Mineralogy and Microbiology at the Juan de Fuca Ridge

The largest aquifer on Earth lies within igneous oceanic crust. This deep environment plays host to microbial ecosystems, with diverse bacterial and archaeal communities living on energy produced when circulating seawater reacts with rocks and minerals.

basalt and life

Earth’s crust is not homogeneous, however, and in a new paper in Geomicrobiology Journal, DCO's Amy Smith, Rick Colwell (Oregon State University, USA), and colleagues investigate how variation in crust mineralogy affects the composition and abundance of deep microbial communities [1].

The authors set up a subseafloor microbial observatory in Integrated Ocean Drilling Program (IODP) Hole 1301A on the eastern flank of the Juan de Fuca Ridge in the Northeastern Pacific Ocean. They used a Circulation Obviation Retrofit Kit (CORK) to incubate sand-sized grains of eight igneous minerals and glasses for four years (2004-2008) in microbial flow cells 280 meters below the seafloor in 3.5 million year old rock. CORKs prevent circulation between seawater and the borehole, ensuring experimental microbial communities are solely influenced by aquifer fluids.

After retrieving the samples, the authors used 16S DNA sequencing to identify bacterial and archaeal taxa associated with each mineral. They confirmed that distinct communities colonize different minerals, and that these communities group by mineral chemistry. Olivine-phase minerals and iron-bearing minerals hosted bacterial and archaeal communities that were significantly different than microbial communities on iron-poor minerals.

The team then compared their results to previous studies on microbial communities from attached rocks and fluid from the same borehole, fluid from another borehole, and seawater. The microbial communities from these experiments are more similar to microbial communities attached to rocks and minerals than to fluid microbial communities, suggesting that there is a common deep microbial community associated with rocks and minerals.

“Structure and function are intimately related,” said Smith. “Through in situ studies such as these, we can now begin to understand what role Earth’s mineral structure plays in determining ecosystem function.”

 

Image: An Environmental Scanning Electron Micrograph of the surface of a basalt sand grain incubated in the subseafloor for four years, with microbes and crusty deposits visible on the surface. Credit: Amy Smith and Teresa Sawyer.

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