> The nutrition raining down from the surface ultimately links back to photosynthesis
This is a key point. The trench lives on the spillover from the surface. If there was no habitable surface full of life, there would be no life in the trench.
Have a read of http://en.wikipedia.org/wiki/Whale_fall - the deep ocean floor may host ecosystems, but this does not mean that planets where the best conditions are like the deep ocean floor are "habitable".
Similarly, the presence of mosses and lichens in Antarctic dry valleys is interesting, but does not prove beyond doubt that such organisms could evolve if the best conditions were like that.
This is a key point. The trench lives on the spillover from the surface. If there was no habitable surface full of life, there would be no life in the trench.
Have a read of http://en.wikipedia.org/wiki/Whale_fall - the deep ocean floor may host ecosystems, but this does not mean that planets where the best conditions are like the deep ocean floor are "habitable".
Similarly, the presence of mosses and lichens in Antarctic dry valleys is interesting, but does not prove beyond doubt that such organisms could evolve if the best conditions were like that.