"he biomass in the deep subsurface is about 15% of the total for the biosphere"
I'd bet that it's a higher percentage. That 15% represents what we have already found. There is likely more at greater depths we haven't found, and perhaps other types of life we aren't yet recognizing yet (a parallel to finding Archaea),
>Assuming that three miles was the limit, and that only 1 percent of the total pore space available in the rocky crust was occupied by microbes, then the total mass of living material there would be about 200 trillion tons. If that material were spread over all the Earth's land surfaces, Dr. Gold wrote, the global layer of microbial sludge would be nearly five feet thick. "This would indeed be more than the existing surface flora and fauna,"...(https://www.nytimes.com/1993/12/28/science/strange-new-micro...)