Coevolution between parasites and their hosts is an endless war. Viruses are the most confrontational provocateur of evolution.
These parasites impose a tight control over the composition of marine microbial communities. ~ French virologists Frédéric Partensky & Laurence Garczarek
There are typically 100 billion viral particles per liter of water in the top 50 meters of most marine ecosystems. Marine viruses outnumber bacteria 10 to 1.
A cyanophage is a virus that infects cyanobacteria. The impact that cyanophages have on victim populations differs. They exert the strongest predation pressure on the most abundant bacteria. Scarcer species are much less affected. Thus, through their culling, viruses help maintain bacterial diversity.
Different strains of the same bacteria have genomic variances that create ranges of susceptibility or resistance to viruses. Hence, only a fraction of a population is killed when attacked by a cyanophage horde.
Several cyanobacterial genes are found in viral genomes. Most often these are employed in energy metabolism, such as photosynthesis.
The viruses themselves may optimize the functioning of these genes. During infection, viruses hijack the translational machinery of their hosts and make the bacteria express the viral version of the genes. This strategy helps maintain host metabolism long enough for the viral replication cycle to be completed.
These viral optimizations may then be picked up by other viruses, and by their bacterial targets. This gyral dynamic drives evolution for both. For cyanobacteria, what doesn’t kill you might make you and your offspring stronger.
Microbes are not the only ones who practice horizontal gene transfer as an intelligence exercise and self-determine their own evolution. Parasitic plants monitor their hosts and selectively pilfer genes. They also themselves evolve genes which mimic their hosts. This diligence pays off in maintaining the ability to evade host defenses and improve nutrient extraction as needed.