It's an idea that sounds straight from the latest science fiction blockbuster. But scientists at the University of Miami have warned that the world's oceans are teeming with'giant viruses', also known as giruses. Most viruses are less than 0.5 per cent the width of a human hair – too small to be seen with the naked human eye. In contrast, the researchers say that the giant viruses are five times bigger, rivaling bacteria in terms of size. Concerningly, all 230 giant viruses are previously unknown to science.

Some giant viruses encode a genome larger than that of some bacteria, but their evolutionary history is a mystery. Examining the genomes within a sample from a wastewater treatment plant in Austria, Schulz et al. assembled a previously undiscovered giant virus genome, which they used to mine genetic databases for related viruses. The authors thus identified a group of giant viruses with more genes encoding components of the protein translation machinery, including aminoacyl transfer RNA synthetases, than in other giant viruses. Phylogenetic analyses suggest that the genes were acquired in an evolutionarily recent time frame, likely from, and as an adaptation to, their hosts.