[The] diversity in the virosphere is also coming as a surprise. There are now thought to be around 100 million types of virus. They boast a more varied biochemistry than cellular life, storing their genetic information as both single and double-stranded DNA and RNA. Recent virus-hunting expeditions have uncovered one with a unique hybrid genome structure, part single-stranded and part double-stranded DNA, plus a menagerie of novel forms - bottle-shaped viruses, viruses with tails at both ends, viruses shaped like droplets and viruses that resemble stalk-like filaments. Most astonishing of all is the giant mimivirus, which is bigger than some bacteria. And we have only scratched the surface. "In terms of diversity, I don't think we even have an inkling yet what's out there," says [one] microbiologist.-- from Viruses: The unsung heroes of evolution by Garry Hamilton. See also:
...All in all, biologists are confronting what may be the biggest advance in evolutionary thinking since the discovery of the gene. Our emerging knowledge of viruses challenges many tenets of evolution, not least that it is driven by competition between selfish genes. Viruses provide a strong argument for the idea that evolution is also driven by fitness boosts gained through give and take.
Viruses have an important role in global biogeochemical cycles, in deep-sea metabolism and the overall functioning of the largest ecosystem of our biosphere.-- from Major viral impact on the functioning of benthic deep-sea ecosystems (doi:10.1038/nature07268).