by Merry Youle
Hic sunt dracones
Figure 1. The marine macrobial dark matter. Hic sunt dracones. Source.
We live in a world run by microbes, the vast majority of which we have yet to identify or name. We can only refer to them collectively as the microbial dark matter (MDM). However you define a prokaryotic species, and however you tally them once identified, there is a huge gap between the 12,000 or so validly-named species and the total number on our planet, currently estimated to be in the millions. The only evidence we have for the existence of that uncultured mob is either a small subunit ribosomal RNA (SSU rRNA) sequence or some hazily-classifiable metagenomic reads. As the speed of sequencing goes up and the cost goes down, this sort of evidence accrues ever more rapidly, further widening the gap. The challenge at hand is to find out more about the organisms that make up that dark matter.