Most of life on Earth today depends on oxygen. But before oxygen appeared on the planet, life forms probably used arsenic instead. To such conclusions scientists came in a new study, having published the results in the journal Communications Earth&Environment.
A key component of the oxygen cycle is that plants and some types of bacteria use sunlight, water, and CO2 to convert them into carbohydrates and oxygen, which are then circulated and used by other oxygen-using organisms. This oxygen serves as a vehicle for electrons, receiving and giving them away in metabolic processes. However, during the first 1.5 billion years of life on Earth, oxygen was absent, and scientists did not understand how all systems on Earth worked, says the study’s lead author, professor of marine sciences at the University of California and Earth Sciences Peter Visher.
Theories about how life processes functioned in the absence of oxygen have largely relied on hydrogen, sulfur, or iron as the elements that carry electrons to meet the metabolic needs of organisms.
As Vischer explains, these theories are disputed; for example, photosynthesis is possible with iron, but researchers find no evidence of this in the fossil record before oxygen around 2.4 billion years ago. Hydrogen is mentioned, but the energy and competition for hydrogen between different microbes show that this is not feasible.
Arsenic is another theoretical possibility, and evidence for this was found in 2008. Visscher says the new evidence came in 2014 when he and his colleagues found evidence of arsenic-based photosynthesis billions of years ago. To further confirm their theory, researchers needed to find a modern analog for studying biogeochemistry and the turnover of elements.
The tricky aspect of working with the fossil record, especially as ancient as some of the stromatolites, is that few of them remain due to the cycle of rocks as the continents move. However, the breakthrough came when the team discovered an active microbial mat that currently exists in the harsh environment at Laguna la Brava in Chile’s Atacama Desert.
These mats have not previously been studied, but represent an otherworldly set of conditions similar to those of the early Earth. Mats are in a unique environment that leaves them in a permanent oxygen-free state at high altitudes, where they are exposed to daily fluctuations in temperature and high UV radiation. They serve as a powerful and informative tool for understanding life in the early earth.
Scientists began working in Chile, where they discovered a blood-red river. The red deposits are composed of anoxogenic photosynthetic bacteria. This water also contains a lot of arsenic. It contains hydrogen sulphide, which is of volcanic origin and flows very quickly over the mats. There is absolutely no oxygen here, the scientists explain.
The team of scientists showed that the mats create carbonate deposits and a new generation of stromatolites. Carbonate materials also showed evidence of arsenic cycling – it serves as an electron carrier, proving that microbes actively metabolize it, like oxygen in modern systems. Visher says these findings, along with the fossil evidence, provide a clear indication of the early states of the Earth.