Tiny cells may best survive hot soils

The center median strip and double yellow lines of PA 61, a closed road that passed through the town of Centralia, which is now overgrown and graffiti-covered. (Credit: Getty Images)

As the planet warms, what life will survive and thrive? If the coal fire-fueled soils around Centralia, Pennsylvania, are any indication, organisms with smaller genomes and cells may do just fine, researchers say.

A new study represents the first time these kinds of microbes have been found afield and clearly shows that, for soil microbiomes, hot temperatures result in both smaller genomes on average and also smaller cell sizes, says Ashley Shade, a microbial biologist at Michigan State University and lead author of the paper, which appears in Nature Microbiology.

This isn’t the case of simply one microbe embracing an economical approach, either; the majority of populations living in the steaming ground have the same traits.

“This isn’t an evolutionary study; we are seeing organisms that are competitive in the hot environment when they already have these traits,” Shade says. “The populations living nearby in the cooler, ambient temperature zones are different than the organisms living in the thermal areas.”

Arctic permafrost

In an interesting twist, the tiny organisms’ genome sizes resemble those found in a completely different region of the world—those found in Arctic permafrost.

“In a comparison with other soils, we found that the average genome sizes in hot soils were most similar to those in ancient permafrost,” Shade says.

A single teaspoon of soil may hold millions of active and dormant microbes.

“Our working hypothesis is that these thermo-tolerant cells were not undergoing active genome reduction, but instead had never experienced substantial genome expansion because they are less derived from an ancestral state,” Shade says.

Soil is one of the world’s most-complex, most-diverse habitats. A single teaspoon may hold millions of microbes—active and dormant. In fact, it’s the dormant microbes that attracted the researchers’ attention, as they appear to be the leading potential source of these thermophile organisms in Pennsylvania.

Extreme conditions

The Centralia coal fire has been burning since 1962. Ignition of the large, underground coal seams has devastated area communities but created an unworldly laboratory, replete with abandoned roads, decrepit structures, and steaming vents that reek of rotten eggs.

For her research, Shade focused on microbes living across a temperature gradient, spanning from normal to thermal.

The thermophile microbes don’t appear to have evolved from their ambient-temperature-loving neighbors. Also, it doesn’t appear that an outside source, like wind, dropped them in.

That leaves dormant microbes, ones simply biding their time for the optimal conditions to animate, as the prime culprit to their origin.

“Centralia is a field environment in which we can observe organisms getting hit with a sledgehammer,” Shade says. “Traveling there allows us to probe extreme conditions—ones that caused a turnover in the entire community toward small genomes. The gradient happens in the same environment, in the same soils.”

Next steps

The scenario leaves the researchers wondering, what’s the minimum requirement for cell and genome? What other dormant microbes in soil, water, or our gut are awaiting to be awakened and identified?

For the next steps of this research, Shade’s team will dive deeper into the source of these microbes and will also examine the populations living on the gradient more closely, identifying areas of overlap and seeing where and how the different populations transition and compete as the fire heats up.

Additional researchers are from Michigan State and Susquehanna University.

Source: Michigan State University