Photosynthesis changed Earth in powerful ways. When photosynthetic organisms appeared, it led to the Great Oxygenation Event. That allowed multicellular life to evolve and resulted in the ozone layer. Life could venture onto land, protected from the Sun’s intense ultraviolet radiation.
But Earth’s photosynthetic organisms evolved under the Sun’s specific illumination. How would plants do under other stars?
Our Sun is a G-type star, sometimes called a yellow dwarf. It seems like a normal star to us, but yellow dwarfs aren’t that common. Only about 7% to 8% of stars in the Milky Way are G-type stars. When it comes to understanding habitability on exoplanets, we need to understand the more plentiful types of stars.
Some scientists propose that K-dwarf stars are the most optimal host stars for habitable exoplanets. They’re between about 50% and 80% as massive as G-type stars, are more abundant and have stable luminosities for billions of years longer than Sun-like stars. The Sun will be stable on the main sequence for about 10 billion years, while K-type stars can be stable for up to 70 billion years. Despite this, much exoplanet habitability research focuses on M-dwarfs, or red dwarfs, which may actually be far more inhospitable to life because of flaring and tidal locking.
In a new study, a trio of researchers simulated the light output from a K-dwarf star and grew two photosynthetic organisms in those conditions to see how they responded. The research article is “Observation of significant photosynthesis in garden cress and cyanobacteria under simulated illumination from a K dwarf star.” It’s published in the International Journal of Astrobiology, and the lead author is Iva Vilovi?, a PhD student in the Astrobiology Research Group at the Technical University of Berlin.
These figures from the article show the spectra for both the Sun and a K-dwarf star, and the simulated spectra for both. Image Credit: Vilovi? et al. 2024.Garden cress, whose Latin name is Lepidium sativum, is a common garden green used in salads, soups, and sandwiches. It’s an adaptable plant that grows rapidly. The cyanobacterium Chroococcidiopsis is an extremophile known for lying dormant for 13 million years and remaining viable. It can resist radiation, desiccation, and extreme temperatures and is of interest in astrobiology.
We expect photosynthesis to play a role in astrobiology. Starlight provides the energy for organisms to synthesize organic compounds. In order to understand photosynthesis in astrobiology, we need to understand how other stars could power photosynthesis. “Therefore, understanding any planet in the context of its stellar environment is an essential step in assessing its habitability,” the authors write.
Astronomers search for Earth-like planets around Sun-like stars because that’s the only life we know of. They also pay special attention to M-dwarfs because they’re so plentiful and are known to host many rocky exoplanets in their habitable zones. Scientists have demonstrated that photosynthetic organisms from Earth can grow under simulated M-dwarf light. But M-dwarf habitability faces a whole host of potential barriers.
Artist’s impression of a flaring red dwarf star orbited by an exoplanet. Red dwarfs can flare violently, which could make planets in their habitable zones unable to support life. Planets in their habitable zones are also often tidally locked, which is another barrier to habitability. Credit: NASA, ESA, and G. Bacon (STScI) In this work, the researchers focused on K-dwarfs. They lack the magnetic activity that appears to generate extremely powerful flaring on M-dwarfs, flaring so powerful that it could sterilize planets in their liquid-water habitable zone. The habitable zones around K-dwarfs are also far enough away that planets wouldn’t be tidally locked, another potential barrier to habitability that affects M-dwarfs. K-dwarfs also become habitable sooner in their lives than M-dwarfs due to their rapidly weakening FUV and X-ray fluxes.
“All things combined, K dwarfs can be considered the ‘Goldilocks stars’ in the search for potentially life-bearing planets,” the authors write.
This table from the research article shows the conditions that the researchers recreated in their study. Image Credit: Vilovi? et al. 2024. The trio of researchers exposed watercress seedlings to three different light regimes: sunlight, K-dwarf light, and no light. Visually, the solar and K-dwarf samples were similar, though most of the time, the seeds sprouted a day or two earlier than under solar light. The K-dwarf sample also had marginally larger leaf surface area.
The researchers grew garden cress (Lepidium sativum) on a sand substrate with one hundred initial seedlings under Solar (effective temperature 5800 K), K dwarf (effective temperature 4300 K) and dark conditions. This image shows the visual results for selected days. Garden cress under K dwarf radiation sprouts sooner relative to Solar and dark conditions. Image Credit: Vilovi? et al. 2024.After seven days, a side view of the samples showed that height and stem elongation were different. Under the K-dwarf lighting, the watercress grew taller.
The watercress grew taller under K-dwarf lighting than under Solar conditions. Image Credit: Vilovi? et al. 2024.The researchers also measured water content and dry mass. Under K-dwarf conditions, the watercress had slightly higher water content, while the dry content was lower compared to solar conditions.
These figures show the water content and dry content for all three garden cress samples. Image Credit: Vilovi? et al. 2024.The researchers also tested the photosynthetic efficiency and found no significant difference between the solar and K-dwarf samples.
The hardy extremophile Cyanobacterium Chroococcidiopsis sp. CCMEE 029 is at the other end of the spectrum from the quick-growing garden cress. It’s a survivor that can withstand long periods of dormancy and extreme growing conditions. The researchers also cultivated it under Solar, K-dwarf and dark conditions.
They measured the average integrated density (IntD) of the cyanobacterium, which is an indicator of culture growth. They found that the K-dwarf sample exhibited higher values than the solar sample, but the differences were not considered significant. Predictably, “Cyanobacteria under constant dark conditions failed to exhibit significantly measurable IntD,” the authors write in their paper.
This figure from the research article shows incremental ratios and integrated densities of the cyanobacterium on selected days under Solar, K dwarf and dark conditions. Though the integrated density was higher under K-dwarf conditions, the difference isn’t significant, according to the researchers. Image Credit: Vilovi? et al. 2024.They point out that their study didn’t replicate natural conditions completely. Sunlight intensity changes throughout the day, but they didn’t include that in their study. “Sunlight intensity on Earth varies throughout the day, with peak intensities occurring during the central hours. This variation is crucial for plants to adapt and respond to changing light conditions, including the activation of non-photochemical quenching (NPQ) to mitigate the effects of excess light,” they write. NPQ helps plants cope with periods of excess light, meaning light above what it can photosynthesize, by dissipating it as heat.
“Understanding the effects of K-dwarf radiation on photosynthesis and growth is of foremost importance not only for the assessment of its viability for phototrophic organisms but also for the interpretation of atmospheric biosignatures outside of the Solar System,” the authors explain. Other research in this area has focused on M-dwarfs, and this trio of researchers say that to the best of their knowledge, theirs is the first to look at photosynthesis and K-dwarfs.
“These results can bring us closer to addressing which stellar environments could be the optimal candidates in the search for habitable worlds,” the authors write. “These findings not only highlight the coping mechanisms of photosynthetic organisms to modified radiation environments but also they imply the principal habitability of exoplanets orbiting K dwarf stars.”
