Photosynthesis as a Biomarker: What Will Give Away Life on Other Worlds ✛ ✦
Context
The search for life beyond the Solar System relies on biosignatures — remotely observable signs of biological activity. Biosignatures of oxygenic photosynthesis, primarily atmospheric oxygen and the spectral ‘red edge’ of vegetation (VRE), have already transformed Earth and are visible from space. Yet the astrobiology community cannot yet robustly link observational data from exoplanets to the presence of photosynthetic life, especially in the context of different atmospheric compositions and spectra of host stars, the study of which began with the pioneering work of Cecilia Payne-Gaposchkin. It is critically important to understand how universal Earth’s photography mechanisms are and what alternatives might have arisen on planets around red dwarfs or Sun-like stars, largely discovered thanks to missions by William Borucki.
Methods
To build a predictive model of phototrophic biosignatures, an interdisciplinary approach is used, combining spectroscopy with Doppler measurement of radial velocities, genomics, synthetic biology, and planetary modeling. Natural pigments — from chlorophylls to retinal-containing rhodopsins — are studied using Einstein's photoelectric equation and the Planck–Einstein relation, which define the energy thresholds of photoconversion. Laboratory experiments simulate illumination from M-dwarfs for cyanobacterial cultures, while climate models calculate the available photon flux and surface chemical context. Space missions such as JWST and future direct-imaging observatories test the detectability of spectral features under noisy conditions.
Results
A wide range of spectral adaptations has been revealed: in oxygenic phototrophs, the long-wavelength limit is extended from 700 nm (chlorophyll a) to roughly 800 nm thanks to chlorophylls d and f, while anoxygenic bacteria can use near-IR up to 1020 nm. Oxygenic photosynthesis likely emerged in the Archean, but atmospheric O2 accumulated only hundreds of millions of years later due to strong geochemical sinks — pointing to the risk of false negatives in the search for life. It has been shown that rhodopsin-based proton pumps, which do not require complex redox chains, are widespread across all domains of life and can dominate certain spectral niches, though their link to autotrophy remains unclear. The analysis also highlights that the dominant pigments on a planet may not coincide with the first evolutionary innovations due to ecological priorities and planetary catastrophes.
Implications
These results are of fundamental importance for planning missions like JWST and the proposed Habitable Worlds Observatory. They show that unambiguous interpretation of biosignatures requires a comprehensive consideration of stellar spectrum, planetary atmosphere, availability of water, and evolutionary context. Understanding the physicochemical constraints on photoconversion allows us to outline a photosynthetic ‘habitable zone’ — the region around a star where energetically efficient photography is possible. Moreover, the research justifies the need to search not only for Earth-like but also for agnostic biosignatures, such as spectral shifts of pigments not tied to a specific chemical composition.
Future development
The topic will evolve toward integrating systems biology and planetary science. Future work includes creating quantum-chemical models to predict absorption spectra of hypothetical pigments in various solvents and protein environments, as well as conducting evolutionary experiments on the synthetic redesign of reaction centers. An important step will be developing a scaling theory that links molecular innovations to the global proliferation of phototrophs and their reflection in the spectra of exoplanets.
Impact
The results will influence the scientific programs of next-generation space telescopes, criteria for selecting target planetary systems, and the interpretation of spectral data. They also stimulate synthetic biology, since the search for alternative photography pathways will require designing and testing artificial photosystems.
Next steps
It is necessary to refine the minimum photon flux supporting growth and the upper illuminance limit causing photodamage for diverse phototrophs under exoplanet-like conditions. A large-scale campaign for spectral screening of terrestrial microorganisms and metagenomic search for new photopigments will also be key.
Key open problems
The research is directly connected to unresolved questions in astrobiology: how did oxygenic photosynthesis arise and why did it appear only once; what delayed the oxygenation of Earth's atmosphere; can photosynthesis be considered an expected step in the evolution of complex life; what are the true boundaries of the habitable zone considering phototrophic productivity.
The knowledge machine's view
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What the work does
The study compiles data on different types of photosynthesis and shows that life can produce noticeable spectral fingerprints beyond just Earth's chlorophyll. The authors analyze pigments, their adaptations to different stars, and possible biosignatures for searching for life on exoplanets.
Where its strength is
A key strength is the interdisciplinary approach, combining spectroscopy, genomics, and planetary modeling to predict phototrophic biosignatures. This allows moving beyond Earth-centric templates and considering a wide range of possible spectral adaptations.
Where it could go next
- One could consider how to distinguish photosynthetic pigments from mineral spectral features using approaches from the work on the red edge of life and its mineral counterparts.based on: The Red Edge of Life and Its Mineral Mimics arXiv:2605.10646
- It would be interesting to apply methods for assessing the impact of cell size on biosignatures, as in the full analysis of microbial factories of exoplanets, to clarify which phototrophs could produce detectable signals.based on: Microbial Factories of Exoplanets: How Cell Size Conducts the Methane Orchestra arXiv:2607.06366
- One could extend the analysis to photosynthesis around red dwarfs, drawing on the work on the lower chances of photosynthesis around such stars, to account for possible limitations and alternative strategies.based on: Why Red Dwarfs Have Fewer Chances for Photosynthesis arXiv:2602.20789
Worth trying
- Test which spectral features of photosynthetic pigments could be confused with mineral analogs under exoplanet conditions.
- Model how the cell size of phototrophs affects the overall spectral signal of a planet for different types of stars.
- Investigate the possibility of photosynthesis beyond the classical range, for example, using near-infrared light around red dwarfs.
Why this may matter
Understanding the diversity of photosynthetic pigments and their spectral manifestations could broaden the criteria for searching for life, reducing the risk of false negatives and opening up new observational strategies for future missions.
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Original abstract
as written by the authors arXiv:2607.03106
Photosynthesis is of prime interest in the telescopic search for life beyond the Solar System, because, on Earth, oxygenic photosynthesis produces two strong "biosignatures," global scale signs of life that can be seen from space: atmospheric oxygen and the Vegetation Red Edge (VRE). The VRE is the spectral reflectance signature of plant leaves, characterized by a step-like increase in reflectance from the red to the near-infrared. The absorption in the red is due to chlorophyll $$\textit{a}$$ (Chl $$\textit{a}$$). While Chl $$\textit{a}$$ dominates our planet, the Earth harbors diverse phototrophic organisms in niche environments possessing other pigments that produce edge-like spectral features across the UV-VIS-NIR, naturally suggesting diverse signatures that could be found on other planets where phototrophic life is adapted to other stars. However, the astrobiology community is very much at an early stage in its ability to constrain the probability that an observation of another planet has detected a sign of photosynthetic life. This white paper identifies critical research questions to advance to a predictive capability the search for phototrophic biosignatures. These questions pertain to the origins, key features, diversity, and potential for alternative adaptations in fundamental aspects of light harvesting; the electron transfer pathway in photosynthesis; rhodopsin-based proton-pumping; and carbon fixation. We discuss the need to constrain how evolution and ecology affect the scaling up of these molecular mechanisms to be potentially detectable by a direct imaging mission. The research questions and recommendations presented here are cross-linked to those posed by the NASA Astrobiology Strategy 2015, and to the Focus Areas of the upcoming NASA Decadal Astrobiology Exploration Strategy (DARES).
The vegetation red edge, which gives away Earth's plants from space, is not just a ‘green’ effect: it results from a sharp drop in chlorophyll absorption in the near-IR and strong scattering inside the leaf — that's why vegetation looks bright red on satellite images!
In the movie ‘Avatar,’ Pandora's flora has unusual colors, echoing the idea that photosynthetic life on other planets may use pigments different from Earth's and create unique spectral biosignatures, similar to what scientists are searching for in real exoplanetary systems.
Key numbers
- vegetation red edge (VRE): 680–760 nm
- record long-wavelength limit of oxygenic photosynthesis: ~800 nm (chlorophyll f)
- maximum wavelength of anoxygenic photosynthesis: ~1020 nm
- minimum photon flux density for survival of oxygenic phototrophs: 0.04 mmol m⁻² s⁻¹
- delay period of O₂ accumulation after the emergence of oxygenic photosynthesis: hundreds of millions of years