Cyanobacteria are single-celled, photosynthetic organisms that have an outsized impact on our planet: these ancestors of modern plant life started transforming Earth’s atmosphere with the oxygen emitted during photosynthesis, making way for modern plant and animal life. Today, cyanobacteria are used to make biofuels and also provide a basis for studying how photosynthesis has evolved over millennia. New research from scientists at the University of Wisconsin–Madison analyzes the photosynthetic machinery of a newly discovered cyanobacterium, giving scientists a novel evolutionary line to explore the history of photosynthesis.
UW–Madison scientists led by biochemistry assistant professor Christopher Gisriel collaborated with scientists at National Taiwan University, Queen Mary University of London, and Yale University to capture the first images of the photosynthetic machinery in the cyanobacteria Anthocerotibacter panamensis, a species discovered in 2021. Their findings are published in the Proceedings of the National Academy of Sciences.
Over time, photosynthetic organisms evolved from cyanobacteria into more complex beings such as algae and plants. To sustain life in a larger body, their cells evolved to include more complex structures such as chloroplasts, and their photosynthetic machinery (photosystems) evolved to efficiently harvest energy from a wider range of light wavelengths. Despite these sophisticated evolutionary modifications, the photosystems of today’s giant trees still bear strong resemblance to that of their ancient cyanobacteria ancestors.
A. panamensis is believed to be an especially ancient strain of cyanobacteria. It’s only the second cyanobacteria discovered that lacks the stacked membrane structures that hold the photosystems in more advanced cyanobacteria. The other, Gloeobacter, is a distant cousin of A. panamensis. Photosystems in A. panamensis and Gloeobacter are nestled in the inner cell membrane.
“The photosystem we see in A. panamensis is probably pretty close to what we would have seen in the ancient ancestor of all cyanobacteria. It preserves only the most conserved features across photosystems,” says Gisriel. “With this structure in hand, we can compare it to other photosystems and see which features are ancient and which are more recent evolutionary innovations.”
To get a closer look at A. panamensis’s photosystem, the researchers used cryo-electron microscopy (cryo-EM), an imaging technique for capturing small biomolecules at a moment in time by rapidly freezing samples, stopping them in action. They also analyzed where A. panamensis falls in cyanobacteria’s genetic tree.
Images revealed that the ancient organism retains some ancestral features not seen in Gloeobacter, including a distinct mix of light-soluble pigments and an ability to capture low-energy light. These differences suggest that primitive cyanobacteria evolved in distinct ways, and, the researchers say, highlights the importance of studying multiple lineages of cyanobacteria to understand early photosynthesis.
Broadening our understanding of the evolution of photosynthesis may have implications for agriculture and sustainability through improved biofuels and plant breeding with protein engineering. “These studies are an important building block,” Gisriel says. “Understanding how the biology of photosynthesis evolved different features can lead us to leveraging that knowledge to actually build similar types of complexes that can harvest light efficiently under different conditions.”
Written by Renata Solan.