29,000,000,000,000,000,000,000,000,000

The ocean floor is home to 2.9×10²⁹ single-celled organisms — that's 10 million trillion microbes for every human on the planet — but vast though it is, this figure is only 8% of the previous estimate of 35.5×10²⁹

-- report 

Only 8% of the previous estimate is still plenty (!) and it exists alongside and/or interacts with/supports a huge amount of larger life even in the coldest darkest places. In Antarctic benthos, for example, researchers have found the Polychaete worm Spiophanes tcherniai  at densities 150,000 to 180,000 per square metre. (Hat tip DSN)