Viruses of Rhodophyta: lack of cultures and genomic resources pose a threat to the growing red algal aquaculture industry

There is a growing global demand for algal-derived products as they offer alternatives that can help to mitigate climate change, support coastal biodiversity and reach food sovereignty. Red algae (Rhodophyta) have been cultivated for hundreds of years and currently support an economically important industry worldwide. However, pathogen outbreaks pose a major potential threat to this growing industry, in some cases already causing more than $1 million loss annually, as in Pyropia (Bangiaceae) farms. Some of the most destructive algal pathogens, although poorly understood, are viruses. They are highly diverse and abundant, exerting strong pressure on the life cycle of their hosts. Knowledge of red algal viruses has developed at a much slower pace than for their green algal counterparts, even though it was in a red algal species, Sirodotia tenuissima (Batrachospermaceae), that the first observation of viruses in eukaryotic algae was made. Furthermore, it was only in 2016 that the first Rhodophyta viruses were isolated, and to date only three have sequenced genomes, all RNA viruses, isolated from the hosts Delisea pulchra, Chondrus crispus and Pyropia suborbiculata. Given that viruses are prevalent and constitute major threats to all groups of life and that genomic resources are still lacking, they could pose a serious menace to ongoing aquaculture endeavours. Therefore, here we report a thorough and comprehensive (though not exhaustive) review of studies on Rhodophyta viruses, suggesting a set of propositions to advance knowledge in the field and to encourage more active focussed investigation. This will benefit not only red algal aquaculture but also shed light on the associated viral diversity, evolution and viral impacts on the life history of their red algal hosts.