Arsenic metabolism in extreme environments

Laguna Diamante

In 2020, a team of astrobiologists found evidence that early Earth microorganisms could have used arsenic as a metabolic energy source by studying current day extremophiles living in hostile conditions similar to ancient ecosystems. Laguna Diamante is a remote lake formed by the Galán Volcano in Catamarca, Argentina. Residing 4,600 meters above sea level, this altitude is accompanied by low oxygen and high ultraviolet radiation. The hypersalinity of the lake makes it very alkaline, with a pH ranging from 9-11. Toxic gasses from this volcanic lake are one of many difficulties to life in the lagoon. Extremely high arsenic concentrations pose another challenge to survival here. Yet, life does persist.

Microbial Mats & Arsenic Cycling

One group of residents in the lagoon may help us understand the evolution of life before an oxygenated planet. Layers of microbes, called microbial mats, build on top rock and sediments in the lagoon. Microbial mats are the first known evidence of life on Earth and are highly adaptable in extreme environments, such as Laguna Diamante. The conditions of the lagoon resemble neoproterozoic Earth, alkaline waters, low oxygen, high UV radiation, and volcanic activity. The goal of these researchers is to find evidence of active arsenic metabolisms in these microbial mats and their preservation in the rock record.

During the process of photosynthesis, inorganic carbon (CO2), water (the electron donor), and sunlight are used to make energy and produce carbohydrates, water, and oxygen as a byproduct. In anoxygenic photosynthesis, other available sources are used as the electron donor. Reducing compounds such as sulfide, thiosulfate, sulfur, ferrous iron, hydrogen, and as the team of astrobiologists are interested in, arsenic. A few sulfur oxidizing microbes could possibly use As(III) as the electron donor in environments abundant in arsenic. When in the right conditions, microbial mats produce calcium carbonates during photosynthesis, creating lithified structures called stromatolites that can be preserved in the rock record.If microbial mats use arsenic as a source for photosynthesis, then both the oxidized, As(III), and reduced, As(V), forms of arsenic could be found in stromatolites. In 2014, the researchers also found similar evidence of Arsenic cycling in 2.7-billion-year-old stromatolites from Tumbiana, Australia that indicated microbes used arsenic for photosynthesis and respiration. Samples were collected from microbialites around the oxygen-depleted shoreline which consisted of a pink crystalline surface, purple and green under-layer, and a carbonate interior. X-ray synchrotron radiation analysis was used to map arsenic distribution in microbialites. The speciation maps found that As(III) was thinly spread throughout and the presence of As(V) in a globular pattern. The two types of arsenic suggest that there is active arsenic cycling in Laguna Diamante microbialites.

Further Evidence & Beyond

In 2020, the team solidified evidence of recent arsenic cycling at Laguna La Brava in Chile's Atacama Desert. Unlike Laguna Diamante, these stromatolite forming microbes live in complete absence of oxygen. Microbial samples were cultured in a lab where they were able to photosynthesize using arsenic. Genetic and X-ray analysis also provided evidence of an arsenic metabolism. Understanding the ability of life to persist in harsh, oxygen-absent environments on Earth may help in the search for life elsewhere. The confirmation of arsenic cycling means arsenic can be a detectable biosignature in areas of similar conditions. If this geochemical pattern is found on another planet, that could be evidence of life at some point in that planet’s history.

References:

Sancho-Tomas, M., Somogyi, A., Medjoubi, K., Antoine Bergamaschi, & Visscher, P. T. (2020) Geochemical evidence for arsenic cycling in living microbialites of a High Altitude Andean Lake (Laguna Diamante, Argentina). Chemical Geology, 549, pp.119681. ff10.1016/j.chemgeo.2020.119681ff. ffhal-02912566f