“Quasigenus” among Phycodnaviridae: A diversity of chlorophyte-infecting viruses in response to a dense algal culture in a high-rate algal pond

This study approaches a high rate algal pond (HRAP) culture by metagenomic sequencing of the viral DNA fraction, this includes the so-called giant virus fraction (phylum Nucleocytoviricota), with the goal of revealing viruses coexisting within an intensified algal culture. A wealth of interesting novel viruses is revealed, including members of Nucleocytoviricota, Lavidaviridae, and polinton-like viruses, which are taxa containing previously characterized algal viruses. Our sequencing results are coupled with a virus targeted qPCR study and 18S rDNA metabarcoding to elucidate potential virus-host interactions. Several species of green algae are identified (Chlorophyta), likely representing the alternating dominant populations during the year of study. Finally, we observe a bloom of viral diversity within the family Phycodnaviridae (Nucleocytoviricota), including highly related but non-identical genotypes, appearing in the HRAP in September and October 2018. This bloom is most likely the cause of a mass mortality event of the cultured algae that occurred during these same months. We hypothesize that these related Phycodnaviridae lineages selectively infect different strains of the same algal species of the Genus Picochlorum that have been identified in the HRAP by metabarcoding and coined this phenomenon a “quasigenus” by analogy to the RNA virus quasispecies concept.