristics of I. galbana and algae evolution for adaptation to blue-green light underwater.