The extremely large chloroplast genome of the green alga Haematococcus pluvialis: Genome structure, and comparative analysis

Abstract

Astaxanthin is arguably nature's most potent antioxidant, powerful to prevent free radical damage and oxidative stress. The microalga Haematococcus pluvialis seems to accumulate the highest levels of astaxanthin in nature and thus is currently the primary industrial source for natural astaxanthin production. The accumulation happens under photooxidative stress to quench radical damage from the photosynthesis, which is considered as one of the main reasons why chloroplast genomes are usually kept the small size and high guanine-cytosine (GC) content. We hypothesize that the outstanding astaxanthin production of the species may reflect its chloroplast genome features. In this study, the complete chloroplast genome of H. pluvialis FACHB-712 was sequenced and investigated. The genome is large, comprising 1.35 Mbp with 83 genes. However, the GC content is counterintuitively as high as 50.1%. The genome shows slight phylogenetic distance and genome structural differences in comparison with that of H. lacustris, whose species nomenclature is confusing and controversial. Moreover, a possible correlation between its powerful antioxidant capacity and the large chloroplast genome maintenance was further investigated. Strains of Chlorophyta and several strains with chloroplast genomes size of more than 500 kbp were used for comparative analysis. The evolutionary mechanism for the large chloroplast genome and and the contribution of non-coding region ratio and GC content to genome amplification is discussed.Large genomes may be associated with harsh habitats, and increased GC content may be used to increase stability.