Could photosynthesis function on Proxima Centauri b?

Abstract

AbstractCould oxygenic and/or anoxygenic photosynthesis exist on planet Proxima Centauri b? Proxima Centauri (spectral type – M5.5 V, 3050 K) is a red dwarf, whereas the Sun is type G2 V (5780 K). The light regimes on Earth and Proxima Centauri b are compared with estimates of the planet's suitability for Chlorophylla(Chla) and Chld-based oxygenic photosynthesis and for bacteriochlorophyll (BChl)-based anoxygenic photosynthesis. Proxima Centauri b has low irradiance in the oxygenic photosynthesis range (400–749 nm: 64–132 µmol quanta m−2s−1). Much larger amounts of light would be available for BChl-based anoxygenic photosynthesis (350–1100 nm: 724–1538 µmol quanta m−2s−1). We estimated primary production under these light regimes. We used the oxygenic algaeSynechocystisPCC6803,Prochlorothrix hollandica,Acaryochloris marina,Chlorella vulgaris,Rhodomonassp. andPhaeodactylum tricornutumand the anoxygenic photosynthetic bacteriaRhodopseudomonas palustris(BChla),Afifella marina(BChla),Thermochromatium tepidum(BChla),Chlorobaculum tepidum(BChla + c) andBlastochloris viridis(BChlb) as representative photosynthetic organisms. Proxima Centauri b has only ≈3% of the PAR (400–700 nm) of Earth irradiance, but we found that potential gross photosynthesis (Pg) on Proxima Centauri b could be surprisingly high (oxygenic photosynthesis: earth ≈0.8 gC m−2h−1; Proxima Centauri b ≈0.14 gC m−2h−1). The proportion of PAR irradiance useable by oxygenic photosynthetic organisms (the sum of Blue + Red irradiance) is similar for the Earth and Proxima Centauri b. The oxygenic photic zone would be only ≈10 m deep in water compared with ≈200 m on Earth. ThePgof an anoxic Earth (gC m−2h−1) is ≈0.34–0.59 (land) and could be as high as ≈0.29–0.44 on Proxima Centauri b. 1 m of water does not affect oxygenic or anoxygenic photosynthesis on Earth, but on Proxima Centauri b oxygenicPgis reduced by ≈50%. Effective elimination of near IR limitsPgby photosynthetic bacteria (<10% of the surface value). The spectrum of Proxima Centauri b is unfavourable for anoxygenic aquatic photosynthesis. Nevertheless, a substantial aerobic or anaerobic ecology is possible on Proxima Centauri b. Protocols to recognize the biogenic signature of anoxygenic photosynthesis are needed.