Photosynthesis In Oceans Existed Earlier Than We Thought
Simple, photosynthesising life forms created an excessof oxygen in the oceans 700 million years earlier than previousestimates suggest, an international team of geologists claim.
The research, published today in the journal Nature Geosciences, pushes back the earliest appearance of photosynthesising organisms from 2.7 to 3.46 billion years ago.
Microscopic organisms such as cyanobacteria create oxygen as a by-product of photosynthesis.
The timing of their first appearance is hotly debated as it provides clues as to how early life on earth evolved.
Crystallised in time
Untilnow, the earliest evidence of photosynthesis was microscopic fossilsfound in shale rocks in Western Australia dating from 2.7 billion yearsago.
Now a team of Japanese, US and Australian scientists, led by Dr Masamichi Hoashi of the Kagoshima University,Japan, have found evidence for oxygen in ancient sea water from marinesedimentary rocks in the Pilbara region of Western Australia.
The evidence comes from tiny crystals of the iron-oxide mineralhaematite in a 160-metre-long core section that forms part of theMarble Bar Chert.
Haematite can form in the presence of aerobic (oxygen-loving)bacteria in the water, or by photo-electric processes in the upper 10metres of seawater.
The researchers say haematite crystals in the Marble Bar Chertformed in water at least 200 metres deep, because microscopic analysisof the rocks show no sign of wave action or other structurescharacteristic of shallow-water sediments.
The orientation and nature of the grains of haematite also show thatit precipitated directly from the seawater, rather than forming laterfrom other processes, such as the movement of groundwater, they add.
"These data strongly suggest that oxygenic photoautotrophsflourished in the photic zone of the 3.46 [billion-year-old] oceans andsupplied molecular oxygen to the deep water," the researchers write.
