DEBI-SELECT: Probing the Subseafloor

Legacy Borehole

The amount of biomass could be huge—a significant portion of that on Earth, more than that found in the Amazon rainforests

     People often speak of the oceans as being largely unexplored, and rightly so. But beneath the oceans there’s another realm even less explored and less understood. In recent years scientists have come to appreciate that within the sediment and rock below the seafloor, microbial life can and does thrive with significant impacts on the world above. A new MSTF-supported project called the Deep Energy Biosphere Initiative-SubsurfaceE LifE Characterization Tool (DEBI-SELECT) is aimed at answering some of the most basic questions about who’s down there and what they are doing.

What’s Going On
Based on the very limited work that’s been done so far, some research suggests that most of the planet’s microbes live in deep subsurface environments. Microbes live in sediments, but sediment is not permeable, meaning that it is not flushed with sewater that can supply nutrients. Basalt rock layers, in contrast, are extremely permeable. So microbes can live on the rock itself, as well as in the water that fills and flows through these spaces, transforming both in the process. Amazingly, some of these microbes live for thousands of years or possibly even longer.

     “The amount of biomass could be huge—a significant portion of that on Earth, more than that found in the Amazon rainforests,” says Geoff Wheat, the DEBI-SELECT lead scientist and a geochemist at the University of Alaska, Fairbanks but based at the Monterey Bay Aquarium Research Institute in California. “The subsurface is just so vast that even small amounts of activity make a big difference on a global scale.”

Why It’s Important
Subsurface organisms have to produce their own food using available chemicals, a process called chemosynthesis. That’s because they are relatively isolated from the sunlight and the flow of plant-based foods that drive most ocean food webs. Nonetheless, seawater does cycle down into the subseafloor, where it is heated and eventually reemerges into the waters above at various types of seeps and hydrothermal vents.

     That connection means organisms in the subseafloor can alter the chemistry of the ocean, and could, for instance, be major players in converting seawater inorganic carbon—the various forms carbon dioxide takes after entering the ocean—into organic carbon. Carbon cycling is now of particular interest because researchers are still working to fully understand the processes that allow the ocean to take up a huge portion of the carbon dioxide in the atmosphere, partially mitigating climate change.  

     Studying the subsurface will improve our understanding of life on Earth and could also lead to benefits such as the discovery of new bacterial species with biomedical or industrial potential. “Every time someone does something in the subsurface they find a whole bunch of novel microbes,” says Wheat, “We sort of know what category they’re in but we don’t necessarily know what they’re doing.” The main reason so little is known is that accessing the sub realm is so difficult, particularly the basalt rock layers found below the relatively more accessible sediments. DEBI-SELECT will help overcome this problem as part of a growing series of experiments and activities aimed at finally understanding the subseafloor.

The Technology
Wheat is working with Bill Kirkwood at TLR Inc.; Chris Kitts at Santa Clara University; and William Hug at Photon Systems Inc. This team is creating an innovative sensor and sampling instrument package that can be lowered into existing holes in the seafloor as part of the long-term Ocean Drilling Program. These holes, which are in water depths from 2,000 to 5,000 meters, offer a unique window into otherwise hidden rock layers.

     There is good reason to believe that life in the boreholes should be the same as would be found in the porous spaces of the inaccessible subsurface, once enough time has passed for normal activity to resume after a hole is drilled—months to years.

Nonetheless, only twice have instrument packages aimed at studying microbial subseafloor life been lowered into these holes, and only one of these was equipped with a sensor that could directly measure life. Neither could collect samples. So, great potential remains open to leverage the billions already invested in drilling programs to answer key questions about life beneath the sea.

     DEBI-SELECT will include a derrick and winch system rated to 5,000 meters that will lower a sensor-laden probe into a borehole. First, the complete package will be lowered from a ship to a spot near a borehole and disconnected. A remotely operated vehicle (ROV) will then position the package over a borehole and then control the winch to lower the probe up to 1 kilometer beneath the seafloor. This feat will require substantial advancements in tether management systems, as it is 7 to 10 times the cable length that has been used in other similar applications. A hole might be characterized in just a couple of days work.

     One key instrument will be an advanced fluorescent probe for detecting microbes. This will be a modified version of a device called DEBI-t used in past research. Fluorescent probes emit light of one wavelength that living cells absorb then reemit at a slightly different wavelength. The unit measures the return of this light as a way to quantify the microbial cells and spores present, in this case either on rock walls or in the associated water. The package will also include a variety of sensors for parameters such as temperature and pH, nitrate and sulfide sensors, devices to collect water samples and microbes on rock surfaces for more detailed analyses, and two high definition cameras with LED lighting. Also, the system was designed from the outset to include standardized connections that will allow the easy addition of more sensors and samplers.

     Much of the DEBI-SELECT data will be stored until the system is retrieved. But the ROV will transmit certain critical data, such as the video feed, directly to the surface in real time so that researchers can make decisions such as where to collect samples. To accomplish this feed, the team will be constructing a modified slip ring system, which allows transmission between the spinning wheel of the winch and the ROV without a directly wired connection.

What We’ll Gain
DEBI-SELECT can be used alone, or in conjunction with other research activities. The data the system will return will be truly unprecedented. Ultimately, DEBI-SELECT will provide information about how water flows through the porous rock, interactions between the water and the rocks, and the influence microbes have on global cycles of carbon, nitrogen, iron, sulfur and other elements. Such information will be critical in guiding future efforts such as the National Science Foundation’s Science and Technology Center Deep Energy Biosphere Investigations (C-DEBI) program.

     Learning more about what lives in Earth’s extreme environments like the subseafloor, and under what conditions they survive may also aid researchers in determining where to look for signs of life on Mars and other planetary bodies.

The Plan
The DEBI-SELECT project began October 1, 2013 and the goal will be to complete the prototype system within three years. After testing, potentially aboard the R/V Falkor, operated by MSTF’s sister organization, Schmidt Ocean Institute, the team hopes to use workshops and other means to familiarize other researchers with the technology so that it can be more widely used. The target boreholes are those cased at the top to prevent upper sediment layers from collapsing in and open at the bottom so the basalt rock layers are exposed for studies. The team also hopes to work in holes that will accommodate the full 1,000 meters of tether. Several that fit this bill are found along the Costa Rica Rift, in the eastern Pacific north of the equator.

     In most scientific fields, research leads to small, incremental improvements in understanding of a particular topic. But the subseafloor is a nearly untapped scientific frontier. That means that data from DEBI-SELECT, along with other projects underway, have the potential to completely transform our understanding of what is almost certainly a realm critical to the ocean’s and the planet’s proper functioning and health. “Everything is extremely wide open,” says Wheat.

                                                                                                                               --by Mark Schrope