Oxidants from above
Hi Larry
The hydrothermal vent animals need oxygen in the water, which has come from the photic regions above. On Europa there's probably a steady supply of oxidants made by radio-chemical processing of the crustal ice, so there could be a pretty vigorous biosphere there...
The Ocean of Europa
Enceladus, and other icy moon oceans, there's somewhat less oxidant supply and less likelihood of a decent biosphere. The irony of Europa is the possible lack of reductants, but fingers crossed...