Lightless Life 

A correspondent brought THIS ARTICLE to my attention. It is based on THIS ACADEMIC PAPER.

Deep underground, abundant microbes produce oxygen in the absence of light - something which was once thought to be impossible. 

This discovery was made while looking at deep aquifers in Alberta, Canada. These have long been studied as they are essential to agriculture in the province. Much was known of their composition, nothing of their microbiology.

And what was found was that some bacteria split nitrites to produce oxygen.

It is estimated that there is more microbes below ground than on the surface!

Lots more in both articles - well worth a read.