Steven D'Hondt, Professor
Graduate School of Oceanography
Steven D'Hondt earned a BS from Stanford University and a PhD from Princeton University. He has been at GSO since 1989. His primary research interest is the interaction between Earth and life.
In the middle of the nineteenth century, biologists
assumed that the deep sea was devoid of life. The crushing hydrostatic pressure
and chilly temperature of the deep sea were thought to be conditions intolerable
for living organisms. This assumption was refuted in 1860 by the discovery of
corals and sponges attached to a transatlantic cable that was hauled up from
the seafloor for repair. From 1868 to 1870, C. Wyville Thomson led multiple
cruises of HMS Lightning and HMS Porcupine to dredge the Atlantic
seafloor at depths as great as 2,500 fathoms (about 4,600 meters). By the end
of his project, he had discovered diverse life on much of the ocean floor. Since
then, marine biologists have come to accept the existence of life on the seafloor
and in the first few meters of deep-sea sediments. However, they continue to
be skeptical of claims that living organisms are buried deep within those sediments.
During the last decade, this perspective has been challenged by the work of John Parkes and his colleagues at the University of Bristol, England, who have reported bacteria buried 800 meters or more below the seafloor. Their work has raised a number of provocative questions. Are these bacteria denizens of the drilled sediment or contaminants from the drilling fluid? If they're truly from the sediment, are they alive? Are they active? Or are they ghost cells? If they're active, what sustains them? If they're not active, can they be resuscitated?
The Ocean Drilling Program (ODP) is an international partnership of scientists and research institutions that samples the deeply buried sediments and rocky crust of the open ocean. On ODP Leg 185 (1999), GSO microbiologist David Smith, newly appointed GSO geochemist Art Spivack (then at the University of North Carolina), Oregon State University marine geologist (and GSO alumnus) Martin Fisk, and their shipboard colleagues Göteborg University microbiologist Shelley Haveman and Scripps Institution of Oceanography geochemist Hubert Staudigel set out to determine if the bacteria reside in the sediments or are introduced during drilling. They injected a distinctive perfluorocarbon molecule continuously into the drilling fluid while drilling old sediments and crust in the western Pacific. The synthetic perfluorocarbon molecule permeates the sediment, is fairly easy to identify and measure in trace quantities, and acts as a tracer for drilling contamination of deep-sea sediments. They also injected bacterium-sized fluorescent beads while drilling representative cores. Although the centers of recovered sediment cores were generally found to be free of the perfluorocarbon and beads, they consistently contained bacterial cells. These studies show that bacteria do truly exist hundreds of meters beneath the seafloor.
The Leg 185 studies did not tell us whether these deeply buried bacteria were alive and active or the ghostly relicts of a long-buried seafloor. To address this, Smith and Spivack joined ODP Leg 190 (2000), where they and their shipboard colleagues measured products of microbial activity in deep-sea sediments of the Japan Sea. Their shipboard work required careful microbiological and geochemical analyses of the buried cells and bulk sediments of uncontaminated cores. Their analyses required a fully equipped shipboard microbiology laboratory. Last year, Smith, three colleagues from other institutions, and I wrote a National Science Foundation (NSF) proposal to construct a microbiology laboratory aboard the JOIDES Resolution. Our colleagues were Andreas Teske, lead investigator and microbiologist at Woods Hole Oceanographic Institution; Richard Murray, a geochemist at Boston University; and Elizabeth Screaton, an earth scientist at the University of Florida. Our proposal was funded and, as a result, microbiologists from any country in the ODP consortium can now go to sea equipped to study the deeply buried marine sedimentary biosphere.
Scott Rutherford (GSO alumnus and post-doctoral researcher at GSO and the University of Virginia), Spivack, and I are tackling the issue of buried microbial activity in a different way. Like larger animals, many buried microbes break down organic molecules (food) and oxygen-bearing molecules to get energy. Unlike larger animals, buried microbial communities generally don't require free oxygen (O2) to break down their food, but instead use various oxidized molecules, including nitrate (NO3-), sulfate (SO42-), and oxidized metals. Of these many molecules, sulfate is the most abundant in the deep ocean and deep-sea sediments. In fact, it's nearly 50 times as abundant as all other oxidants combined. Sulfate, nitrate, and free oxygen diffuse down into deep-sea sediments from the overlying ocean. Consequently, the concentrations of these chemicals in sediments result from the balance between diffusion from above and reduction by microbes within the sediments. Because rates of diffusion can be estimated from dissolved concentrations and a few other sedimentary properties (such as sediment porosity), we can use concentrations of dissolved sulfate in deep sea sediments to estimate the amount of microbial respiration that occurs in deeply buried sediments. For nearly thirty years, geochemists have measured the concentrations of dissolved sulfate (and sometimes nitrate and oxidized metals) in Deep Sea Drilling Project and ODP drill holes throughout the world. Rutherford, Spivack, and I are compiling this data to develop global maps of microbial activity in deep-sea sediments. This will allow us to assess whether deeply buried microbes are active or relict cells.
Our understanding of deeply buried life remains incomplete and many of the questions that I raised at the beginning of this article remain unanswered. Furthermore, there are other, deeper questions to be addressed. What are the relationships between buried microbial activity and the surface world? How do microorganisms adapt physiologically to burial under extreme conditions? What is the biotechnology potential for these organisms? Are there unknown and possibly ancient types of bacteria to be found? How do deeply buried bacteria get around? Can rock-locked bacteria survive and evolve in isolation for millions of years? And last, but not least, if bacteria survive in deep oceanic sediments and in hot or cold oceanic crust, can similar life forms also survive on other planets? Can we learn to recognize molecular and chemical signatures of life in Earth's deeply buried sediments and crust? Will we be able to distinguish the elusive evidence of life in nonterrestrial samples?
The deeply buried biosphere is likely to be one of the next major research topics for the earth and life sciences. Recent advances in molecular biology, geochemistry, and deep-sea drilling make the detailed study of deeply buried life possible. However, much like Wyville Thomson's nineteenth-century study of seafloor life, study of subseafloor life in the twenty-first century will require an ambitious program of multiple drilling cruises spaced over several years. Fortunately, the Ocean Drilling Program and the international ocean drilling community are committed to advancing such study. It's going to be interesting.
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