It's the basement apartment like no other. Life has been found 1.6 kilometres beneath the sea floor, at temperatures reaching 100 °C.
The discovery marks the deepest living cells ever to be found beneath the sea floor. Bacteria have been found deeper underneath the continents, but there they are rare. In comparison, the rocks beneath the sea appear to be teeming with life.
John Parkes, a geobiologist at the University of Cardiff, UK, hopes his team's discovery might one day help find life on other planets. He says it might even redefine what we understand as life, and, bizarrely, what we understand by "age".
Parkes has been hunting for deep life for over 20 years. Recently, he and his colleagues examined samples of a mud core extracted from between 860 metres and 1626 metres beneath the sea floor off the coast of Newfoundland.
They found simple organisms known as prokaryotes in every sample. Prokaryotes are organisms that often have just one cell. Their peculiarity is that, unlike any other form of life, their DNA is not neatly packed into a nucleus.
About 60% of the cells Parkes and his team found were alive. They are related to organisms found in deep-sea hydrothermal vents. Depending on the depth, between one in 20 and one in 10 of the cells were dividing, which is the normal way prokaryotes reproduce.
Where cells living so far beneath the sea floor could have come from remains a mystery. They may have been gradually buried in sediment as millions of years passed by, and adapted to the increasing temperatures and pressure, he says.
Another possibility is that they were sucked deep into the mud from the sea water above. Hydrothermal vents pulse hot water out of the seabed and into the ocean. This creates a vacuum in the sediment, which draws fresh sea water into the marine aquifer.
It is important to understand the way the cells got down there, because that has implications for their age. The cells are not very active and according to Parkes they have very few predators. "We find very few viruses, for example, down there," he says. "At the surface, if you don't divide you get eaten. But if there are no predators, the pressure to reproduce decreases and you can spend more energy on repairing your damaged molecules."
This means it is conceivable – but unproven – that some of the cells are as old as the sediment. At 1.6 km beneath the sea, that's 111 million years old. But in an underworld where cells divide excruciatingly slowly, if at all, age tends to lose its relevance, says Parkes.
Parkes' interest in prokaryotes goes far beyond those that are buried deep in the Earth. He thinks the cells found there could lead to life on other planets.
Previously, he has shown that the rocks beneath the oceans could be home to the largest population of prokaryotes on Earth, and account for one tenth of all living carbon. He estimates the combined undersea biomass could be equivalent to that of all the plants on Earth.
"We are all dominated by our surface existence where everything relates to photosynthesis and oxygen," he told New Scientist.
The possibility that there could be more forms of life beneath the surface than above it suggests that they have different and effective ways of surviving – ways that could be independent of light and oxygen. And if these "new" forms of life exist on Earth, they could exist on other planets too.
Dense at depth
"That's what really excites me. This is not just about the deepest, hottest, oldest – but also that we may have misunderstood life."
Life beneath the continents is very different. The temperature increases more slowly with depth in the continental crust, which allows life to go deeper.
"We have recovered living cells from depths of 3.2 km to about 5 km in South Africa," says Tullis Onstott of Princeton University. "But what I find most interesting in Parkes' samples is the high density of microbial cells. They are about 100 to 1000-fold greater than in our terrestrial environments at comparable depths or temperatures."
In 2002, Parkes had found prokaryotes at 842 metres beneath the seabed, the previous record, and it seems likely he will be finding life deeper yet in years to come. "The more you look the more you find," comments Karsten Pedersen of the Deep Biosphere Laboratory at Göteborg University, Sweden.
[Via New Scientist]