Life Under the Sea Floor
By Bill O’Neill
Before my train of thought was interrupted by that exciting news I described in the last issue about the evidence for a lot of water frozen just beneath the surface soil of Mars, I had planned to write a bit about life under the sea - in particular, life beneath the floor of the sea. During the past Spring term at UW, I had the privilege of auditing an oceanography course about life at the liquid-solid interface of the seafloor – the benthos, as such bottom dwellers are known collectively. Those critters live on and in the sediment that has recently fallen from the sea above and from run-off near the continents. More appropriate for this column, which concerns the possible characteristics of extraterrestrial life, I thought I’d try to summarize some of what’s known about organisms that inhabit the subseafloor deep beneath the sediments where most benthic creatures creep and dig. Subseafloor organisms are more akin to the deep subterranean microbes that I wrote about in the May-June issue. The name given to this netherworld is the deep biosphere. Literally, such life exists under enormous pressures.
One perspective of the deep biosphere is obtained from hydrothermal vents like those that John Delaney illustrated in his lecture here last April. These occur along mid-ocean sites of seafloor spreading, where new crust is being formed and hot dissolved minerals precipitate to form towers known as “smokers.” While the smokers certainly are surrounded by unusual life forms, whose food chains derive from chemical energy and are not linked to the solar photosynthesis most life depends upon, those more obvious organisms are not the aspect of the deep biosphere with which we’re concerned here. The few times that Delaney and his colleagues have been able to observe new eruptions on the seafloor, large quantities of organic matter were ejected along with the expected inert material. The earth’s crust surrounding these regions is riddled with cracks, and seawater permeates deeply into this deep subsurface fracture zone where it becomes heated, then percolates vigorously back to the seafloor. At recently-formed vents, before a new ecological community has had time to become established, a virtual “snow storm” of organic particles (presumably derived from microorganisms that previously lived within the underlying rock) is observed to accompany the ejected inorganic solids, solutions and gases. DNA gathered from the emissions of such vents is being studied by researchers at UW and elsewhere. Both DNA and intact microbes, detected by fluorescence, turn out to be most abundant in the effluent from those smokers that emit hotter fluids, which probably tap deep subsurface fracture zones most directly.
The second approach to exploring the deep biosphere involves boring into the seafloor directly, as has been done over the past 15 years in the Ocean Drilling Program (ODP) and the Deep Sea Drilling Program (DSDP). Microorganisms have been recovered from depths as great as half a mile below the seafloor. Like other microbes buried more than a short distance in marine sediments, they probably use sulfate, a universal constituent of seawater, to oxidize their nutrients, because no free oxygen is available. There is evidence that they respire 10,000 times more slowly than bacteria that inhabit the earth’s surface. Samples of 15million-year-old volcanic glass recovered from beneath 1200 feet of sediment, where the ambient temperature was about 40oC, show thin, irregular channels (1 mm diameter, 20-40 mm long) that appear to have been created by microorganisms capable of digesting the basalt.
Although the concentrations of organisms dispersed in deep submarine niches are far lower than those in more hospitable environments, the volume of such environments has given rise to estimates that the deep biosphere accounts for between ten and fifty percent of the biomass on earth.