In Chile’s Atacama Desert called Laguna La Brava, scientists have been studying a purple band of photosynthetic microbes that live in a high salt lake that never has oxygen. < / P > < p > “I’ve been working on microbial mats for 35 years,” says Pieter vische, a geographer at the University of Connecticut. “It’s the only microbial mat I can find on earth that can live without oxygen at all.” The microbial mats fossilized into stromatolites have existed on earth for at least 3.5 billion years. However, during the first billion years of its existence, there was no oxygen available for photosynthesis on earth. How these life forms can survive such extreme conditions is still unknown. But after studying stromatolites and extremophiles that still exist, the researchers found a few possibilities. < / P > < p > although iron, sulfur and hydrogen have long been considered as alternatives to oxygen, arsenic has not become a potential competitor for oxygen until the discovery of arsenotrophy in salty lakes Sears and mono in California. Since then, stromatolites from “tumbiana formation” in Western Australia have shown that light and arsenic capture was also an effective way of photosynthesis in the Precambrian. But iron and sulfur do not. < / P > < p > even Laguna La Brava’s life form is associated with an ecothiorhodospira sp. The purple sulfur bacteria are very similar. The bacterium, recently discovered in an arsenic rich lake in Nevada, appears to be able to oxidize compound arsenite into another form of arsenate for photosynthesis. < / P > < p > although more studies are needed to verify whether the microbes of Laguna La Brava also metabolize arsenite, preliminary studies have found that there are large amounts of hydrogen sulfide and arsenic in the water around these microbial mats. < / P > < p > if the author is correct and Laguna La Brava’s microbes are indeed “breathing” arsenic, these will be the first life forms to breathe arsenic in a permanent and completely anaerobic microbial mat, similar to the Precambrian environment we predicted. < / P > < p > although genomic studies have shown that Laguna La Brava’s microbial mats have the tools to metabolize arsenic and sulfur, the authors also suggest that their arsenate reduction seems to be more effective than sulfate reduction. < / P > < p > in any case, the researchers say there is strong evidence that both pathways exist, which is enough to support the survival of a large number of microbial mats in the early stages of life on earth. < / P > < p > “when looking for evidence of life on Mars, scientists will focus on iron, and maybe in the future they can also look at arsenic,” vische said. Global Tech