What are the biochemical effects of arsenic

Biochemistry: bacteria with arsenic in their DNA

How strange can life get? Astrobiologists ask themselves that. Now they have succeeded in breeding bacteria that live with arsenic (which is poisonous for us) instead of phosphorus.

Arsenic, the chemical element with atomic number 33, is decreed as a poison, and not only in combination with lace. In fact, its compounds are highly toxic. Millions of people in Bangladesh suffer from chronic poisoning from arsenic in drinking water, and arsenic compounds were used in chemical warfare agents during the First World War.

Arsenic compounds only act as stimulants in very small doses. Rosstäuscher use arsenic (As2O3) to stir up tired horses, and in the US southern states, but also in the Alpine region, it has long been used as a drug by "arsenic eaters", in Tyrol under the name "Hüttrauch". There it is said to have served murderous purposes, see Helmut Qualtinger's “Ahndlvertilgung”.

 

Chemically similar to phosphorus

A good part of the poisonous effect of arsenic is probably due to the fact that it is directly below phosphorus in the periodic table of the elements. So it has similar chemical properties, just a little bit more metallic. Like phosphorus, it has five electrons in the outermost shell; like this it forms oxygen compounds in which it is pentavalent. The counterpart to phosphate (PO43–), for example, is arsenate (AsPO42–). A number of metabolic processes cannot differentiate between the two; arsenic is built into biomolecules instead of phosphorus, with mostly fatal consequences.

 

Life with other elements?

Can you imagine life that consistently uses arsenic instead of phosphorus? In which, for example, there is no phosphate but arsenate in the DNA? A wild thought that comes to people who professionally consider what life “elsewhere”, for example on distant planets, could look like. Whether the "canonical elements" of (earthly) life - carbon, hydrogen, nitrogen, oxygen, phosphorus, sulfur - are really irreplaceable everywhere and always.

Such people work for NASA's Astrobiology Institute; Felisa Wolfe-Simon is one of them. In 2009 she published a corresponding article in the International Journal of Astrobiology with the title: "Did nature also choose arsenic?"

 

"Strange Life" on Earth?

Since the chemistry on distant planets can hardly be researched for the time being, astrobiologists are interested in extraordinary organisms that thrive on earth, but in very special, in our terms "extreme" biotopes. They speak of “weird life” and of a “shadow biosphere”. One looks for (and finds) one mostly in hellish areas, such as underground volcanoes, where archaebacteria feed on sulfur.

Wolfe-Simon and her colleagues found what they were looking for in a much brighter, almost idyllic landscape: at a Californian lake called Mono Lake, which is very salty and alkaline and also contains a lot of arsenic, especially in the form of arsenate. Your working hypothesis: bacteria that live there have to tolerate a lot of arsenic. Perhaps they can be bred to tolerate even more? That they replace their phosphorus entirely with arsenic?

The astrobiologists took samples from the mud at the bottom of the lake and drew cultures of bacteria from it. They then gradually transplanted these bacteria (for connoisseurs: Halomonadaceae) into nutrient media that became richer in arsenic and poorer in phosphorus. “I held my breath with every step,” says Wolfe-Simon. It worked. The bacteria not only survived and grew, but built the arsenic into their biomolecules.

 

Arsenylated amino acids

The researchers proved this by radioactive labeling. The amino acids that are normally phosphorylated (serine, tyrosine, threonine) were partially arsenylated in the cultivated strains. And also in the DNA of the bacterium arsenic had partly taken the place of phosphorus (Science, 330, p.1302). Apparently the bacterium “doesn't care” whether it gets arsenic or phosphorus.

"Our find shows that life is much more flexible than we can normally imagine," says Wolfe-Simon. "If it can do something so unexpected on earth - what else can life do that we have not yet seen?"

Arsenic in art

As "royal yellow" served arsenic sulfide (As2S3) Dutch painters of the 17th century. Unfortunately, it makes the images crumbly as it turns into As2O3 over time. Copper arsenates were added to the "Parisian green" with which wallpapers were printed. Napoleon is said to have suffered poisoning from arsenic released from the wallpaper.

("Die Presse", print edition, December 3rd, 2010)