Current fusion reactor schemes require deuterium and the plan is to extract it from the ocean where it can be found in small abundance. Does deuterium serve some purpose in the oceans? What would happen if there was none left?
Replacing 20% or more of the normal water in ones body with heavy water results is toxic (Kushner 1999). Then again, a number of vitamins are toxic in high doses, but vitamins are essential to human life. Could deuterium in small doses be essential to life? That's exactly what Somlyai et al. argue in their paper (Somlyai 1993).
However, it's a bit premature to start shouting "peak deuterium." We don't even know if ITER will prove to be successful, let alone point the way to commercially successful fusion. Even if it does prove to be successful, the deuterium consumption rate will be tiny. By the time our children's children's ... children would have to shout peak deuterium, we'll have gone on to even bigger and better things (if humanity lives that long).