Somewhere in the sediments and rocks beneath the ocean floor, it gets too hot for living things. But how far down? Even after drilling kilometers into the ocean floor, scientists have found that microbes persist. “We keep digging and digging and digging deeper and have not hit the bottom of the biosphere,” says Jan Amend, a geochemist at the University of Southern California in Los Angeles.
A new ocean drilling expedition will try to settle the question by drilling into crust where high temperatures are found unusually close to the sea floor, bringing life’s thermal limit within reach. On 13 September, the research vessel Chikyu will set sail from Shimizu, Japan, and sink its bits into a patch of ocean floor where the sediments should reach 130°C at the maximum drill depth of 1260 meters. Somewhere along the way, the team expects, life should succumb to rising temperatures.
Finding that limit—the goal of the 62-day T-Limit campaign, part of the International Ocean Discovery Program—could guide estimates of the abundance and diversity of ocean floor microbes, which play large roles in biogeochemical cycles. For instance, the deep microbes, estimated to hold one-third of Earth’s total biomass, take carbon out of the ocean and sequester it when they die. “We need to understand who they are and how they make a living,” says Bo Jørgensen, a geomicrobiologist at Aarhus University in Denmark.
Over the past decade, awareness of the abundance of life in the watery pores of the deep biosphere has turned microbiology into a primary focus of scientific ocean drilling. Expeditions have discovered microbes in nutrient-poor clays and hard bedrock. And, in 2012, the same team behind the T-Limit expedition found microbes 2.5 kilometers beneath the sea floor, a record depth, eking out a living from coal beds fossilized more than 20 million years ago.
T-Limit’s target is the Nankai Trough, perched atop the subduction zone where the Philippine Sea Plate dives beneath the Eurasian Plate. That tectonic action boosts temperatures in the layers of mudstone and volcanic ash that fill in the trough. T-Limit will drill all the way through this sediment and then 50 meters into the basalt underneath. It’s not an easy target, as the drill may encounter clays transformed into cementlike pockets by high temperatures. “They’re drilling through some really awkward sediment,” says Beth Orcutt, a biogeochemist at the Bigelow Laboratory for Ocean Sciences in East Boothbay, Maine. The drilling itself will heat nearby sediments, so the team will leave a probe in place for a year to discover the borehole’s true heat gradient.
This is a fantastic challenge for geochemistry and microbiology.
Demonstrating that life really is absent below a certain depth will be tough. Beyond philosophical debates about proving a negative, contamination in core samples has been a chronic problem. But the Chikyu will use a new drilling technology, stabilizing and lubricating its borehole with seawater instead of the drilling muds that have often caused contamination in the past. The team will also x-ray the cores to choose those whose interiors appear pristine, evacuating those by helicopter to the Kochi Core Center in Nankoku, Japan. There, any microbes will be sequenced and cultured.
All told, the T-Limit team expects to detect traces of life as sparse as six microbial cells per cubic centimeter of sediment. “This is a fantastic challenge for geochemistry and microbiology,” says Kai-Uwe Hinrichs, one of the expedition’s lead scientists and a geochemist at the University of Bremen in Germany.
If the effort does reveal a thermal limit to life, it won’t be the only boundary. Other hostile conditions deep in ocean sediments, such as scarce nutrients, high pressure, or extreme salinity, probably set life’s limit in some places. And the Nankai Trough sediments are starkly different from another high-temperature environment, the hydrothermal vents on midocean ridges. Bacteria and archaea discovered at these vents have been grown in the lab at up to 122°C. But the vents, which are rich in energy sources for microbes, are poor proxies for most ocean floor sediments, where scarce nutrients could mean a lower thermal limit. It would be a surprise to see life in the Nankai linger at temperatures close to that lab-set record, Jørgensen says.
The best existing evidence for a heat limit beneath the ocean comes from the oil and gas industry, says Victoria Orphan, a geobiologist at the California Institute of Technology in Pasadena. Researchers learned decades ago that petroleum reservoirs formed at 85°C or higher underwent “paleo-pasteurization,” which killed off microbes that would have degraded the oil. Crude oil found at cooler temperatures, even deep down, often bears the sulfuric detritus left by busy microbes.
The Nankai cores could reveal more than a temperature limit. Microbes cultivated from them could also help settle whether bacteria or archaea are more dominant in the subsurface, and they could also offer a glimpse of the microbes’ lifestyles. Current evidence suggests they live extraordinarily long lives thanks to glacial metabolisms. Some reproduce as little as once a century, Jørgensen says. But that evidence, based on metabolic products and cell counts, is uncertain: It’s possible that only a fraction of the bugs are active and the rest dormant. If so, the metabolisms of active microbes could resemble those of their surface cousins.
Nor do investigators know precisely how heat limits life in a nutrient-poor environment. At high temperatures, DNA and amino acids become difficult to maintain: The bond between DNA’s backbone and bases tends to fail, and amino acids become more prone to flipping into their mirror image structures, which are mostly unusable for building proteins. Jørgensen suspects the failure of mechanisms that repair such damage could be a primary reason for a thermal limit.
He looks forward to learning whether the inhabitants of the deep cores confirm his hunch. But most microbiologists are eagerly awaiting what amounts to the ultimate bottom line: an end to Earth’s habitable zone. As Steve D’Hondt, a geobiologist at the University of Rhode Island, Narragansett Bay, puts it, “Whatever their temperature limit is, it sets a new boundary.”