University researcher contributes to study into limits of life on Earth

A University of Plymouth researcher is part of an international team that has provided new in­sights into the tem­per­at­ure lim­its of life be­neath the ocean floor.

Dr Hayley Manners, Lecturer in Organic Chemistry, and colleagues from 29 different institutes found single-celled microorganisms living in sediments more than a kilometre into the ocean floor - and at a temperature of 120°C.

The study, published in Science, was carried out during a two-month research expedition in 2016 - in which Dr Manners participated - and forms a part of the work of Expedition 370 of the International Ocean Discovery Program (IODP).

It focused on the Nankai Trough off the coast of Japan, where the deep-sea scientific vessel Chikyu drilled a hole 1,180 meters deep to reach sediment at 120°C.

In detailed analyses of the samples, scientists found the con­cen­tra­tion of ve­get­at­ive cells de­creased sharply to a level of less than 100 cells per cu­bic centimetre of sed­i­ment at over 50°C.

However, the con­cen­tra­tion of en­dospores - dormant cells of cer­tain types of bac­teria that can re­act­iv­ate and switch to a live state whenever con­di­tions are favourable again - in­creases rap­idly and reaches a peak at 85°C.

The research was led by scientists at MARUM, the Centre for Marine and Environmental Sciences at the University of Bremen in Germany.

Like the search for life in outer space, de­term­in­ing the lim­its of life on the Earth is fraught with great tech­no­lo­gical chal­lenges. Tem­per­at­ures of 120°C are nor­mally en­countered at about 4,000 meters be­low the sea floor, and using the Chikyu is the only way in the world for sci­ent­ists to ob­tain samples from such great depths.

The sampling site used in this study lies in a wa­ter depth of 4.8km, but be­cause of the steeper-than-av­er­age geo­thermal gradi­ent, it was pos­sible to reach a tem­per­at­ure of 120°C in a hole only 1,180 meters deep. The pro­cessing of samples was mon­itored us­ing strict con­tam­in­a­tion con­trols, and for par­tic­u­larly crit­ical work the samples were trans­por­ted by heli­copter to the clean­room labor­at­or­ies at the IODP core re­pos­it­ory in Ko­chi, Ja­pan.

Study leader Kai‐Uwe Hinrichs, of MARUM, said:

"Only a few scientific drilling sites have yet reached depths where temperatures in the sediments are greater than 30°C. The goal of the T-Limit Expedition, therefore, was to drill a thousand-meter deep hole into sediments with a temperature of up to 120°C - and we succeeded."

• The full study - Heuer et al: Temperature limits to deep subseafloor life in the Nankai Trough subduction zone - is published in Science, doi: 10.1126/science.abd7934.