Heavy Metal-Loving Microbes (2)


Unless you’re someone like biologist David Lipson, a handful of the dark and often wet soil in Barrow, Alaska may not look very exciting. But if you know what Lipson knows,  you understand something else: the dirt is rocking, millions of microbes dancing to the tune of a heavy metal. These tiny bugs use iron, specifically iron oxide, to live in a waterlogged environment where oxygen is absent.  In addition to iron, the microbes also take advantage of the ancient mix of complex organic matter that makes soil dark brown or black (“humic substances”)—to complete their metabolic processes.

And because of these unique abilities, microbes play a key, controlling role in the Arctic carbon cycle—a rock-star role, if you will. As these millions of tiny microbes gulp iron oxide and soil, they release gasses, including carbon dioxide, in copious quantities. In some cases 30 percent or more of the carbon dioxide released from the Arctic’s landscape is a result of this microbial metabolic processes in soil.

Because of their importance in the vital exchange of carbon—and because of the potential of carbon dioxide to trap heat in the atmosphere—scientists like Lipson are very interested in understanding the contributions of these heavy metal-loving microbes to carbon exchange processes and looking at ways a warmer climate, and warmer, more iron-rich soils, may impact the Arctic in the future.

The Curious Professor

Lipson, a professor of biology at the University of California, San Diego, leads a team investigating this biochemical process in low oxygen, or suboxic, lakebed soils in and around Barrow with support from the National Science Foundation.

“These cold, northern soils have a lot of carbon and microbes control how much carbon dioxide can leave the soils. People think that if there’s no oxygen, that nothing is really happening and that’s just not true,” Lipson said. “Iron and humic substances affect the carbon cycle dramatically.

Iron is key in the Arctic where permafrost—ground that remains frozen year round—prevents oxygen-rich water from draining into the deeper soil layers. The result is an oxygen-free, or anaerobic, soil environment.

“It [iron oxide] is helping the microbes make a living and is important globally because it is related to climate change. Both carbon dioxide and methane are important green house gasses and the microbes in Arctic soils could emit large amounts of both of these gasses,” Lipson explained.

Lake Basin Study Sites

Northern Alaska has the perfect terrain to study microbial biochemical processes in oxygen-free soils. Empty, low-lying lake basins, ranging from young (50 years old or less) to ancient (around 5,000 years old or more), dot at least half of the area’s landscape.  Their oval shape and the underlying permafrost means more water is trapped in the top layers of the soil and thus less oxygen is present.

lake basins
lake basins

“There is this wonderful age gradient and that’s a great natural experiment to study soil development,” Lipson said. “We can see how the soil properties change with age and it gives us a really nice range of soil types to study the process.”

Digging in the Mud

When Lipson and his team visit these sites, they are mostly interested in the top layer of the soil, or roughly the first 30-35 centimeters. This is the layer that thaws every yearand is most biologically active.

The team gathers soils and microbial samples in a variety of ways. There’s seasonal soil core drilling that allows the researchers to study the oxidation rate at various times of year and under different environmental conditions. The team also collects readings on different variables like soil pH and other electrochemical measurements that tell a lot about what’s happening in the soil. Soil water samples, extracted with tiny vacuum tubes, are also collected at the sites.


“There’s a lot of walking up and down the landscape collecting soil samples, water samples and electrochemical measurements,” Lipson said.

Researchers analyze samples both in Lipson’s San Diego lab as well as in the field.

Partners at Stanford University and Cornell are joining Lipson in his efforts, researching the bioelectrical systems that result from these microscopic metabolic processes, as well as doing unique analyses of the iron minerals themselves.


A Flood of Ideas

Lipson has long had an interest in soil microbiology. He started studying microbial communities and how they change with the seasons and interact with plants in Colorado’s alpine country. Then, at the invitation of a colleague at the University of California San Diego, he made his way to Alaska to study far-northern soils and microbes. That NSF-funded study involved flooding parts of the landscape to look at the water table.

“This gave me the opportunity to explore and we sort of stumbled on to this iron reduction work,” Lipson explained. “Basically, because we flooded the landscape, I knew that anaerobic conditions would be very important. So we started looking at anaerobic processes and discovered that iron was really important.”

Upcoming Summer Field Season

This summer members of Lipson’s team will return to Barrow to focus on how methane fits into this cycle. When iron or humic substances are not available to the microbes they rely on other elements in the soil to respire. This summer one of Lipson’s graduate students will investigate the controls over methane from the landscape.

“She will measure methane fluxes after adding more iron, humic substances and doing different things to see if she can predict changes in the amount of methane coming out of the soil,” Lipson said.

Ultimately, all this digging in the mud in northern Alaska will shed light on a major control over the carbon cycle in the Arctic. Lipson hopes to expand this work to other parts of the Arctic and also sees its relevance to wetland areas.

“People are starting to recognize that these processes are important all over the place. In the Arctic, one thing that could be really important in terms of climate change is how might this change if the permafrost starts to melt—and the active layer gets deeper—that might increase the amount of iron available and so it could be that these processes become even more important in the future.” Lipson said. And that could make these tiny bugs a very big deal.