How do you go about building a working Earth-like ocean ecosystem from scratch in Europa's subsurface ocean?
On Earth, abyssopelagic species have evolved to live more than 6km below the surface. Although Europa's ocean is estimated to be up to around 200km deep, hydrostatic pressure at the seafloor would be 130-260 MPa, corresponding to 13-26 km depth of a theoretical Earth's ocean. The water beneath the ice shell is stirred by vigorous thermal convection, with heat flow at base of ocean about 8 mW/m2.
Assume that we can engineer abyssopelagic Earth life to handle these depths and Europan temperatures as a result of a culture employing advanced, but limited, genetic engineering and biotech.
Assume that oxygen levels are abundant, and that Europa's ocean is absent complex life beyond a microecology of archaea-type organisms clustered around thermal vents. These organisms may offer some adaptive advantages that can be spliced into Earth life introduced to Europa.
Salinity is handwaved with nanotech. Acidity can be regulated by adapted Earth organisms such as Haloquadratum walsbyi, Noctiluca scintillans and Rimicaris hybisae which prey on (Noctiluca scintillans) or coexist with (Rimicaris hybisae) Europan archaea-analogues.