Connection between microbes and serpentinite?
The most common explanation for microbial activity in serpentinites is the exploitation of abiogentic hydrogen and methane formed during the serpentinisation process.

Quoting the abstract of a recent authoritative review:


  The process of serpentinization creates strongly reducing conditions and produces fluids that are highly enriched in molecular hydrogen and methane. Some microorganisms are able to exploit these compounds to gain metabolic energy and to generate biomass, leading to the development of biological communities based on chemical energy rather than photosynthesis. The abundance of chemical energy and favorable conditions for organic synthesis make serpentinites a strong candidate for the site of the origin of life on Earth, as well as a prime target in the search for life elsewhere in our Solar System.


From Serpentinites, Hydrogen, and Life (may be paywalled).

You might also find this review interesting: Serpentinization, Carbon, and Deep Life (should be free to access).

These two papers present the current thinking in this subject.