Comparisons between Complete Genomes of the Eukaryotic Extremophile *Galdieria sulphuraria* Reveals Complex Nuclear Chromosomal Structures

Abstract

Extremophiles, while typically bacteria and archaea, are also found in the eukaryotic domain of life. The eukaryote Galdieria sulphuraria is a thermoacidophilic red alga belonging to the class Cyanidiophyceae, an especially unique class as it comprises the basal clade of eukaryotic extremophiles. Galdieria species can grow both photosynthetically and heterotrophically on a variety of carbon sources, thriving down to pH 0 and temperatures up to 56 °C, while tolerating high levels of reactive oxygen species and high levels of heavy metals. Here we report whole-genome sequencing of three G. sulphuraria strains, uncovering a compact (13.1 Mb – 16.0 Mb) nuclear genome with 72-73 chromosomes, dependent on the strain.Comparative analyses of the macro synteny revealed significant structural rearrangement between G. sulphuraria isolates and the genome shows signs of sexual recombination. This, along with the large number of nuclear chromosomes compared to the genome size, reveals a mechanism of intrinsic adaptability in this eukaryotic extremophile, uncovering how G. sulphuraria can thrive in a rapidly changing extreme environment.