Abstract
Drivers of algal bloom dynamics remain poorly understood, but viruses have been implicated as important players. Research addressing bloom dynamics has generally been restricted to the virus-infection of the numerically dominant (i.e. bloom forming) taxa. Yet this approach neglects a broad diversity of viral groups, limiting our knowledge of viral interactions and constraints within these systems. We examined hallmark virus marker genes in metatranscriptomic libraries from a seasonal and spatial survey of a Microcystis aeruginosa bloom in Lake Tai (Taihu) China to identify active infections by nucleocytoplasmic large DNA viruses (NCLDVs), RNA viruses, ssDNA viruses, bacteriophage, and virophage. Phylogenetic analyses revealed a diverse virus population with seasonal and spatial variability. We observed disproportionately high expression of markers associated with NCLDVs and ssRNA viruses (consistent with viruses that infect photosynthetic protists) relative to bacteriophage infecting heterotrophic bacteria or cyanobacteria during the height of the Microcystis bloom event. Under a modified kill-the-winner scheme, we hypothesize viruses infecting protists help suppress the photosynthetic eukaryotic community and allow for the proliferation of cyanobacteria such as Microcystis. Our observations provide a foundation for a little considered factor promoting algal blooms.
Highlights
Harmful algal blooms (HABs) are extreme biological events that have detrimental environmental and socioeconomic effects on both fresh and salt waters
Freshwater HABs commonly manifest as a summer bloom of cyanobacteria that follows from a winter/spring population of eukaryotic phytoplankton (Ke et al, 2008; Niu et al, 2011; Edgar et al, 2016)
Biological success can arise during suppression of competitors, and there is less known about the decline of the eukaryotic population and factors that constrain these populations as cyanobacterial blooms form
Summary
Harmful algal blooms (HABs) are extreme biological events that have detrimental environmental and socioeconomic effects on both fresh and salt waters. There is less focus on numerically less-abundant protists that may be subject to virus-mediated suppression In part this may be due to the general consensus that viruses of prokaryotes are the most abundant and active in aquatic systems (Weinbauer, 2004), or that many of the viruses infecting protists contain RNA genomes (Moniruzzaman et al, 2017) and are not detected by DNA-based metagenomics. These “neglected” viruses have the potential to control community dynamics, reducing the success of competitors. In addition to observations of active eukaryotic viruses, determinations of putative host diversity and statistical co-occurrence analyses of “who infects whom” reveal potential advantages for Microcystis that account for part of its ability to form large bloom events in fresh waters
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