Is Extraterrestrial Life Suppressed on Subsurface Ocean Worlds due to the Paucity of Bioessential Elements?

The availability of bioessential elements for "life as we know it", such as phosphorus (P) or possibly molybdenum (Mo), is expected to restrict the biological productivity of extraterrestrial biospheres. Here, we consider worlds with subsurface oceans and model the dissolved concentrations of bioessential elements. In particular, we focus on the sources and sinks of P (available as phosphates) and find that the average steady-state oceanic concentration of P is likely to be lower than the corresponding value on Earth by a few orders of magnitude, provided that the oceans are alkaline and possess hydrothermal activity. While our result does not eliminate the prospects of life on subsurface worlds like Enceladus, it suggests that the putative biospheres might be oligotrophic and perhaps harder to detect. Along these lines, potential biospheres in the clouds of Venus may end up being limited by the availability of Mo. We also point out the possibility that stellar spectroscopy can be used to deduce potential constraints on the availability of bioessential elements on planets and moons.