Life in Extreme Environments

Lying beneath the ocean at typical water depths of 2000-3000 m are the mid-ocean ridges. Here at great depth and pressure, in total darkness, and with no oxygen is an environment where life abounds.

Drawing by Paul Oberlander.

Hot rock wells up at mid-ocean ridges to form new ocean crust. Water moves through cracks into the ocean crust, gets heated by the hot rock, and then rises back to the seafloor. The heated water jets out of the crust at spots on the seafloor called hydrothermal (hot water) vents. The environment around these vents can be described as “extreme” and this is one of the places where biologists study life that forms in extreme environments on Earth (others are in sea ice and at hot springs on land). Because of the conditions in which they live, organisms in these environments are also often called “extremophiles”. The information that scientists learn at these deep sea communities might hold clues to the possibility of life on other planets such as Mars.

One of the most exciting and recent discoveries about life at mid-ocean ridges was that microorganisms (microbes) were abundant at the vents, thrived in very hot water (up to 105o C), and lived on minerals that would be toxic to most other life on Earth. Although some of the microbes fall within the taxonomic group called Bacteria, the majority belong to a different taxonomic group.The discovery of these organisms made it necessary to reconfigure the “Tree of Life” so that a new top level was designed that contained three “domains”: Archaea, Bacteria and Eucarya. Archea includes the microorganisms found in these extreme environments. Eucarya includes the plants, animals, protists, and fungi.

Scientists are only beginning to learn about microbes at hydrothermal vents and why they thrive there. New species continue to be discovered. Microbes have been found within the chimneys built above the vents and within the water that is ejected at the vent. There is also the potential that microbes live deep within the ocean crust, perhaps as much as several kilometers below the seafloor.

Evidence that microbes exist in the deep subseafloor comes from clouds of abundant biogenic material that are ejected when a volcanic eruption occurs on the seafloor, and from rock cores obtained on cruises as part of the Ocean Drilling Program.

The following is an article from the Ocean Drilling Program Volume “Greatest Hits” published in 1997 that explains what evidence has been found for microbial activity in drill cores from the deep sea.

EVIDENCE FOR MICROBES IN OCEANIC BASALTS: GLASS-EATING BACTERIA?

Martin R. Fisk, College of Oceanic and Atmospheric Sciences, Oregon State University

Until recently, conventional wisdom held that Earth’s subsurface was a sterile place, devoid of life. Exciting new results, however, indicate that this is far from the truth. Scientists have discovered evidence of organisms deep beneath Earth’s crust on both continents and ocean floors. By studying new microbial life-forms, and the incredibly wide range of environments in which they live, we gain a much better understanding of how life began and evolved on Earth, and possibly other planets.

To this end, ODP (Ocean Drilling Program) has led the way in collecting subseafloor microbes to evaluate the exciting new paradigm of the so-called deep biosphere. The size of this biosphere is difficult to determine, and will require additional drilling to constrain. The concentration of living material in the oceanic crust is small, but because of the huge global volume of this material, it may contain a significant fraction of Earth’s biomass [Parkes et al., 1994]. About 5% of oceanic crust consists of volcanic glass, intuitively a material inhospitable to life. Nevertheless, new microscopic examination and application of molecular genetic techniques on DSDP (Deep Sea Drilling Program) and ODP basalts collected near the Mid-Atlantic Ridge [Bougault et al., 1985] suggests that the rocks contain ample evidence of microbial life.

Photomicrograph shows 15 million year old volcanic glass with both thin, irregular and smooth, broad channels thought to have been created by microorganisms [ Furnes et al., 1996; Giovannoni et al., 1996]. The volcanic glass also has a crack filled with clay (orange). The sample is from basalt recovered by drilling about 200 km west of the Mid-Atlantic Ridge beneath 400 m of sediment. Rock temperature before drilling was about 40° C.

The idea is that microbial activity, indicated by pitting of the glass, and the formation of intricate and branching burrows, helps weather and erode this volcanically derived material. The microbes may even be “eating” the glass, using it as an energy source. The most typical texture observed microscopically is thin irregular channels, about one mm in diameter and extending 20 to 40 mm into the glass (see photomicrograph). A better understanding of Earth’s subsurface biosphere will result by examining other crustal rocks and new samples from future drilling that are free of contamination and are specially preserved immediately after collection. Microbes in volcanic crust may turn out to be important catalysts of chemical change. In this role, they would help regulate the cycling of elements between seawater and the oceanic crust. Microbes that derive their energy from inorganic chemical reactions suggests that life may thrive in previously unsuspected places, such as on Mars and Europa.

References:

Parkes, R.J., B.A. Cragg, S.J. Bale, J.M. Getcliff, K. Goodman, P.A., Rochelle, J.J. Fry, A.J. Weightman, and S. M. Harvey, Deep bacterial biosphere in Pacific Ocean sediments, Nature, 371, 410-413, 1994.

Bougault, H, S.C. Cande, et al., Init. Repts. DSDP, 82, 1985. Furnes, H., I.H. Thorseth, O. Tumyr, T. Torsvik, and M.R. Fisk, Microbial activity in the alteration of glass from pillow lavas from Hole 896A, ODP Proc. Sci. Res., 148, 191-206, 1996.

Giovannoni, S.J., M.R. Fisk, T.D. Mullins, and H. Furnes, Genetic evidence for endolithic microbial life colonizing basalt glass/seawater interfaces, ODP Proc. Sci. Res., 148, 207-214, 1996.