Arsenic and (Very) Old Life

This normally toxic substance might have been useful in the oxygen-deprived environment of early Earth.

Laguna La Brava in the Atacama desert of Chile. (Pieter Visscher)
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We usually think of arsenic as a poison, the Borgias’ murder weapon of choice when they wanted to do away with enemies. But according to a recent paper by Pieter Visscher from the University of Connecticut, Groton, and his colleagues, arsenic may have been a critical element for early life on Earth.

The researchers studied sulfur and arsenic-rich waters feeding into the salt lake called Laguna La Brava in the Atacama Desert of northern Chile, which has many similarities to early Earth. Both environments were characterized by high amounts of ultraviolet irradiation, huge temperature variations, a lack of oxygen, and water produced by precipitation as well as volcanic exhalations.

The microbial mats that Visscher analyzed are purple in color, suggesting that they formed before oxygen levels rose dramatically on Earth about 2.3 billion years ago. The mats may help us understand the metabolic pathway microbes relied on to survive before cyanobacteria pumped huge amounts of oxygen into Earth’s oceans and atmosphere. Visscher’s team showed that the microbes in these mats metabolize both arsenic and sulfur. In fact, arsenic (in the form of arsenates) is actually better in terms of the energy it supplies.

Based on studies of Mono Lake in California, we already knew that microbes can metabolize arsenic. But that was generally thought to be a very special adaptation of bacteria to extreme conditions in an unusual soda lake. That view is now changing very quickly, following a 2019 study by Jaclyn Saunders from the University of Washington, Seattle, supporting the idea that the biological use of arsenic is not only intended to lessen its toxicity, but also to gain metabolic energy. Saunders showed that some microbes use arsenic for respiration in the open tropical ocean, in places where there is no available oxygen.

A note of caution, though, before we start thinking of arsenic as some kind of wonder food for microbes. An earlier assertion that some microbes may prefer arsenic over phosphorous for growth and substitution in their DNA has largely been refuted. Nevertheless, it seems that using arsenic to gain critically needed energy is not only a way for life to gain a foothold in niche environments like Mono Lake, it may have been very important for early life on Earth. The same should hold true for life in any oxygen-depleted natural environment, including Mars.

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