My story premise is that a human expedition to Europa has happened and the travellers have tunnelled through the ice and formed a colony in the ocean.
When they breached into the ocean, they discovered that the underside of the ice was covered by glowing organisms that - through further research, turned out to be using some form of fibre optic tubes to the surface to gather light and other energies from Jupiter and the sun.
This energy is enough to cause the organisms to glow a variety of colors which - given the sheer abundance of them - produces a moderate quantity of light, something which is then used by other native biology. The evolution of these organisms can be tracked back to similar glowing organisms that live near hydrothermal vents, explaining how life first arose and that light is a factor in the evolution of Europan life.
While I understand that the energies captured through such a process probably wouldn't be enough for the organism to re-emit light (and that emitting light probably isn't in the best interests for these organisms), my main issue is working out how exactly these organisms created these optical tunnels.
Could ice be modified in such a way through chemical and/or physical processes to collect and direct light towards the organisms? How would the organisms act on the ice over long distances? Could the ice be modified in such a way that it filters the incoming light to only let UV and longer wavelengths through, while blocking X-Ray and gamma radiation?
I'm assuming that the Europan ocean contains many of the same elements found on Earth and that the organisms can generate complex compounds from these. The organisms could also potentially move or grow in ways that (probably) aren't found on Earth, and may even be able to bud off sub-organisms to do their work.