Traces of ancient microbial communities have been found in rock samples of Earth’s mantle from a seafloor hydrothermal system that was active more than 100 million years ago during the Lower Cretaceous – the earlier of the two major divisions of the Cretaceous period – when the supercontinent Pangaea was breaking apart and the Atlantic Ocean was just about to open. This discovery, published in the Proceedings of the National Academy of Sciences, confirms a long-standing hypothesis that interactions between mantle rocks and seawater can create potential for life even in hard rocks deep below the ocean floor.
These rocks, recovered by the Ocean Drilling Program on board the drilling vessel JOIDES Resolution, are from the Earth’s upper mantle that underwent intense alteration by heated seawater. The rocks show a systematic change in color from rusty brown (top) to green and black (bottom), reflecting the chemical gradients across the fluid mixing zone. These chemical gradients played a key role in supporting microbes with chemical energy and the substrates they needed to thrive. Fossilized microbes were found in white veins consisting of the minerals calcite and brucite. Image credit: Ocean Drilling Program.
“We were initially looking at how seawater interacts with mantle rocks, and how that process generates hydrogen,” said lead author Dr Frieder Klein of the Woods Hole Oceanographic Institution.
“But during our analysis of the rock samples, we discovered organic-rich inclusions that contained lipids, proteins and amino acids fossilized in the surrounding minerals.”
This study focused on mantle rocks that were originally exposed to seawater approximately 125 million years ago when a large rift split Pangaea. The rift, which eventually evolved into the Atlantic Ocean, pulled mantle rocks from Earth’s interior to the seafloor, where they underwent chemical reactions with seawater, transforming the seawater into a hydrothermal fluid.
“The hydrothermal fluid likely had a high pH and was depleted in carbon and electron acceptors. These extreme chemical conditions can be challenging for microbes,” Dr Klein explained.
“However, the hydrothermal fluid contained hydrogen and methane and seawater contains dissolved carbon and electron acceptors. So when you mix the two in just the right proportions, you can have the ingredients to support life.”
The rock samples analyzed by Dr Klein and his colleagues were originally drilled from the Iberian continental margin off the coast of Spain and Portugal in 1993.
During that expedition scientists drilled through almost 2,300 feet (700 m) of mud and sediment deposited onto to the ocean floor to reach the ancient seafloor created during the break-up of Pangaea and the opening of the Atlantic Ocean. The samples had been stored in core repositories at room temperature for more than 20 years, before Dr Klein’s team began their investigation and discovered the fossilized microbial remains.
“Colonies of bacteria and archaea were feeding off the seawater-hydrothermal fluid mix and became engulfed in the minerals growing in the fractured rock. This kept them completely isolated from the environment. The minerals proved to be the ultimate storage containers for these organisms, preserving their lipids and proteins for over 100 million years,” Dr Klein said.
In the lab, samples from the rock interior had to be extracted since the outside of the drill core was stored under non-sterile conditions. So the scientists took careful steps to ensure the integrity of the sample interior wasn’t compromised, and then analyzed the rocks with microscopes, a confocal Raman spectrometer and a range of isotope techniques.
While Raman spectroscopy enabled the team to verify the presence of amino acids, proteins and lipids in the samples, it did not provide enough detailed information to correlate them with other hydrothermal systems.
The lipids were of particular interest to the researchers since they tend to be better preserved over long timescales, and have been studied in a wide range of seafloor environments. They ran the lipids through an advanced liquid chromatography-based mass spectrometer system to separate out and identify their biochemical components.
Around 100 million years ago hydrothermal fluids rich in hydrogen and methane mixed with seawater 210 feet (65 m) below the seafloor. This process supported bacteria and archaea in what scientists call the deep biosphere in mantle rock. Conditions for microbial life were nearly ideal, the study showed, in this seemingly inhospitable environment. Image credit: Jack Cook, Woods Hole Oceanographic Institution / Ron Blakey, Colorado Plateau Geosystems.
The analysis led to a remarkable discovery: the lipids from the Iberian margin match up with those from the Lost City hydrothermal field, which was discovered in 2000 in the Mid-Atlantic Ridge during an expedition on board the WHOI-operated research vessel Atlantis.
This is significant because scientists believe the Lost City is a present-day analog to ancient hydrothermal systems on early Earth.
“Confirmation that life is possible in mantle rocks deep below the seafloor may have important implications for understanding subseafloor life across a wide range of geologic environments,” Dr Klein said.
“All the ingredients necessary to drive these ecosystems were made entirely from scratch. Similar systems have likely existed throughout most of Earth’s history to the present day and possibly exist(ed) on other water-bearing rocky planetary bodies, such as Jupiter’s moon Europa.”
The results reinforce the idea that life springs up anywhere there is water, even in seemingly hostile geological environments – a tantalizing prospect as scientists find more and more water elsewhere in the Solar System.
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Frieder Klein et al. Fluid mixing and the deep biosphere of a fossil Lost City-type hydrothermal system at the Iberia Margin. PNAS, published online August 31, 2015; doi: 10.1073/pnas.1504674112
