New Microbial Phylum Discovered in Earth's Deep Soil Zone

[kevinaggner]

Scientists have identified a previously unknown phylum of microbes living in Earth's Critical Zone—the deep layer of soil that plays a key role in filtering water and maintaining environmental health. The discovery is detailed in a study titled "Diversification, niche adaptation and evolution of a candidate phylum thriving in the deep Critical Zone," published in the Proceedings of the National Academy of Sciences.

Leonardo da Vinci once remarked that humanity knows more about the stars than the soil beneath our feet—a sentiment echoed by Michigan State University microbiologist James Tiedje, who is working to deepen our understanding of this vital yet underexplored part of the planet.

“The Critical Zone extends from the canopy of trees down through soil layers to depths of around 700 feet,” Tiedje explained. “It governs essential Earth processes, including soil formation, nutrient and water cycling—all fundamental to sustaining life, food production, and clean water.” Despite its importance, the deep layers of this zone remain a largely untapped frontier.

While studying these deeper layers, Tiedje and his team discovered an entirely new microbial phylum, named CSP1-3. This group was found in soil samples collected from both Iowa and China, at depths of up to 70 feet. These regions were chosen due to their similarly deep and rich soils, with the goal of determining whether such microbes are more widespread than previously thought.

DNA analysis revealed that CSP1-3’s ancestors once thrived in aquatic environments like hot springs and freshwater systems. Over millions of years, they transitioned first into surface soils and eventually into deep subterranean environments. Importantly, the research showed that CSP1-3 microbes are not dormant but actively growing, albeit slowly, in these challenging conditions.

Contrary to expectations, CSP1-3 was not a rare microbial presence in these soils—it was dominant. In some cases, it made up over 50% of the microbial community, a figure virtually unheard of in surface soils. Tiedje believes this dominance stems from the phylum’s evolutionary adaptations to the nutrient-poor deep-soil environment.

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These microbes also play a crucial ecological role. As rainwater percolates through the soil, it becomes purified through various natural processes. While surface layers handle much of this filtration, CSP1-3 microbes complete the job in deeper layers, feeding on residual carbon and nitrogen compounds that escape the upper zones.

“CSP1-3 are essentially the last line of defense,” said Tiedje. “They clean up what passes through the upper soil layers. They have a specific job, and they’re doing it well.”

Looking ahead, Tiedje’s team aims to culture CSP1-3 in the lab to better understand the metabolic and physiological traits that allow them to thrive in such harsh conditions. This won’t be easy, as replicating their natural environment is a major challenge. However, insights from their genomes—particularly their connection to hot spring habitats—are guiding experiments involving high-temperature cultivation and other specialized conditions.

This discovery not only expands our understanding of microbial diversity but also highlights the importance of deep soil ecosystems in supporting life and maintaining environmental health.