The oldest evidence of photosynthetic structures reported to date has been identified inside a collection of 1.75-billion-year-old microfossils, a paper published in the journal Nature reveals. The discovery helps to shed light on the evolution of oxygenic photosynthesis.
Oxygenic photosynthesis, in which sunlight catalyses the conversion of water and carbon dioxide into glucose and oxygen, is unique to cyanobacteria and related organelles within eukaryotes. Cyanobacteria had an important role in the evolution of early life and were active during the Great Oxidation Event around 2.4 billion years ago, but the timings of the origins of oxygenic photosynthesis are debated owing to limited evidence.
Catherine Demoulin from the University of Liège, Liège, Belgium and others present direct evidence of fossilised photosynthetic structures from Navifusa majensis. The microstructures are thylakoids; membrane-bound structures found inside the chloroplasts of plants and some modern cyanobacteria. The authors identified them in fossils from two different locations, but the oldest, which come from the McDermott Formation in Australia, are 1.75 billion years old.
N. majensis is presumed to be a cyanobacterium. The discovery of thylakoids in a specimen of this age suggests that photosynthesis may have evolved at some point before 1.75 billion years ago. It does not, however, solve the mystery of whether photosynthesis evolved before or after the Great Oxidation Event. Similar ultrastructural analyses of older microfossils could help to answer this question, the authors say, and help to determine whether the evolution of thylakoids contributed to the rise in oxygen levels at the time of the Great Oxidation Event.
Thylakoids represent direct ultrastructural evidence for oxygenic photosynthesis metabolism. Thylakoid membranes are dense, mostly galactolipid, protein-containing bilayers in which photosynthesis occurs in photosynthetic organisms. “The discovery of preserved thylakoids in N. majensis from both the Shaler Supergroup and Tawallah Group provides direct evidence for oxygenic photosynthesis, for a cyanobacterial affinity and for metabolically active vegetative cell rather than a cyst (akinete) stage for these specimens,” they note.
“This discovery extends their fossil record by at least 1.2Ga and provides a minimum age for the divergence of thylakoid-bearing cyanobacteria at roughly 1.75Ga,” they write. “We predict that similar ultrastructural analyses of well-preserved microfossils might expand the geological record of oxygenic photosynthesisers, and of early, weakly oxygenated ecosystems in which complex cells developed,” they write.
