There are some genomes that have been reported to be positively coiled instead of negatively supercoiled, which is the status of most genomes that we have studied. The genomes that are positively supercoiled seem to belong to viruses and cells that exist at very high temperatures. Why might positive supercoiling be an advantage at high temperatures

Positive supercoils prevent thermal denaturation, thereby avoiding the degradation of the virus genome.

Explanation:

DNA positive supercoiling is the process for which the DNA molecule is twisted strongly so double helix conforms a series of knots. All thermophilic species synthesize a type of reverse DNA topoisomerase referred to as DNA gyrase that has the ability to introduce positively supercoiled DNA coils. These positive supercoils make to the DNA molecule more resistant to high temperatures, and thus these organisms can survive in higher temperature conditions.