Observation of significant photosynthesis in garden cress and cyanobacteria under simulated illumination from a K dwarf star

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Stars with about 45 to 80% the mass of the Sun, so-called K dwarf stars, have previously been proposed as optimal host stars in the search for habitable extrasolar worlds. These stars are abundant, have stable luminosities over billions of years longer than Sun-like stars, and offer favourable space environmental conditions. So far, the theoretical and experimental focus on exoplanet habitability has been on even less massive, though potentially less hospitable red dwarf stars. Here we present the first experimental data on the responses of photosynthetic organisms to a simulated K dwarf spectrum. We find that garden cress Lepidium sativum under K-dwarf radiation exhibits comparable growth and photosynthetic efficiency as under Solar illumination on Earth. The cyanobacterium Chroococcidiopsis sp. CCMEE 029 exhibits significantly higher photosynthetic efficiency and culture growth under K dwarf radiation compared to Solar conditions. Our findings of the affirmative responses of these two photosynthetic organisms to K dwarf radiation suggest that exoplanets in the habitable zones around such stars deserve high priority in the search for extrasolar life.

Light is the fundamental energy source for photosynthesis, enabling the synthesis of organic compounds. Over billions of years, photosynthetic organisms have profoundly transformed our planet into a diverse global ecosystem (Kiang et al., Reference Kiang, Segura, Tinetti, Govindjee, Blankenship, Cohen, Siefert, Crisp and Meadows 2007a, Reference Kiang, Siefert, Govindjee and Blankenship 2007b). Therefore, understanding any planet in the context of its stellar environment is an essential step in assessing its habitability. The only life we know of so far has developed around our Sun – a G dwarf star in the Harvard spectral classification system with an effective temperature of 5770 K (Williams, Reference Williams 2013). In addition to the search for Earth-like planets around Sun-like stars, much of the focus in the search of extraterrestrial life (Anglada-Escudé et al., Reference Anglada-Escudé, Amado, Barnes, Berdiñas, Butler, Coleman, de la Cueva, Dreizler, Endl, Giesers, Jeffers, Jenkins, Jones, Kiraga, Kürster, López-González, Marvin, Morales, Morin, Nelson, Ortiz, Ofir, Paardekooper, Reiners, Rodríguez, Rodrίguez-López, Sarmiento, Strachan, Tsapras, Tuomi and Zechmeister 2016; Gillon et al., Reference Gillon, Jehin, Lederer, Delrez, de Wit, Burdanov, Van Grootel, Burgasser, Triaud, Opitom, Demory, Sahu, Bardalez Gagliuffi, Magain and Queloz 2016, Reference Gillon, Triaud, Demory, Jehin, Agol, Deck, Lederer, de Wit, Burdanov, Ingalls, Bolmont, Leconte, Raymond, Selsis, Turbet, Barkaoui, Burgasser, Burleigh, Carey, Chaushev, Copperwheat, Delrez, Fernandes, Holdsworth, Kotze, Van Grootel, Almleaky, Benkhaldoun, Magain and Queloz 2017; Rajpurohit et al., Reference Rajpurohit, Allard, Rajpurohit, Sharma, Teixeira, Mousis and Kamlesh 2018; Gebauer et al., Reference Gebauer, Vilović, Grenfell, Wunderlich, Schreier and Rauer 2021) has been placed on the smallest, least massive and coolest type of star. These red dwarf stars (M dwarfs) possess a range of favourable attributes that are conducive to the detection and potential development of life (Adams et al., Reference Adams, Bodenheimer and Laughlin 2005). It has even been demonstrated that photosynthetic organisms from Earth can grow and photosynthesize under simulated M dwarf radiation, highlighting the potential viability of modified spectra for biological processes (Claudi et al., Reference Claudi, Alei, Battistuzzi, Cocola, Erculiani, Pozzer, Salasnich, Simionato, Squicciarini, Poletto and La Rocca 2020; Battistuzzi et al., Reference Battistuzzi, Cocola, Claudi, Pozzer, Segalla, Simionato, Morosinotto, Poletto and La Rocca 2023a, Reference Battistuzzi, Morlino, Cocola, Trainotti, Treu, Campanaro, Claudi, Poletto and La Rocca 2023c). That being said, planets orbiting M dwarfs face challenges such as intense tidal forces (Barnes et al., Reference Barnes, Jackson, Greenberg and Raymond 2009; Heller et al., Reference Heller, Leconte and Barnes 2011), significant water loss during the super luminous pre-main sequence phase (Luger and Barnes, Reference Luger and Barnes 2015) and exposure to substantial levels of extreme UV radiation and high-energy particles. These factors can lead to the photochemical destruction of atmospheric biosignatures and atmospheric erosion (Scalo et al., Reference Scalo, Kaltenegger, Segura, Fridlund, Ribas, Kulikov, Grenfell, Rauer, Odert, Leitzinger, Selsis, Khodachenko, Eiroa, Kasting and Lammer 2007; Barnes et al., Reference Barnes, Jackson, Greenberg and Raymond 2009).

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