Thursday, October 28, 2010
Megavirus vs Bicosoecid
Viruses lurk at the hairy edge of what we generally consider to be 'living' things. Small infectious structures, they replicate only by digging into a host's intracellular environment and hitching a ride. The smallest are barely 10 nanometers across - the size of a wavelength of ultraviolet light - and coded for by only a few thousand nucleotide base-pairs (compared to the more than 3 billion that code a human). Vast numbers of viruses exist in nature. In marine environments there can be 250 million individuals per milliliter of water, most of them so-called bacteriophages - targeting microbial hosts. Viruses play fast and loose with genetic material, clipping, swapping, dropping and incorporating at a mind-boggling rate. There is no doubt that they have played a critical role in the molecular evolution of life on Earth over the past 4 billion or so years. Our human genome contains millions of fragments of viral DNA - accumulated by our distant, distant ancestors and ourselves. Viruses bring into question even the very notion of 'species', we are all molecular ragdolls, a button sewn on here, a piece of thread incorporated there.
A newly discovered virus steps up the ante (and adds to the zoo of such things). Infecting a single-celled marine organism known as Cafeteria roenbergensis (good name, apparently it's a voracious eater) this virus is a monster. Its genetic makeup is 730,000 base-pairs, with 500 regions that look like bona-fide genes, many likely involved in making protein structures - something most viruses don't bother with. Like all viruses it's not a cellular organism, but here it is doing much of what regular living things do. It not only seems to have genes that could help make cell membranes, it even seems to have stolen genes directly from bacteria.
The line between this remarkable structure and 'life' is thin indeed and I think makes it very clear that there is more of a continuum than any dramatic 'jump' between complex molecules and what we clearly recognize as organisms. We have barely scratched the surface in our understanding of the relentless activity of the microscopic world. Intriguingly this Megavirus infects one of the major predators of the marine microbial environment. In that context it's hitching a ride with an organism that hoovers up both bacteria and viruses as food. What an excellent smorgasbord of genetic material it must see, one cannot help but wonder if that opportunity has helped it build its own massive library.
In the quest for life in the cosmos, and particularly as we continue to poke around in our own solar system, we need to think carefully about what we are looking for. Fragments of DNA from a formerly aqueous environment on Mars - should they ever be found - would likely offer a window into this borderworld between molecules and self-contained organisms. It's going to be messy.