New York, March 24 (IANS): We all know that photosynthesis, the process used by plants and other organisms to convert energy derived from light into chemical energy, maintains the oxygen levels in the atmosphere necessary for life on earth.

Cyanobacteria, the blue-green algae, that evolved photosynthesis were thought to have appeared 2.5 billion years ago when the 'Great Oxidation Event' (a spike in oxygen levels) happened.

But a new research suggests that first photosynthetic organisms lived on earth three billion years ago, long before the 'Great Oxidation Event'.

Noah Planavsky, a geochemist at Yale University and his colleagues analysed levels of molybdenum chemical and iron in 2.95-billion-year-old rocks from South Africa.
The metals serve as markers of photosynthesis.

Molybdenum isotopes, or elements with the same number of protons but a different number of neutrons, track manganese oxidisation - a process that requires high levels of oxygen, Planavsky said.

The rocks were laid down in water, in a shallow ocean setting near the shore.
The chemical traces in the rocks indicate cyanobacteria were producing oxygen in ocean surface water, Planavsky added.

“Our study is telling you that there was localised cyanobacteria production in the oceans,” he was quoted as saying.

Another study, also on South Africa's Pongola rocks, found that atmospheric oxygen was about 100,000 times higher than could be explained by non-biological chemical reactions.

“The two studies are quite complementary,” Planavsky said in the study that appeared in the journal Nature Geoscience.