During photosynthesis, plants and cyanobacteria convert water and carbon dioxide into sugar and oxygen using sunlight. This process is the basis for almost all life on our planet. However, little is known about the evolutionary origins. Researchers have now identified structures in 1.75 billion year old microfossils that are still responsible for photosynthesis in plants today. The discovery of these ancient thylakoid membranes provides the oldest direct evidence of photosynthesis to date.
In Earth's early days, the atmosphere contained only small traces of oxygen. That changed around 2.4 billion years ago with the so-called Great Oxygen Catastrophe. For reasons not yet fully understood, oxygen levels in the oceans and atmosphere increased rapidly, fundamentally changing the conditions on our planet. Most of the life forms that had existed until then died out. This is where the evolution of life as we know it today began. Scientists believe that photosynthesis, which produces oxygen as a waste product, played an important role in the sudden increase in oxygen. Early cyanobacteria, which were the first to develop oxygen-producing photosynthesis, were probably responsible for this. However, when photosynthesis emerged in its current form is controversial.
Membrane invaginations for effective photosynthesis
“Knowledge about how cyanobacteria differentiated is crucial to understanding the evolution of our planet and life,” writes a team led by Catherine Demoulin from the University of Liège in Belgium. What is particularly important is a structure that is found in most photosynthetically active organisms today: the so-called thylakoids. These are invaginations in the membrane of cyanobacteria or chloroplasts in plants. These invaginations contain a particularly large number of light-collecting complexes that enable effective photosynthesis.
“Among the cyanobacteria living today, there are groups with and without a thylakoid membrane,” explain Demoulin and her team. “Based on the genetic differences between these groups, it is assumed that they diverged between 2.7 and two billion years ago.” However, no fossil evidence from this period has yet been found. It is therefore also unclear whether the thylakoids developed before the Great Oxygen Catastrophe - and perhaps helped trigger it - or whether they only emerged afterwards.
Insights into the interior of the microfossils
Demoulin and her team have now examined fossilized remains of the microorganism Navifusa majensis. This was probably an early type of cyanobacteria. The oldest specimens come from the McDermott Formation in Australia and are dated to around 1.75 billion years old. Using a special electron microscope, the team managed to make the structures inside the fossil microorganisms visible. And indeed: There were clearly separated, stacked layers – the typical structure of a thylakoid membrane.
“This discovery extends the fossil record by at least 1.2 billion years and demonstrates that thylakoid-bearing cyanobacteria originated at least 1.75 billion years ago,” the team writes. Even though it is still not clear whether the thylakoids were formed before or after the Great Oxygen Catastrophe, the researchers assume that their method could help clarify this question in the future. “The ultrastructure of cell walls and organelles has received little attention,” they explain. “In the future, similar analyzes of even older microfossils could be used to identify early photosynthetically active organisms and gain new insights into the ecosystems of that time.”
Source: Catherine Demoulin (University of Liège, Belgium) et al., Nature, doi: 10.1038/s41586-023-06896-7