A team from BIAM, in collaboration with the Laboratoire de Chimie Bactérienne (LCB), proposes a new scenario for the origin of photosymbioses. Their work suggests that oxygen produced by photosynthesis, rather than sugars, may constitute the initial advantage of these associations that profoundly transformed the biosphere.
Photosymbioses—the intimate associations between a non-photosynthetic organism and a partner capable of capturing light energy—are at the origin of the modern biosphere. Algae, corals, and plants, including agricultural crops, rely on this ability to produce organic carbon from carbon dioxide. Until now, the supply of photosynthetic sugars was considered the cornerstone of these relationships. Yet, the evolutionary mechanisms that enabled the durable establishment of a photosynthetic organism inside another remained largely unknown.
Tracing Back to the First Steps of Symbiosis
To explore these events from the past, that still evolve today, researchers developed an original experimental approach. They brought together a predatory eukaryotic microorganism, the ciliate Tetrahymena thermophila, with photosynthetic prey: cyanobacteria or Chlorella microalgae. This system allowed them to reconstruct, in real time, cellular interactions likely mimicking the earliest stages of photosymbiosis.
Oxygen: An Unexpected Evolutionary Driver
The study’s central finding upends conventional wisdom: “In hypoxic conditions, common in shallow, organic-rich aquatic environments [settings that may have favored the emergence of the first photosymbioses], the presence of photosynthetic prey significantly improves the predator’s survival,” explains Christophe Robaglia, professor at BIAM and co-author of the study. “This protection does not rely on sugar supply but on local oxygen production by photosynthesis,” he points out. “Furthermore, in a carbon-poor but well-oxygenated environment, we observe little to no immediate benefit to the predator from the carbon metabolites produced by the photosynthetic prey.”
These observations suggest that photosynthetic oxygen may constitute a key selective force in the evolution of photosymbioses, with carbon supply being gradually selected as an additional advantage over the course of evolution (what is known as exaptation). This hypothesis resonates with Earth’s history: emerging around three billion years ago in bacteria, oxygenic photosynthesis first created a hostile atmosphere before enabling aerobic metabolism and the emergence of new forms of life, such as multicellular organisms.
Experimenting with Evolution Using Advanced Technologies
Beyond the conceptual breakthrough, the work relies on complementary approaches combining microbiology, dissolved gas measurements, flow cytometry, fluorescence microscopy, and reconstruction of simplified ecosystems in the lab. This success highlights the importance of interdisciplinarity: BIAM’s expertise in photosynthesis ecology, combined with LCB’s technical capabilities for culturing and analyzing microorganisms under controlled conditions, was crucial for designing and leveraging this experimental model.
Future Applications, towards life in Extreme Environments?
In the longer term, this model could serve as a basis for identifying the cellular and molecular mechanisms enabling the stable establishment of a photosynthetic symbiont
inside a host cell. Speculative applications are envisioned, from synthetic biology to designing organisms capable of locally producing oxygen, or even scenarios for colonizing hostile extraterrestrial environments.
But for the researchers, one of the major takeaways remains the importance of fundamental research. The project stemmed from a chance encounter between two scientists on an apparently distant topic, driven by the same curiosity: “to understand and reconstruct, with contemporary tools, crucial events in the history of life,” enthuses the scientist. An approach that today opens new avenues, both for exploring natural hypoxic environments and for revisiting, in a new light, the origins of the oldest biological alliances.
REFERENCES
Loïc QUEVAREC; Rachel BONNARDE, Christophe ROBAGLIA; Gaël BRASSEUR.
doi.org/10.1016/j.cub.2026.01.010