Weather and human activity impact lake extremophiles Gaby Clark Scientific Editor Andrew Zinin Chief Editor The Great Salt Lake is separated by a 20-mile (32-kilometer) causeway into a super-salty north arm and a less-salty south arm, with each hosting distinct communities of extremophiles. These organisms mingle at a gap in the causeway, designed to regulate salinity, that has provided microbiologists with an unusual and rich opportunity to study how human activities and weather disturbances affect microbial communities in extreme environments. Scientists began observing microbial communities near the breach in the fall of 2022, when water levels hit a record low after years of extreme drought.

In the spring, as snow melted in the surrounding mountains, floodwaters poured in. As water levels fell and rose, government agencies installed a berm in the channel to regulate water flow. The height of the berm could be changed to manage the salinity in the less saline south arm.

Water flow reshaped microbial mixing This week in Applied and Environmental Microbiology, the researchers report results from the first year of seasonal sampling. They found that weather- and human-driven changes directly affected the mixing of the microbes, not only in the immediate vicinity of the breach but also far from the causeway. An analysis of the system using hydrodynamic modeling showed that water flow largely drove the community changes.

"This is the first study conducted in every season across one year in the Great Salt Lake," said Amy Schmid, Ph.D., a molecular biologist at Duke University who led the study. Her work focuses on how biological networks from molecules to microbes interact and develop resilience to stress. Their analysis showed that during the 2022 drought, microbial diversity in the north arm remained mostly unchanged but dropped significantly in the south arm, likely because of shifts in water flow around the underwater berm.

"It really seems to go in one direction," said Alex Phillips, Ph.D., a microbial ecologist at Duke and first author on the study. "We're seeing the extreme hypersaline microbes from the north arm mixing into the less dense water column from the south arm, but not really the other way." In future analyses, he said, the researchers plan to investigate why the microbes respond in the ways they do. A lake nearing a tipping point The Great Salt Lake is a terminal lake, which means water flows in from rivers and leaves only by evaporation.

It's one of the largest in the world, and it's also in peril: Decades of redirecting rivers for agricultural use, combined with prolonged droughts, have driven water levels to persistent lows. The two arms of the lake represent different stages of drying, Schmid said, adding that the saltier north arm may be at its final stage before becoming a fully terrestrial environment. Studies of microbial responses to stressors like human intervention and extreme weather may help inform policy to protect the ecosystem.

"If we understand how the lake would die, we can save it," Schmid said. "We have the opportunity to understand that tipping point for the south arm.