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Bacteria incorporate arsenic into lipids

Microbes in seawater are taking in low levels of potentially noxious arsenic and making it into lipids.

by Raleigh McElvery, special to C&EN
November 3, 2021


Two researchers collecting seawater samples.
Credit: Randelle Bundy
Katherine Heal and her colleagues sampled seawater in the North Pacific Ocean.

For centuries arsenic has been synonymous with poison and peril. However, it is also ubiquitous in the Earth’s crust, and runoff from natural deposits and industrial processes has spread it throughout the environment, including into the oceans. As a result, sea life around the globe is exposed to low concentrations of this potentially toxic element. Over the years, lab experiments have hinted at how the ocean’s smallest residents might cope. Now, a new study suggests that bacteria in their natural marine environments can detoxify arsenic by converting it into lipid compounds (Limnol. Oceanogr. Lett. 2021, DOI: 10.1002/lol2.10216).

In lab studies, researchers have determined that microbes readily take up arsenic because it’s easy to mistake for phosphorus, a nutrient they need to survive. They can either immediately flush out the arsenic, or string carbons and hydrogens onto it to make a variety of organic compounds, including lipids. These arsenolipids have also been observed in larger marine life, such as seaweed.

But Katherine R. Heal, a scientist at Integral Consulting Inc., wanted to know what organisms did in the open ocean. Heal’s team secured five large samples holding hundreds of liters of seawater each. Using a combination of liquid chromatography and two types of mass spectrometry, they quantified the total arsenic taken up by the organisms in each liter. They then characterized the specific compounds they saw. In doing so, they identified a rare lipid called an acyl ether glycerol (AEG). But these AEGs were special—they contained arsenic. To Heal’s knowledge, this is the first report of an “arseno-AEG.”

Although she couldn’t be sure exactly which microorganisms she’d captured in her samples, AEGs are considered biomarkers for bacteria.

Kevin A. Francesconi, an expert in environmental arsenic chemistry from the University of Graz, says this finding—along with previous work he was part of, identifying arsenolipids in plankton (Environ. Sci. Technol. 2021, DOI: 10.1021/acs.est.0c06901)—clearly demonstrates that marine microbes can produce arsenolipids in the ocean. The microbes may generate arsenolipids simply to detoxify arsenic, or the compounds could carry out important functions—like building cellular membranes—when there’s not enough phosphorus around. In their recent study, Francesconi, Ronald A. Glabonjat, Georg Raber, and coworkers estimated that ocean microbes could transform as much as 50,000-100,000 tons of arsenic into arsenolipids each year.

“It’s tantalizing to think that the organisms are actually producing these compounds specifically for a purpose we haven’t identified yet,” Francesconi says.

The broad effects of this chemical cycling on marine ecosystems are still to be determined, and likely depend on each location’s unique phosphate to arsenic ratio. Scientists also don’t yet know what happens to the arsenolipids inside microbes once the organisms die and sink.



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