Phosphorus and Life on a Water World

Abstract

On Earth, the major mechanism for providing the vital limiting nutrient phosphorus necessary to fuel biological productivity and the long arc of evolution is weathering of exposed continental rocks. It has been presumed that life may not be present on exoplanets with substantially more water than Earth. Many of these “Water Worlds” exist, but without exposed land mass for weathering, there is not a viable mechanism for nutrient delivery and climate stabilization. In novel laboratory experiments performed in chambers designed to mimic the weathering of seafloor basalts in anoxic conditions, Syverson et al. (2021, https://doi.org/10.1029/2021GL094442) found that silicate weathering in these conditions release an adequate amount of phosphorus to fuel a robust biosphere, at least in an idealized system. Perhaps we shouldn't rule out “Water Worlds” as potential harbors for life after all?

Key Points

• Many “water worlds” exist in the galaxy, but it has been presumed that they lack an adequate phosphorus supply to maintain life

• To test this assumption, Syverson et al. (2021) performed anoxic chamber weathering experiments on basalts

• Syverson et al. (2021) found significant phosphorus weathering occurs in this setting, and thus water worlds may sustain prolonged life

Plain Language Summary

Based on chamber experiments, Syverson et al. (2021, https://doi.org/10.1029/2021GL094442) show that an adequate amount of phosphorus can be weathered rom seafloor basalts in anoxic settings to supply biological productivity and support a long-lived ecosystem required for evolution on a water world.

1 Introduction

On Earth, there have always been several non-negotiable ingredients for the support of life, and the slow evolutionary pathway from Archaea to Homo Sapiens. Water is obviously one of them, but water has been plentiful for much of Earth's history. But one essential ingredient that has always been in short supply, and whose geochemical cycle results in it frequently being sequestered in forms that are not readily available to biota, is phosphorus. The Earth never started off with much of this element—average crustal concentrations of phosphorus are about 0.1 wt. percent—and as long as the planet has been oxygenated, much of the phosphorus made available by continental weathering is readily trapped by iron oxidation (e.g., Filippelli, 2016). In fact, the only new source of phosphorus to the global biosphere is via chemical weathering of terrestrial minerals. So, if Earth were a “water world” planet, covered completely by ocean with no possibility of terrestrial supply of phosphorus, would life have ever proliferated here? This is not just a rhetorical question, but one that is relevant to conjecturing about life on other planets, including those that have been detected that seem largely to be covered with water.

In a combination of geochemical chamber experiment/thought piece on exobiology, Syverson et al. (2021) ask this question and come to the intriguing conclusion that a water world would indeed have enough phosphorus released from an unlikely source to sustain life—submarine basalt weathering. Many models of astrobiology are based on Earth analogs, and certainly reasonable arguments could be made that the lack of terrestrial phosphorus weathering on a water world would significantly reduce phosphorus weathering production, perhaps making it as much as 1,000-fold lower (Glaser et al., 2020). In the deep ocean on Earth, hydrothermal vents and the oxides that form from emitted vent fluids are certainly net consumers of phosphorus, and massive ones, comprising approximately 20% of the marine export flux for phosphorus. But as Syverson et al. (2021) argue, if the water circulating through submarine vent systems and the deep ocean into which they emit were anoxic, the oxidative phosphorus loss factor be removed. Additionally, based on lab experiments, the weathering of submarine basalts in these anoxic conditions might produce significant amounts of phosphorus (Figure 1).

Figure 1

A schematic of the global phosphorus on today's Earth and a “Water World” planet. The key difference between the two scenarios is that vast majority of new phosphorus to the marine system to fuel productivity comes from terrestrial weathering of exposed landscapes on today's Earth, whereas phosphorus from a “Water World” might be provided by submarine weathering of seafloor basalt under conditions of deep ocean anoxia in a hydrothermal spreading system.

1.1 The Global Phosphorus Cycle

The global phosphorus cycle, or more accurately that before humans began perturbing with the widespread application of chemical phosphate fertilizers, is simple. Having no volatile phase, like the nitrogen cycle, the only new source of phosphorus to global biogeochemical cycles is through the chemical weathering of continental rocks. Once released from its mineral form, phosphorus is transformed in the soil profile into a variety of fractions that have been chemically defined and crudely reflect their reactivity— “occluded” phosphate bound to oxyhydroxides, organic phosphate, and “non-occluded” or adsorbed phosphate Filippelli, 2016. Natural soil ecosystems are highly efficient in retaining phosphate as it cycles between its various geochemical forms. But significant loss of phosphate from the landscape occurs with land disturbance, such as glaciation and deglaciation, and in modern chemical agricultural systems where fertilizers are typically over-applied, and the phosphate runoff causes significant water quality issues through eutrophication.

A portion of the phosphorus that eventually makes its way to the ocean is unreactive in the marine environment, but that portion that is reactive sets marine biological productivity on longer timescales, and thus modulates and net organic carbon export from the ocean (Figure 1). This is not to say that the internal marine phosphorus cycle is simple, and indeed, there is significant action around suboxic and anoxic portions of the ocean within the water column and in sediments. Long ago termed the “iron-phosphate pump,” oxide-bound phosphorus is released in sub/anoxic systems and becomes dissolved phosphorus, available for biological uptake (Wheat et al., 2003). Thus, a system that is sub/anoxic can lead to significant internal recycling of phosphorus and to higher net carbon export should other limiting nutrients be present in adequate concentrations. This is the scenario that has been proposed for driving significant organic carbon burial in various times in the past, with the net result being a loss of atmospheric carbon dioxide and lower global temperatures. But until now, little attention has been paid to what might happen if deep ocean water itself was anoxic (perhaps because this is not a situation seen today?), and circulates through seafloor hydrothermal systems as a weathering agent for phosphorus, instead of an oxide-rich phosphate scavenging environment as it is now.

1.2 Water World Scenario

To determine the extent and rate of anoxic phosphorus weathering from seafloor basalts, Syverson et al. (2021) conducted a range of long-duration chamber experiments. This study used well-characterized basalt samples in anoxic reaction chambers, with the weathering aspects followed by an enriched dissolved ²⁹SiO₂ tracer. This tracer provided a direct relationship between silicate weathering and phosphorus mobilization. Oxygenic experiments were also conducted as reaction chamber controls.

The experiments lasted approximately two months, and the results were striking—indeed, not only is the phosphorus weathering release high in anoxic conditions, but it is comparable to the current weathering release of phosphorus from continental rocks. The reaction of dissolved CO₂ being consumed during submarine basalt weathering is balanced by the weathering release of phosphorus and the consequent uptake and eventual burial of organic carbon. One product of this reaction is O₂. Syverson et al. (2021), using a host of reasonable (but largely unconstrained) assumptions, calculate that this process yields a biospheric O₂ flux of ∼10 Tmol y⁻¹, similar to the ∼10–20 Tmol y⁻¹ determined for the modern Earth (Catling & Kasting, 2017) and greater than biospheric O₂ fluxes of 2–5 Tmol y⁻¹ calculated for the Proterozoic soon after the Great Oxidation Event (GOE; Ozaki et al., 2019).

Several critical implications result from this study. First, although a number of studies have focused on phosphorus cycling in the early anoxic ocean (both pre-GOE and at various intervals of the Paleozoic), they've been using the model of the modern anoxic zones as a frame. In the modern anoxic systems, significant phosphorus recycling occurs from oxide-bound phosphorus as it is dissolved in the water column or sediments. This process drives the assumption that in a mostly anoxic ocean, the phosphorus concentration would be higher because of this process alone, and yet the origin of that marine phosphorus is assumed to be terrestrial weathering (Figure 1). Submarine on and off-axis basalt weathering has not typically been included in calculations of the phosphorus cycle at these times because of the assumption terrigenous weathering dominating the global phosphorus cycle.

A second implication of this study is that an exoplanet with significantly more water (per mass) than Earth may not only have adequate phosphorus for life, but also a carbonate-silicate cycle with feedbacks not altogether unlike those with exposed landmasses. This becomes a critical and perhaps groundbreaking finding for astrobiology, where the habitable zone is defined as distance envelope around a star where a black body planet would have a temperature between 0 and 100 degrees Celsius—i.e., a planet with liquid water. The distance window is defined by the energy output of a given star, and would in reality be modified by the greenhouse gas composition of a planetary atmosphere and the albedo of that planet. But the classical concept of a habitable zone has been considered moot for planets with global oceans (Abbot et al., 2012; Foley, 2015), because it was presumed that the normal atmospheric stabilization of carbon dioxide in the atmosphere was achieved by carbonate-silicate weathering cycle (i.e., the Kasting et al., 1993 model), and a planet without exposed landmasses for silicate weathering would not qualify for a robust self-regulating climatic system required for the slow timescales of biological macroevolution. This ruled out a tremendous number of candidate exoplanets, as “water worlds” are common (e.g., Mulders et al., 2015). It seems that there is some justification to rule them back into the realm of potential life-harboring planets.

2 Limitations and Further Work

The P release rate from submarine basalt weathering needs a bit of a reality check. I understand that they are analyzing it on a CO₂-normalized basis, which is a fine way to capture the weathering reaction. I also understand that they did a back-of the envelope calculations for actual global terrestrial weathering P/CO₂ and found the ratio comparable. But there is a substantial distance between the experimental results and the actual modern values, both in terms of spatial and temporal scales. The reality check is that even if the P/CO₂ relationship for the reaction chambers used in the Syverson et al. (2021) experiments is comparable to terrestrial, the extent of actively weathering submarine basalt in the modern ocean is quite small, confined largely to the region around Mid-Ocean Ridges—much of the rest of the basalt stops weathering significantly after moving away from the ridge and being buried by sediments. One might assume the same for the ancient ocean, such as the late Devonian when marine anoxia was widespread. So, one could think of Syverson et al. (2021) experimental reactions as the maximum possible phosphorus release, which occurs only on a small amount of the ocean floor. Thus, the net phosphorus input flux rate from basalt for the full ocean would not reflect these narrow, extreme experimental results.

Acknowledgments

This work was supported by the National Science Foundation (EAR-1850878) and the Donors of the American Chemical Society-Petroleum Research Fund.

Open Research

Data Availability Statement

No new data was used in this paper.