No Sunlight Reaches The Lakes
The lakes lie underneath glaciers that can be a mile thick or more. While ice may have a clear appearance in minimal amounts, this dense layer of ice has an opaque appearance that is solid white all the way through. You cannot see through a glacier.
As a result, no sunlight can reach the subglacial lakes. They are pitch black, so photosynthesis – the process whereby plants and some microbial life forms synthesize energy from sunlight – is unable to happen there. By all accounts, there should not be any life that exists down there.