By weaving together multiple threads encapsulating evolutionary biology, functional genomics and geochemistry, an international team has documented the evolutionary history of arsenic resistance systems facilitating radiation of life into a primordial toxic world.

Headed by ZHU Yongguan from the Institute of Urban Environment of the Chinese Academy of Sciences and Barry P. Rosen from the Herbert Wertheim College of Medicine at Florida International University, and other scientists from Helmholtz Centre for Environmental Research – UFZ and Georgia Institute of Technology, the team has published the results in a paper in PNAS entitled "The Great Oxidation Event expanded the genetic repertoire of arsenic metabolism and cycling".

Poisoning Event in the Early Earth 

Life evolved in an environment rich in toxic heavy metal(loid)s, and one of the most notable events in the early Earth occurred between 2.45 and 2.35 billion years ago, during which the atmospheric oxygen rose to appreciation levels (~ 1% of oxygen present in the atmosphere today). This episode, known as the Great Oxidation Event (GOE), reorganized redox cycling of toxic metal(loid)s, including arsenic.

Arsenic existed predominantly in its reduced form (i.e., arsenite) in oxygen-poor environments preceding the GOE. When oxygen built up in the atmosphere, the oxidized arsenic species (i.e., arsenate) took over.

"The introduction of new toxicant – arsenate – into the anaerobic biosphere must have represented a catalysm in the history of life," said Prof. ZHU, co-corresponding author of the research article "How our ancestors overcame it remains largely unexplored".

Modern-day Genomes Bear Imprints of Early Evolutionary Events 

Part of the challenge in reconstructing the evolutionary history of arsenic detoxification systems lies with the paucity of fossil records, precluding studies on Precambrian life. "Can modern-day genomes serve as Precambrian fossils? Will ancient biogeochemistry events leave imprints on the composition of extant genomes?" asked the postdoc researcher Dr. CHEN Songcan in UFZ, lead author of the study.

Following this direction, the research team compiled genetic data representing a full range of hitherto characterized arsenic resistance pathways from 786 living organisms and mapped their evolutionary origin onto a geological timeline using relaxed molecular clocks approaches.

The results imply a genetic expansion of microbial arsenic resistance systems in response to the GOE. Prior to the GOE, microbial communities utilized anoxic pathways; following the GOE, microbes invented new resistance genes for detoxification of oxidized arsenic species.

"Genetic innovations enabled ancient microbes to explore more complex ecosystems with stratified arsenic chemistry ensuing the GOE" said Barry P. Rosen, who initiated this study.

Open Questions Remain 

"Our results provide new insights into major questions of arsenic biology," said Prof. ZHU, "further studies that benchmark of geochemical records against the evolutionary timeline present here are required to refine the interactions of metalloid redox chemistry, evolution and planetary history."