Teeming with heat-loving microbes, samples of fluid drawn from the crustal rocks that make up most of the Earth's seafloor are providing the best evidence yet to support the controversial assertion that life is widespread within oceanic crust, according to H. Paul Johnson, a University of Washington oceanographer. Johnson is lead author of a report being published March 25 in the American Geophysical Union's publication Eos about a National Science Foundation-funded expedition he led last summer.
Fifteen-foot-long hypodermic needles – strong enough to penetrate the volcanic rocks that make up the Earth's crust – were among the novel devices used to collect samples from sites on the Juan de Fuca plate 200 miles off the coast of Washington and Oregon.
Scientists have known for 20 years of microorganisms that thrive in the acidic iron-, sulfur- and heavy-metal-rich fluid environments in areas where seafloor is being created at mid-ocean ridge spreading centers. These areas are subject to frequent volcanic eruptions and can have fields of hydrothermal vents that pour superheated water as hot as 750 F into the oceans.
As visually spectacular as such areas can be, they represent only a tiny area of the seafloor. Far more of the seafloor is tens of millions of years old.
"The types of seafloor environments we sampled last summer are found everywhere in the ocean. This argues, although it doesn't prove, that oceanic crust may be a microbial incubator of global proportions," he says.
Scientists still haven't sampled widely enough to say for sure, and it was just Jan. 3 when the first report of microbes living in 3.5 million-year-old crust was published in the journal Science by Johnson and co-authors from University of Hawaii, Oregon State University and University of Illinois.
Now Johnson and another group of scientists report in Eos that they retrieved and are actively growing microbes from both old and young seamounts – "old" being 3.5 million years in age and "young" being new enough to be volcanically active. They also are culturing microbe samples from similarly old and young seafloor that is unaffected by the growth of seamounts or disturbed by fracture zones or tectonic forces – what Johnson refers to as "normal" seafloor.
Ninety percent of the world's seafloor consists of normal seafloor or has seamounts similar to the types of crust recently sampled, Johnson says. And most of that – even seafloor that's much older, 100 million years or more – is similar to the 3.5-million-year-old sites sampled as far as porosity of the rock, the sediment cover and rock temperatures of between 100 and 160 F, he says. The rocks at the sites on the Juan de Fuca plate are at the higher end of the range for 3.5 million-year-old crust because sediments covered them at a very young age.
University of Washington doctoral student Julie Huber and her advisor, John Baross, are working with samples of live microbes from the expedition. Some UW laboratory work shows bacteria extracted from seamount flanks grow best at hot temperatures, 190 F, which is considerably higher than the 68 F fluid they were collected with and the 140 F temperature of the rocks in that area. This means that the fluid and microbes are coming from deeper within the ocean crust, perhaps as deep as half a mile below the seafloor.
Cell counts at most of the sites are higher than cell counts from normal seawater, a strong indication the scientists were sampling a crustal environment and that their samples were not contaminated with bottom seawater.
Another important concern involves possible contamination from the process of drilling. This is a critical question even for a hole drilled by the International Ocean Drilling program in 1997 that has been gushing copious crustal fluids since then. There is a chance that drill hole, sampled for work reported in both the Science and Eos articles, was contaminated during drilling or that microbes being collected for these studies were growing in the artificial environment of the steel drill pipe in the bore hole.
To try to avoid this Johnson and engineers with the UW's Applied Physics Laboratory designed probes that look remarkably like giant hypodermic needles. The 15-foot-long stainless steel probes were driven into the summit of a 3.5-million-year-old seamount. Two of the hollow probes immediately began venting warm crustal fluid.
The successful insertion of the probes and the development of a new barrel sampler meant scientists could take very large samples, 25 gallons at a time, of uncontaminated fluid to measure extremely dilute organic compounds that would tell how long the fluid was within the crustal rocks. These quantities are 200 times larger than normal hydrothermal fluid samples and the scientists may have accidentally spilled more hydrothermal fluid than is collected during other expeditions, Johnson says. Woods Hole Oceanographic Institution's new remotely operated vehicle Jason II was used for the seafloor work.
All 24 scientists on the expedition are named as co-authors on the Eos paper. In addition to the UW, they represent NOAA's Pacific Marine Environmental Laboratory in Seattle, University of Victoria, Oregon State University, University of Chicago, Field Museum of Chicago, University of California Santa Cruz, University of South Carolina, Dartmouth University and Woods Hole Oceanographic Institution.
While work continues on the chemical and microbiological analyses, Johnson and his colleagues have been intrigued by research reported by Andrew Fisher of the University of California, Santa Cruz, and 12 co-authors in the Feb. 6 issue of Nature.
That work describes how two seamounts in the North Pacific appear to share the same underground plumbing so that cold seawater being taken in at one seamount is venting as warm hydrothermal fluid at another – and the two are 25 miles apart.
The seamount at the venting end of this system is the one where Johnson and his team drove their hypodermic needles and found abundant microbial life.
"If crustal fluid can flow over large distances in old oceanic crust and can nurture these large microbial populations," Johnson says, "then the chances are good that there is a global-scale biosphere living within the upper oceanic crust. This oceanic crustal biosphere would live at a wide range of temperatures and fluid flow rates, have different chemical environments, have unique entrance and exit ports and would have been exposed to completely different formation histories.
"It's like finding an undiscovered world."
Materials provided by University Of Washington. Note: Content may be edited for style and length.
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