Their strategy for staying alive is to be barely alive at all. Their metabolism is dialed down to almost nothing, an adaptive advantage in a place with so few resources. The bacteria that survive are the ones that can satisfy themselves with minute traces of oxygen and a parsimonious diet of organic material laid down millions of years ago.
Such buried bacteria have been found before, but a new study, published Thursday online by the journal Science, has provided the clearest look at their glacial pace of existence. The conclusion, in short, is that microbes can putter along at extremely low rates of oxygen respiration, their numbers limited only by the paucity of energy available in the buried sediment.
“These organisms live so slowly that when we look at it at our own time scale, it’s like suspended animation,” said Danish scientist Hans Roy, a biologist at Aarhus University and the lead author of the study. “The main lesson here is that we need to stop looking at life at our own time scale.”
An ancillary message is that human beings should not be too chauvinistic about what constitutes, or characterizes, a living thing.
There are a lot more nuances to nature than scientists realized just a few decades ago.
The ingenuity of life gives hope to researchers looking for evidence of life beyond Earth. Extraterrestrial life could conceivably be detected by robotic probes, for example, in the Martian subsurface, or in an ice-
covered ocean on a cold moon farther out in the solar system.
‘Life in the slow lane’
Scientists now believe that much of the life on Earth is barely able to fog a mirror, as it were. The deep-sea microbes may be an extreme example of a laid-back norm.
Most of Earth life, measured by numbers, is not rambunctious and charismatic like life in the sunshine, nor is it akin to the microbes that grow quickly in a laboratory petri dish.
Rather, it’s kind of boring — living out of sight, below the surface, in total darkness, using energy slowly and efficiently.
NASA research scientist Tori Hoehler, who was not involved with the new study but who has investigated such microbes, said, “I think this is a window into life in the slow lane, which, far from being a niche thing, is probably the average condition on Earth.”
Only in recent decades have scientists come to realize that life on the surface is but the flashy veneer of the biosphere.
Life, we now know, will find a way to survive in the unlikeliest places, from deep-sea hydrothermal vents and ancient salt deposits to Yellowstone’s hot springs. There are microbes living on aerosolized particles high in the atmosphere.
But the subsea, buried bacteria demonstrate a form of extremophiles that are at the end of the spectrum of vibrancy.
Their ability to survive in such impoverished conditions supports the belief that bacteria could also survive in material ejected into space from a planet by an asteroid impact. Such space-borne microbes could later be deposited, as stowaways on a meteorite, on a different planet.
Scientists in recent years have speculated that life might have originally appeared on Mars when it was warmer and wetter about 4 billion years ago, and then been transported via meteorite to Earth.
A desert in the ocean
The scientific team led by Roy cruised to a spot in the northern Pacific far from the nearest land mass, and thus removed from most wind-borne dust or stream-carried sediments. The scientists picked a place where a current called the North Pacific Gyre inhibits the influx of sediments from far away. This ensured that the sea floor would be essentially a desert, with little organic material raining down.
In effect, the scientists used the Pacific as a laboratory in which they could study millions of years of biological activity by drilling cores roughly 100 feet below the sea floor. The bottom of the sediment cores contained material laid down 86 million years ago.
What they found is the latest example of life’s ingenuity. As expected, there were far more microbes at the top of the sediment column than at the bottom. But even at rock bottom, in red clay that hadn’t been exposed to oxygen or new nutrients — much less sunlight — since the Cretaceous Period, a smattering of bacteria found a way to eke out an existence.
“These communities have not received input or new food since the dinosaurs walked the planet,” Roy said. “Those that are left down there are the ones that can deal with the lowest amount of food.”
An old-age question
The research pivots on the measurement of oxygen in the sediment. By calculating the rate at which oxygen disappears, the scientists can determine the metabolic speed of the organisms.
Key questions remain: How rapidly do these bacteria reproduce? Have they evolved and developed new traits in these eons in the subsea muck, or do they survive thanks to traits that were already there when they first inhabited the sea floor?
And how old are they, exactly?
It’s a big number, the scientists think.
“The notion of being several thousand years old is really a minimum of what is going on,” Hoehler ventured.
One possibility is that, over time, these bacteria have slowly reproduced, and thus today’s population is countless generations removed from the original population. But another idea is that the bacteria are incredibly old and are the last, hardy remnants of the population that flourished when the sediments first settled on the sea floor.
Living isn’t easy for any organism, and there is wear and tear on cellular structures. Thus any very old bacteria would have had to repair themselves. In theory, they could be millions of years old.
“If we look at how fast they metabolize, it would take them a thousand years just to reproduce themselves. They may be much older than this. There’s no way of knowing,” Roy said.
The slow rate of reproduction means that they cannot evolve at the same speed as bacteria in friendlier, energy-rich, nutrient-thick settings. That means, in turn, that they may preserve more primitive genetic features than other bacteria, says Robert Hazen, a scientist at the Carnegie Institution’s Geophysical Laboratory in the District and author of the new book “The Story of Earth.”
“If we’re looking at deep life, we’re looking at ancient life because they appear to evolve more slowly,” Hazen said. “You might be able to see more deeply rooted aspects of biochemistry. Some of the genes that are present may be just intrinsically much, much older.”
The meaning of life
Hazen has speculated that the definition of life may need to be expanded — that perhaps there are growing films of molecules deep below the surface that have characteristics similar to those of traditional organisms.
The basic definition of life has long been a matter of debate, though life as we know it has a couple of key features.
First, living things have a metabolism, meaning they use chemical reactions to convert energy to useful purposes.
Second, they have a genetic code that prescribes their structure and enables them to reproduce. The inexact nature of reproduction, due to mutations, allows life to undergo Darwinian evolution.
There are other features of living things: They have an inside and an outside. They use liquid water as the medium for their chemistry. And they’re built on a foundation of carbon, which is useful for forming long, complex molecules (such as DNA) that are stable at a variety of temperatures.
The weird microbes in the red clay below the deep Pacific abide by these basic rules of life. They just take their sweet time about it.