Searching for the oxygen footprint of light-harvesting organisms

The discovery of more than 4000 exoplanets in the last 25 years has rapidly established a whole new field of study: namely, that of exoplanetary sciences. The longstanding philosophical issue on the origin and the distribution of life beyond Earth has recently abandoned the realm of speculation and entered the scientific debate: for the first time in history, we do have the means to discover extraterrestrial life.

The remote detection of life is based on the fact that life uses chemical reactions to extract, store and release energy, producing by-products in its metabolic processes that, under the right conditions, can build up in the atmosphere to a detectable concentration. This is the case of Earth’s molecular oxygen (O2), the waste product of the photosynthetic activity of autotroph organisms that, without a continuous biological source, would disappear in just a few million years. Detecting oxygen in exoplanetary atmospheres has been considered for decades as a tracer of life, even if caution must be taken to definitely rule out alternative abiotic explanations: a stronger constraint would be, for instance, the concurrent detection of O2/O3 and a reduced gas like CH4 or N2O. Searching for the footprint of life in exoplanetary atmospheres will be feasible thanks to next-generation space missions like NASA’s JWST and ESA’s ARIEL and ground-based facilities like SPHERE@VLT, GPI@GEMINI and PCS@E-ELT,
retrieving the atmospheric transmission, reflection and emission spectra of extrasolar planets.

While terrestrial oxygenic photosynthesis exploits just a quite narrow window of the electromagnetic spectrum (PAR, 400-700 nm), it has been suggested that the upper limit might be pushed to 1050 nm or even 1400 nm, allowing organisms to make the most of the irradiation of the coolest M stars, too. Theoretical arguments, taking into account astrophysical, chemical, climatic and biological processes, indicate that light-harvesting processes, and in particular oxygenic photosynthesis, should be universal features of life, because they rely on an exceptionally abundant source like water and represent an effective way to harvest enormous amounts of energy, extremely prone to strong positive evolutionary selection.

Whether photosynthesis leads to oxygen buildup is another matter: oxygenation time depends sensitively on the balance between processes creating ("sources") and destroying ("sinks") oxygen, and Earth’s history reminds how a world with photosynthetic organisms does not have to be highly oxygenated.
We developed a toy model that, starting from the oxygenation history of our planet, tries to discern the key processes dictating the long-time evolution of molecular oxygen: it turns out that free O2 is a small residue of interactions involving atmosphere, lithosphere, hydrosphere and biosphere. The main oxygen sinks are volcanic and metamorphic outgassing, weathering and serpentinisation. Starting from present values of sources and sinks, analytical forms for their time evolution were searched. The biomass term was modelled through the logistic function, commonly used to describe bacterial growth, and allowing for up to four different episodes of accretion. Despite many simplifications, the model yielded a good fit to the reconstructed O2 time evolution, with four biomass bumps at t = 2.3 Gyr, t = 720 Myr, t = 370 Myr and t = 260 Myr ago, consistent to paleontological evidence.

Although the assumptions behind the model are necessarily Earth-based, an attempt was made to generalise it to exoplanets in order to get some insights on possible mechanisms preventing or enhancing the plausibility of oxygen accumulation. Geological factors, like the presence of plate tectonics and of a strong magnetic field, are highly uncertain and rely on models lacking, as we know, any observational verification; scaling relations, based on recent work, were used to model sources and sinks. As regards the biological source, building on the work by Lingam & Loeb (2019), we posit that the maximum biomass sustainable by a world is dictated by the availability of light and nutrients. In contrast to Earth, some worlds can receive too little PAR to sustain Earth-like biospheres; as a result of the decreased O2 source, they would have longer oxygenation time or even, if Fsource < Fsink, accumulate no free oxygen at all.

A clear distinction has emerged between nutrient-limited and light-limited worlds. Nutrient-limited worlds are virtually unaffected by the spectral class of their host star and follow a similar evolutionary history as the Earth. Larger planets than the Earth may take several billion years before evolving land life, perhaps posing a physical limitation to higher habitability. The effect of a  different water coverage fo is a delicate balance between varying ocean primary production and continental weathering: worlds with fo < 0:6 do not accumulate enough oxygen to develop land life. Light-limited worlds, which should be common around M stars if the PAR window were the same as Earths, never managed to overcome the critical O2 threshold for land colonisation. Nonetheless, concentrations as high as 0.65 PAL were reached in some cases, and the Mars-sized planet orbiting an M2 primary looks particularly intriguing, because the convergence of a high ocean productivity and small sinks causes a very stable and easily detectable O2 signal for over 10 Gyr.

Chimera planets, defined as those with a light-limited land NPP and a nutrient-limited ocean NPP, manage to accumulate significant amounts of oxygen, sometimes even higher than 1 PAL, even when the primary is an M6 star. These worlds might offer conditions for a high-paced biological evolution, even though their O2 reservoirs are extremely sensitive to biomass variations, possibly leading to lethal, anoxic transients.

The main result of our simulations is that biotic oxygen can indeed accumulate in a wide variety of planetary environments, including light-starving M-star systems, even if an O2-rich atmosphere is not the inevitable outcome of photosynthesis. Only if the biotic source outweights sinks, can oxygen begin to accumulate.