Light-to-electricity nanodevice reveals how Earth’s oldest surviving cyanobacteria worked

A cryo-electron microscopy structure of Photosystem I isolated from the bacterium Anthocerotibacter panamensis. Credit: Han-Wei Jiang et al./PNAS

An international team of scientists have unlocked a key piece of Earth’s evolutionary puzzle by decoding the structure of a light-harvesting "nanodevice" in one of the planet’s most ancient lineages of cyanobacteria. The discovery, published in Proceedings of the National Academy of Sciences, provides an unprecedented glimpse into how early life harnessed sunlight to produce oxygen – a process that transformed our planet forever. 

The team, including Dr Tanai Cardona from Queen Mary University of London, focused on Photosystem I (PSI), a molecular complex that converts light into electrical energy, purified from Anthocerotibacter panamensis – a recently discovered species representing a lineage that diverged from all other cyanobacteria roughly 3 billion years ago. Remarkably, this living relic shares almost no close relatives, with its nearest known evolutionary "sister" species parting ways some 1.4 billion years ago. 

“We cannot travel back three billion years to observe the cyanobacteria on Earth,” said Dr Ming-Yang Ho of National Taiwan University, lead author of the study. “That is why the early-branched A. panamensis is so crucial; it lets us glimpse what occurred in the past.” 

Most cyanobacteria, plus all algae and plants, pack their photosynthetic machinery into stacked membrane sheets called thylakoids: imagine several layers of solar panels. A. panamensis lacks thylakoids, confining its entire photosynthetic toolkit to a single membrane layer. That restriction limits photosynthesis, so these thylakoid-less cyanobacteria grow slowly and tolerate only dim light in the lab. 

“With this PSI structure in hand,” added co-author Dr Christopher Gisriel from University of Wisconsin-Madison, “We can compare it to others and see which features are ancient and which are recent evolutionary innovations.” 

The team found that, although the protein sequences have drifted like those in any bacterium, PSI’s architecture is almost unchanged: three PSI units join in a three-leaf-clover arrangement, collectively carrying more than 300 light-absorbing pigments such as chlorophylls and carotenoids. 

Dr Tanai Cardona concluded, “Even three billion years ago, photosynthesis appears to have reached a remarkable degree of sophistication. To find the true origin of oxygen-producing photosynthesis, we’ll have to look even further back—before cyanobacteria themselves evolved.” 

The study was funded National Science and Technology Council (Taiwan), NIH, U.S. Department of Energy and UKRI.