About 65 million years ago, massive volcanic eruptions spread lava over an estimated 1.5 million square kilometers in India creating the Deccan Traps. In the millions of years since, the Traps have eroded into impressive layered rock formations and microbes have colonized their fissures and fluids. Only recently have scientists been able to access these microbes through scientific drilling expeditions and figure out how they survive within these deep, nutrient-limited rocks.
In two new publications, DCO Deep Life Community members Avishek Dutta, Pinaki Sar, Himadri Bose (all at Indian Institute of Technology Kharagpur, India), Douglas Bartlett (University of California San Diego, USA), and colleagues further illuminate these deep communities. The researchers explored the archaea living within the Deccan Traps basalt and the underlying 2.5 billion-year-old granite basement layer. They also investigated how these “extremophiles,” which are microbes that survive in seemingly inhospitable environments, survive the heat and intense pressure of the Deccan Traps. Despite the challenging conditions, microbes persist in this environment and actively contribute to deep biogeochemical cycles.
“The deep subsurface realm underneath the Deccan Traps represents an extreme habitat for life,” said Sar. “Apart from their low-nutrient nature, these deep basalt layers and underlying granitic rock are at extreme temperature and pressure regimes.” For every 1000 meters of rock pressing down, the pressure increases by more than 250 times the pressure at sea level and the temperature rises by 25 degrees Celsius in basalt and by 15 degrees in granite.
In a new paper  in Frontiers in Microbiology, the researchers reported two distinctly different communities of archaea within the basalt and granite which reflect the different characteristics of the two rock types. “Members of the domain Archaea are considered to have evolved as the most extremophilic microorganisms on our planet, yet their diversity and community composition in deep terrestrial igneous crusts are least explored,” said Sar.
The researchers collected rock samples from different depths up to about 1400 meters below the surface using cores from exploratory drill holes in the Koyna–Warna region of the Deccan Traps in western India. They identified archaea living within the rocks through DNA sequencing.
The basalt layers hosted acid- and heat-tolerant archaea that metabolize iron and sulfur for energy, and which are known to colonize other areas with geothermal, tectonic, and volcanic activity. In the deeper granite layers, methane-oxidizing archaea and archaea that produce methane dominate, indicating a greater role for methane metabolism in the hotter, organic carbon-lean granite.
The distinct community of archaea in the granite layer suggests that those populations may have existed there since before the eruption of the Deccan Traps. In contrast, the researchers suspect that archaea in the basalt layers trickled in from the surface. A few of the organisms appear to be related to marine microbes, making it possible that seawater may have infiltrated the area at some point in the geological past.
In a second recent paper  in Extremophiles, the researchers enriched communities of microbes from Deccan Traps fluids under pressure in the lab and identified genes that likely enable their high-pressure lifestyles. The microbes came from a 100-meter deep aquifer within the Traps, but they may actually originate from farther down because water movement connects the aquifer to deeper fluids.
DNA sequencing of the pressurized cultures detected multiple “stress response” genes that are known to help microbes survive other extremes, such as high heat, acid, or salt. These genes are involved in important housekeeping functions like nutrient uptake, protein folding, and DNA repair. Scientists have not yet identified genes specific for pressure-tolerance, but microbes from other high-pressure environments often carry similar stress response genes. Further experiments are necessary to verify the role of these genes in tolerating extreme pressures.
In their current work, the team is going deeper and investigating the microbial inhabitants of even lower layers of granite crust, using rock cores from up to 3000 meters deep. They also have cultured numerous microbes that thrive in the hot, oxygen-free subsurface by metabolizing hydrogen, carbon dioxide, or methane. Now they are attempting to piece together the carbon and hydrogen cycles occurring in the deep terrestrial subsurface beneath the Deccan Traps.
Main image: The Western Ghats hills at Matheran in Maharashtra, India are part of the vast Deccan Traps, leftover from massive eruptions 65 million years ago. Credit: Nicholas (Nichalp) via Wikimedia Commons