Scientists drilled down through more than a mile of underwater sediment and, very carefully, isolated the microbes that live in the coal seams down there. Just finding these microbes (while avoiding contamination) was impressive, so we don't know everything about them yet. But we do find them much more abundant in the coal layers, where they can eat the carbon-rich rocks.
Most of the characteristics of these extremely deep microbes are what we expected. For reasons discussed in Chapter 6 of A World from Dust, these microbes use nickel and heme to process their food, and they emit tiny clouds of methane. They seem slightly out of place, because a limited genetic analysis finds they look more like surface soil bacteria than shallow sediment bacteria, but they made have been carried there from when the sediments were first laid down.
The only real surprise so far is how few of these microbes live down there. There's plentiful food, including hydrogen that, for some reason, they eschew, leaving it on their plate like uneaten broccoli. Something else is holding life back down there. The authors note that proteins and DNA fall apart more readily at high temperature and suggest that the hot, high-pressure environment makes the constant repair too costly. If this is so, then a deep underwater coal seam would be like a planned community in Las Vegas after the housing bust of 2008: plenty of rooms but no one willing to move out there because of the heat.
What this says to me is that the limits on life look a little more stringent than we thought. Whether it's the cost of repair, or something else related to high pressure, or something else entirely, what we have here is a reasonably good environment for methanogens that they don't fully exploit. With more investigation, maybe we can find out what's missing, and that will help us figure out just how prevalent life may be in extreme environments, both on this planet and on others. If life can't live well in this environment, then the same thing may be wrong with other environments that right now look OK to us. This one clue makes the universe seem a bit more lonely.
Reference: Science, July 24, 2015, "Exploring deep microbial life in coal-bearing sediment down to ~2.5 km below the ocean floor." DOI: 10.1126/science.aaa6882