Sulphur-oxidising bacteria which lived on a very young Earth—back when the oxygen concentration in the latter’s atmosphere was almost zero—have been found fossilised on rocks in South Africa. The paper is published in the journal Geology.
Life on Earth is (mostly) characterised by the need for oxygen. And, then, we have exceptions to the rule, such as certain types of bacteria which do not use oxygen at all. The existence of these organisms actually predates the Great Oxidation Event, that is, before oxygen levels increased. 4.5 million years ago, when our planet had just come into being, the atmosphere contained barely any oxygen, with under one-thousandth of 1% of the current level, and this phase probably included bacteria. However, up until now, we had not found much evidence of these early bacterial life forms, nor did we understand how they survived without oxygen. The new paper provides insight into this very topic: the team who authored it mention evidence of the oldest-documented sulphur-oxidising bacteria in South Africa.
The fossils were spotted in two regions of South Africa’s Northern Cape Province.
“These are the oldest reported fossil sulphur bacteria to date,” says Andrew Czaja from the University of Cincinnati. “And this discovery is helping us reveal a diversity of life and ecosystems that existed just prior to the Great Oxidation Event, a time of major atmospheric evolution.”
The bacteria fossils are 2.52 billion-year-old from a geologic time called the Neoarchean Eon. Their main features include a specially large size, a spherical shape, and smooth walls. These organisms are considerably bigger than modern bacteria. However, they bear close resemblance to some single-celled organisms currently residing in our oceans, in regions with high sulphur concentrations and with almost no oxygen.
“These fossils represent the oldest known organisms that lived in a very dark, deep-water environment,” says Czaja. “These bacteria existed two billion years before plants and trees, which evolved about 450 million years ago. We discovered these microfossils preserved in a layer of hard silica-rich rock called chert located within the Kaapvaal craton of South Africa.”
As Czaja explains, these findings constitute the first direct proof of the existence of life forms thriving in deep water with no exposure to sunlight and oxygen.
The authors suggest that these early bacteria were feeding on volcanic hydrogen sulfide, and thus emitting sulphate. This process would be similar to what modern bacteria do nowadays upon recycling decaying organic matter.
“The waste product from one [bacteria] was food for the other,” adds Czaja.
“While I can’t claim that these early bacteria are the same ones we have today, we surmise that they may have been doing the same thing as some of our current bacteria,” says Czaja. “These early bacteria likely consumed the molecules dissolved from sulfur-rich minerals that came from land rocks that had eroded and washed out to sea, or from the volcanic remains on the ocean’s floor.
Czaja is also of the opinion that these bacteria were immediately followed by other bacteria living in shallow waters which started releasing an increasing amount of oxygen into the atmosphere, marking the beginning of the Great Oxidation Event.