Viruses of the habitat-forming kelp ecklonia radiata of Sydney, Australia

Abstract

Marine environments and organisms harbour an enormous diversity and abundance of viruses, which increasingly are being shown to play a crucial ecological role in the oceans. Interactions between viruses and some marine hosts such as bacteria, phytoplankton or corals are beginning to be understood, but the role of viruses in many other marine systems is largely unexplored. This includes macroalgal forests comprised of kelps and other seaweeds, the dominant habitat-forming organisms on temperate rocky shores. Viral diversity and function in seaweed forests is little studied, although viral pathogens have been suggested to be associated with global declines in these algae. The aims of this research were to 1) describe the DNA virome of a dominant Australian brown alga, the kelp Ecklonia radiata, and to then 2) analyse the virome for possible indications of elements that are possibly associated with pathogenesis. To address these aims, transmission electron microscopy and comparative metagenomic analyses were carried out on tissue samples from asymptomatic (healthy) and abnormal (bleached) E. radiata from two sites near Sydney, Australia. All samples revealed virus-like particles of many sizes, all with rounded morphology. Viromes contained bacteriophages similar to taxa typically found in oceanic planktonic samples, such as the Caudovirales and the Phycodnaviridae, which dominated the eukaryotic viral component in all samples. Further analysis of the viromes revealed genes and potential functions known from other brown algae and their viruses, including genes involved in signal transduction, influx and efflux of metabolites, and the growth and reproductive cycle of the host. Most viral taxa did not differ significantly in abundance overall between non-bleached (healthy) and bleached kelp tissues, but the profiles of functional genes contained within8 the viromes differed between kelp from different locations. One group of small ssDNA viruses, the Circoviridae, were significantly overrepresented in bleached kelp viromes. This is the first elucidation of the virome of a kelp through shotgun sequencing of the entire viral fraction of healthy and putatively diseased tissue, providing insights into the interaction of the kelp host and its viruses and a starting point for investigation into viral pathogenesis in kelp.