The search for extraterrestrial life begins with biosignatures—traces left behind by living organisms. Oxygen is a key signature of plants, algae, and bacteria on Earth. Most of it is produced through photosynthesis using the visible light emitted by our Sun, and scientists have searched for the element in the atmospheres of other worlds. But most stars in our galaxy are red dwarfs, which emit far-red light. Whether they can fuel significant oxygen-generating photosynthesis is unknown.
Scientists recently identified the possible genes behind some cyanobacteria species’ ability to use far-red light to perform photosynthesis—a process that researchers think could point to oxygen biosignatures on other worlds.
For decades, scientists have been interested in Acaryochloris, a cyanobacterium found on Earth that uses far-red light for photosynthesis. Unlike most plants and algae, which produce chlorophyll a pigments to turn light into oxygen, Acaryochloris utilizes chlorophyll d (Chl d).
“These organisms break the rule for Earth.”
Earlier studies showed that Chl d, which harvests light at wavelengths of 700–750 nanometers, is produced by some cyanobacteria in a very small amount. In Acaryochloris’s case, those pigments are instead abundant. “Somewhere along the way, for these Acaryochloris, the gas pedal was stuck down, and they just made tons of them,” said astrobiologist Wesley Swingley of Northern Illinois University, a coauthor of the research.
“These organisms break the rule for Earth,” Swingley said. “It is a system that is meant to be turned on only under certain conditions.” All organisms that produce Chl d live in areas where visible light is scarce or where far-red light is strong.
Until now, which genes or pathways are activated in Acaryochloris to produce Chl d has been a mystery. Without that, it is difficult to know the environmental conditions that would constrain their production and to evaluate the likelihood of finding them in other star systems.
Swingley and his colleagues exposed two different groups of cyanobacteria—one that produces Chl d and another that does not—to far-red light and white light. They found that those with Chl d produced more oxygen under far-red light than those that lacked the pigment. This result confirms that Chl d is a necessary component for oxygenic photosynthesis under far-red light.
To decipher the genetic underpinnings behind Chl d production, the scientists exposed the same organisms to high-intensity light that damages their chlorophyll. As they removed the damaging light, these cyanobacteria began replenishing their chlorophyll supply. The researchers monitored the difference in gene expression patterns for their respective chlorophyll types and identified candidate genes responsible for the biosynthesis of Chl d. They presented their results at the 2024 Astrobiology Science Conference in Rhode Island.
Far Red in Far Worlds?
“Life is going to have its own history on other planets,” said astrobiologist Antígona Segura-Peralta from Universidad Nacional Autónoma de México, who was not involved in the study. Finding oxygen on another world would not guarantee life’s existence because the molecule can be produced through nonbiological processes, she noted.
Regardless, Segura-Peralta said the findings were exciting. “If we have enough information about the components in the atmosphere of the planet and the emission of the star, then we can know what to expect (or look for) from a planet.”
Finding candidate genes behind Chl d production was the first step to answering more questions, said Rohan Shiradhonkar, a graduate student in biotechnology science at Northern Illinois University and the first author of the study.
“Can these organisms exist on other planets? Or can organisms on other planets perform a chemical reaction like photosynthesis?” he asked. Now that the candidate genes have been identified, Shiradhonkar and his colleagues can study their functions and validate whether the same photosynthetic process could take place under a simulated red dwarf’s light.
—Kristel Tjandra (@KristelTjandra), Science Writer