Arsenic and Its Biological Role: From Early Earth to Current Andean Microbial Ecosystems

Abstract

Arsenic (As) is present in the Earth's crust and is widely distributed in the environment. It is frequently a component of sulfidic ores in the form of arsenides of nickel, cobalt, copper, and iron. The main sources of As are natural, mostly associated with volcanic areas and hydrothermal vents; further, it can also originate as a result of human activities: mining, waste treatment, and industrial activities, among others.Arsenic is a redox-active metalloid and exists in nature in four oxidation states: arsine [As(-III)], elemental [As(0)], arsenate [As(V)], and arsenite [As(III)]. These states vary according to changes in pH and the redox environment. The first two forms are relatively rare; naturally, As is found as As(V) or As(III). As(V) is predominant in oxygenated aqueous environments, while As(III) is found in reduced or anoxic conditions. Arsenic is a highly dangerous element, as As(III) is 100 times more toxic than As(V). Its greatest toxicity is due to the fact that it can bind to sulfhydryl groups, affecting the correct functioning of many enzymes and proteins. As(V), on the other hand, is a chemical analog of phosphate, so it can interact and eventually replace it in early steps in different ways. On our planet, there are environments with a high arsenic content.Our research group has worked in bioprospecting in Andean microbial ecosystems (AMEs) in the Atacama Desert, Bolivian Altiplano, and Argentine Puna (the so-called Puna?High Andes region). In all of these places, there are hypersaline lakes at altitudes higher than 3000 m above sea level (asl). These lakes share the common characteristic of high concentrations of arsenic, normally ranging between 12 and 230 mg L−1. This range of As concentrations is one of the highest ranges reported for hypersaline lakes.