Adaptive evolution of viruses infecting marine microalgae (haptophytes), from acute infections to stable coexistence

Collectively known as phytoplankton, photosynthetic microbes form the base of the marine food web, and account for up to half of the primary production on Earth. Haptophytes are key components of this phytoplankton community, playing important roles both as primary producers and as mixotrophs that graze on bacteria and protists. Viruses influence the ecology and diversity of phytoplankton in the ocean, with the majority of microalgae-virus interactions described as 'boom and bust' dynamics, which are characteristic of acute virus-host systems. Most haptophytes are, however, part of highly diverse communities and occur at low densities, decreasing their chance of being infected by viruses with high host specificity. Viruses infecting these microalgae have been isolated in the laboratory, and there are several characteristics that distinguish them from acute viruses infecting bloom-forming haptophytes. Herein we synthesise what is known of viruses infecting haptophyte hosts in the ocean, discuss the adaptive evolution of haptophyte-infecting viruses -from those that cause acute infections to those that stably coexist with their host - and identify traits of importance for successful survival in the ocean.