A key component of the oxygen cycle is what plants and some species of bacteria use
Theories about how life processesfunctioned in the absence of oxygen, mostly 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, andevidence of this was found in 2008. Visscher says 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 analogue for studying biogeochemistry and the turnover of elements.
The challenging aspect of working with the fossil record,especially such ancient ones as some stromatolites, is that there are few of them due to the circulation 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 of Laguna la Brava in Chile's Atacama Desert.
These mats have not previously been studied, but representis 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 started working in Chile, where they foundblood 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.
A team of scientists has shown that mats createcarbonate 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.
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