Picochlorum species (Trebouxiophyceae) are small halotolerant and thermotolerant unicellular green alga with multiple biotechnological applications. Here, we sequenced the 13.5-Mb of P. costavermella, isolated from the estuary of a river flowing into the Mediterranean Sea.. genome of Picochlorum costavermella

Picochlorum species (Trebouxiophyceae) are small halotolerant and thermotolerant unicellular green alga with multiple biotechnological applications. Here, we sequenced the 13.3-Mb genome of P. costavermella, isolated from the estuary of a river flowing into the Mediterranean Sea. Comparative genome analysis revealed a severe genome reduction, first by a reduction of intergenic size in both nuclear and organellar genomes and by a gene family contraction. In P. costavermella cultures grown long-term at 35°C, more of the gene families containing over-expressed members were recently evolved than expected and less originated at the ancestor of green plants. Inversely, there was an excess of ancient gene families containing transcriptionally repressed genes. Moreover, more than a third of the gene families up-regulated at 35°C were also up-regulated under variable salt concentrations in Picochlorum SE3 and a third of the down-regulated gene families were commonly down-regulated in both conditions. Phylogenetic analysis revealed twenty-six genes of horizontally transferred origin in P. costavermella, of which five were differentially expressed at higher temperature. Altogether, these results give new insights about how the genomic basis of adaptation to halo- and thermotolerance evolved in the green lineage prior to the evolution of land plants.