Description:
<jats:title>Abstract</jats:title><jats:p>Could 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 Chlorophyll<jats:italic>a</jats:italic>(Chl<jats:italic>a</jats:italic>) and Chl<jats:italic>d-</jats:italic>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<jats:sup>−2</jats:sup>s<jats:sup>−1</jats:sup>). Much larger amounts of light would be available for BChl-based anoxygenic photosynthesis (350–1100 nm: 724–1538 µmol quanta m<jats:sup>−2</jats:sup>s<jats:sup>−1</jats:sup>). We estimated primary production under these light regimes. We used the oxygenic algae<jats:italic>Synechocystis</jats:italic>PCC6803,<jats:italic>Prochlorothrix hollandica</jats:italic>,<jats:italic>Acaryochloris marina</jats:italic>,<jats:italic>Chlorella vulgaris</jats:italic>,<jats:italic>Rhodomonas</jats:italic>sp. and<jats:italic>Phaeodactylum tricornutum</jats:italic>and the anoxygenic photosynthetic bacteria<jats:italic>Rhodopseudomonas palustris</jats:italic>(BChl<jats:italic>a</jats:italic>),<jats:italic>Afifella marina</jats:italic>(BChl<jats:italic>a</jats:italic>),<jats:italic>Thermochromatium tepidum</jats:italic>(BChl<jats:italic>a</jats:italic>),<jats:italic>Chlorobaculum tepidum</jats:italic>(BChl<jats:italic>a + c</jats:italic>) and<jats:italic>Blastochloris viridis</jats:italic>(BChl<jats:italic>b</jats:italic>) 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 (<jats:italic>P</jats:italic><jats:sub>g</jats:sub>) on Proxima Centauri b could be surprisingly high (oxygenic photosynthesis: earth ≈0.8 gC m<jats:sup>−2</jats:sup>h<jats:sup>−1</jats:sup>; Proxima Centauri b ≈0.14 gC m<jats:sup>−2</jats:sup>h<jats:sup>−1</jats:sup>). 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. The<jats:italic>P</jats:italic><jats:sub>g</jats:sub>of an anoxic Earth (gC m<jats:sup>−2</jats:sup>h<jats:sup>−1</jats:sup>) 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 oxygenic<jats:italic>P</jats:italic><jats:sub>g</jats:sub>is reduced by ≈50%. Effective elimination of near IR limits<jats:italic>P</jats:italic><jats:sub>g</jats:sub>by 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.</jats:p>