Windows of Nutrient Bioaccessibility: Phosphorus as a Key Constraint on the Chemical Potential for Life Beyond Earth

Phosphorus plays a central role in the chemistry of life as known on Earth, yet its availability depends on environmental conditions that vary widely among planets and moons. On Earth, transformation of phosphorus from mineral-bound reservoirs into soluble and reactive forms is controlled by weathering regimes, redox states, water activity, and mineral energetics. These factors point to a broader principle explored in this review: in aqueous, Earth-like chemical systems, habitability is governed not only by solvent availability, energy gradients, carbon and nitrogen sources, or nutrient inventories, but by environmental windows of nutrient bioaccessibility that determine whether essential elements can be mobilized, transformed, and sustained within dynamic geochemical systems over meaningful timescales. Comparisons among terrestrial, meteoritic, martian, and icy-moon materials show that phosphorus bioaccessibility reflects thermodynamic and kinetic constraints that are neither unique to Earth nor uniformly expressed across planetary environments. From this perspective, phosphorus emerges as a geochemical bottleneck not because it overrides other requirements for life but because, unlike more volatile elements, it lacks an efficient gaseous reservoir and depends on mineral dissolution, aqueous transport, and environmental renewal. This framework helps interpret planetary chemical data and assess one key dimension of the chemical potential for life beyond Earth.