Bacteria using “silicate sunscreen” provide a glimpse into Archean Earth

If you had the misfortune of becoming stranded for any length of time in the Atacama desert, you’d probably like nothing better than to see a clear blue pools of water stretching out across the landscape. Upon closer examination, however, these pools may begin to look a bit funny, perhaps a tad too crystalline. Known as el Tatio, this strange geyser field used to be a popular site for guides to stop and allow sweltering groups of tourists to get a refreshing drink of clean, clear Chilean water. It was only recently discovered that el Tatio contains the highest natural concentrations of arsenic on Earth- hundreds of times higher than the World Health Organization’s “recommended maximum limit” of ten micrograms/liter.

el Tatio geyserfield, Atacama desert, Chile

An abundance of arsenic is not the only strange thing about el Tatio- high concentrations of silicate minerals give the geyser an unusually glassy look (another good reason that the water is really not fit for human consumption!) Strange environments produce strange biology. Scientists studying el Tatio are now discovering some of the strangest- and perhaps oldest- microbial adaptations on Earth to cope with the stress on a dessicatingly dry, blazingly sunny environment awash in arsenic.

The microbes that have chosen to reside in this harsh environment are primarily arsenic reducers- they use energy from redox reactions involving arsenic to synthesize organic compounds. If this sounds at all familiar to you, it should. Plants do something very similar when they allow sunlight to stimulate their photosystems, releasing electrons that are later used to reduce carbon dioxide into organic carbon sugars. These bugs belong to a rare collection of microbes known as chemolithautotrophs- organisms that produce their own food, much like plants, using an alternate electron acceptors to eventually fix carbon dioxide.

Taking advantage of an abundance of arsenic to produce their own organic food source solves one of the problems associated with living in el Tatio- a scarcity of bioavailable carbon in the environment. It doesn’t, however, solve the problem of the intense sunlight that literally bombards the Atacama desert with powerful UV radiation all day. If the microbes in el Tatio didn’t have some way to protect themselves, their photoreceptors would quickly bleach and their DNA would be destroyed.

It turns out that this problem is solved by taking advantage of the peculiarly high levels of silica in el Tatio’s water. For most microorganisms, silicate mineral concentrations at these levels would be fatal- the precipitation of sharp silicious minerals would literally puncture any soft, free floating cell that tried to make a living. (Imagine living suspended in water, with giant, jagged pieces of glass floating around everywhere). These hardy microbes, however, precipitate a tough polysaccharide on their outer cell membranes that is actually able to capture silica particles and assemble them into a protective coating. Rather then let shards of silicate minerals destroy them, they assemble a house. This protective silicate coating serves two essential defensive purposes. Building silica shells protects the microbes from the silica itself, but it also deflects damaging UV radiation, preventing UV rays from damaging their cellular machinery.

Sounds creative, right? Actually, it may just be one of the oldest tricks in the book. Dr. Philip Bennett and colleagues at the University of Texas, Austin, are now  suggesting that sunscreening oneself up with silica may be  an ancient adaptation to survive in a world with a very thin atmosphere. Remember, when microbes first appeared some 3.5 billion years ago, the world was a very different place. There was very little oxygen in the atmosphere to absorb and deflect harmful UV radiation. Anyone alive today would have suffered acute radiation poisoning. Before the oxygenation of the atmosphere, some strange defenses must have been in place to allow early life to survive. The isolated bugs of the Atcama desert may be giving us clues as to how autotrophy originated on Archean Earth.

Credit: Dr. Philip Bennett, University of Texas at Austin. “Microbial Geochemistry: Coupling microbial ecology and mineral chemistry” . Oral presentation.

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