Viruses in the family Phycodnaviridae infect aquatic algae and they are present in inland, coastal and marine environments throughout the world, sometimes in very high concentrations. Consequently, phycodnaviruses contribute to microbial composition and diversity, nutrient cycling, carbon flow, and other biogeochemically-important processes in aqueous environments. The viruses are relatively large and have 170–450 kbp double-stranded (ds) genomes that encode ~200 to ~550 proteins and as many as 16 tRNAs. The algal viruses encode many interesting and unexpected proteins but over half of the predicted virus-encoded proteins do not match proteins in the databases and so their functions are unknown.