Hi All

Robert I think the bit about clathrates explains the oxygen's origin - it has accumulated from dissolved gases in the ice coming out of their molecular cages in the ice and into solution in the high pressure, cold water at the bottom. The fact that so much has accumulated means that there is little use of it by Lake Vostock's biota.

The oxygen and nitrogen had to come from somewhere and the fact is there is no other source than the atmosphere. On Europa the oxygen might come from ice convection carrying down from the irradiated surface layers above - but it can only accumulate if it isn't used.

Complex life needs A LOT more power than tidal forces and rock chemistry can provide. By way of comparison the tidal power dissipated by Earth's oceans is ~ 2.4 terawatts, but the Earth receives from the Sun (after ~ 30% bounces off) ~ 120,000 terawatts. Europa's tidal power is closer to the former and not the latter.

Adam