Just how many species are there? Measuring the earth's biodiversity is a tricky thing, since most of it is bacterial. If you were going to check anywhere, though, the ocean is probably a good place to start. The first indications of life are from ancient shorelines, meaning that life has probably had the longest time to diversify in the seas. The ocean is also a very hospitable habitat, with samples suggesting that there are between 104 and 105 bacteria in every mililiter of water. But that density of life brings up its own issues: how do you get a grip on all of those bacteria? So far, it's been done by estimating species by DNA sequencing of common genes, but that's limited by manpower and money—there are only so many sequences that we can afford to obtain and analyze. The authors of a new survey of diversity (open access PDF) summarize the results of earlier surveys as follows:
Two inescapable conclusions emerge from these phylogenetic, genomic, and metagenomic analyses: (i) microbes account for the majority of genetic and metabolic variation in the oceans and (ii) the genetic diversity, community composition, relative abundance, and distribution of microbes in the sea remain under-sampled and essentially uncharted.
They then go on to do their best to chart it. Using a new, massively-parallel sequencing technology (which we've noted before), they have built the most complete survey of marine bacterial diversity to date. They sequenced nearly 120,000 individual gene fragments from different regions of two sites: deep in the Atlantic, and at a Pacific mid-oceanic ridge. The results are remarkably consistent, in that nearly half of the sequences in every single sample represented a unique species. A few species would typically dominate the sample (one genus accounted for nearly 50 percent of the sequences in one of the samples), but the diversity of rare species was staggering.
The authors propose that, within the larger oceanic ecosystem there is a "rare biosphere" that is comprised of species that, due to their miniscule population size, are essentially immune to the competition and predation of the dominant species. They suggest that the rare biosphere makes for a more robust ecosystem, as some of those rare bugs will almost certainly find that any environmental changes to their liking. In other words, even if an environmental catastrophe wipes out the dominant species, something will almost certainly be able to do well under the new conditions.