Phosphate availability and implications for life on ocean worlds

Abstract

Several moons in the outer solar system host liquid water oceans. A key next step in assessing the habitability of these ocean worlds is to determine whether life's elemental and energy requirements are also met. Phosphorus is required by all known life and is often limited to biological productivity in Earth's oceans. This raises the possibility that its availability may limit the abundance or productivity of Earth-like life on ocean worlds. To address this potential problem, here we calculate the equilibrium dissolved phosphate concentrations associated with the reaction of water and rocks-a key driver of ocean chemical evolution-across a broad range of compositional inputs and reaction conditions. Equilibrium dissolved phosphate concentrations range from 10^-11 to 10^-1 mol/kg across the full range of carbonaceous chondrite compositions and reaction conditions considered, but are generally > 10^-5 mol/kg for most plausible scenarios. Relative to the phosphate requirements and uptake kinetics of microorganisms in Earth's oceans, such concentrations would be sufficient to support initially rapid cell growth and construction of global ocean cell populations larger than those observed in Earth's deep oceans.

Fig. 1. Phosphorus abundances in chondrites, basalts, and hydrothermal fluids.

a Histogram of phosphorus concentrations (wt%) for all carbonaceous chondrite compositions in the chondrite database (n = 1800) and Mid Ocean Ridge Basalt database (n = 3598). All chondrite compositions are depicted at 10% opacity to highlight the large degree of overlap among different chondrite classes. b Phosphorus concentrations from the submarine hydrothermal vent database (n = 34).

Fig. 2. Total dissolved phosphate concentration (mol/kg) for averaged carbonaceous chondrite composition for different chondrite classes and mid-ocean ridge basalts with 0–2 wt% C.

a–c CI, CM, CR are closer to solar stoichiometry and d–f CV, CO, CK have higher Ca concentrations. For comparison with Europa, vertical lines are inferred from hydrothermal models and the horizontal line is the lower bound on water/rock ratio inferred from salinity estimates. For comparison with Enceladus, shaded regions show estimates from Si nanoparticles while horizontal lines track modeled [Na] corresponding to observations of NaCl observed in the E-ring (additional pH constraints Fig. 3).

Fig. 3. Dissolved phosphate and phosphate and carbonate mineral deposition at range of water-rock ratios and temperatures.

a–c Total dissolved phosphate concentration (mol/kg) for averaged CI carbonaceous chondrite composition for different redox states of carbon. Ca and P bearing minerals for water/rock ratio of 100 d–f and 1 g–i.

Fig. 4. Biological context of CI chondrite dissolved phosphate concentrations.

Cellular abundance a and doubling times b assuming Earth-like phosphorus concentrations within cells and the most conservative Prochlorococcus uptake kinetics. Relevant observational constraints for Enceladus and Europa are annotated^,,,.