Here's a website with an estimate that maybe 10% of life on earth is bacteria in rock strata kilometers below the ocean floor. That was the result of a quick search. Maybe not authoritative.
But these are certainly not all autotrophic. There's a lot of organic stuff in waters, sometimes thousands of ppm, especially in oil fields, to feed on. I emailed Blankenship, who BTW is a good, sound scientist, to ask if he knows if there are volatile fatty acids in the water from these vents. No answer so far.