I'm not an expert on comets, so I might be wrong, but I guess that we talk about recycled ice; that would be sterilized each time the comet orbit pass closest to the sun and the water is (partially?) boiled, to become frozen again later.

There would be probably other consequences if a such large comet will fall in a planet. I can't see any happy ending for this organism to surviving its travel just by chance. If the comet is small or disintegrates in smaller pieces the plant is roasted and dead. Otherwise if fallen in one piece, all the life in the impact area of such large comet should be crushed by the pressure wave and vaporized also, including our plant