> The presence of dissolved O2, nitrate (NO3−), phosphate (PO4−), and dissolved inorganic carbon (DIC) throughout the sedimentary sequence from the seafloor to the volcanic basement indicates that cell abundance and activity are not limited by availability of electron acceptors or dissolved major inorganic nutrients. From the Redfield stoichiometry of net dissolved O2 reduction to net nitrate production in the sediment, the microbial cells have been inferred to consume oxygen coupled to oxidation of marine organic matter at extremely slow rates.

-- D’Hondt, S. et al. Presence of oxygen and aerobic communities from sea floor to basement in deep-sea sediments. Nature Geosci. 8, 299–304 (2015).

As they were buried deeper and deeper in the sediment, presumably oxygen levels fell lower and lower.

That will have caused selective pressure to manage to survive with so few resources. Those that can't simply become energy for others who can.

There are cells that don't need oxygen to survive. The fact that there is no oxygen in the environment doesn't mean that certain forms of bacteria cannot survive.