How hot is too hot for life deep below the ocean floor?

At what depth be­neath the seabed does it be­come so hot that mi­cro­bial life is no longer possible?

An ex­ped­i­tion by In­ter­na­tional Ocean Dis­cov­ery Pro­gram scientists has provided new insights into the tem­per­at­ure lim­its of life be­neath the ocean floor. The find­ings, funded in part by the U.S. National Science Foundation, are published in the journal Science.

Like the search for life in outer space, de­term­in­ing the lim­its of life on Earth is fraught with tech­no­lo­gical chal­lenges. Tem­per­at­ures of 120 de­grees Celsius (248 degrees Fahrenheit) are nor­mally encountered at about 4,000 meters be­low the seafloor.

There is only one way for sci­ent­ists to ob­tain samples from such great depths -- with the deep-sea sci­entific drilling ves­sel Chikyu. The scientists selected a loc­a­tion in the Nankai Trough off the coast of Ja­pan.

The site lies in a wa­ter depth of 4.8 kilo­met­ers (nearly 3 miles), but be­cause the geothermal gradi­ent is steeper than aver­age, it was pos­sible to reach a tem­per­at­ure of 120 de­grees Celsius in a drill hole only 1,180 meters (three-quarters of a mile) deep.

While the con­cen­tra­tion of some cells de­creased sharply to a level of less than 100 cells per cubic cen­ti­meter of sed­i­ment at more than 50 de­grees Celsius, the concen­tra­tion of endospores in­creased rap­idly and reached a peak at 85 de­grees Celsius. En­dospores are dormant cells of certain types of bac­teria that can re­act­iv­ate and switch to a live state whenever con­di­tions are fa­vor­able again.

Much of the re­search for this pro­ject was car­ried out at the fringes of tech­nical feasibility, the scientists said. In the past 20 years, tech­niques for the de­tec­tion of life have been im­proved, with some now as much as 100,000 times more sens­it­ive.