This is not the first time that living bacteria have been discovered many meters into marine sediments. But a new Science article this week, by Danish and German scientists, describe bacteria breathing oxygen at least 30 meters in deep red clay at the bottom of the Pacific ocean north of Hawaii. These bacteria have not had exposure to sunlight or organic matter for 86 million years! I will say it again: the last time these microbes saw fresh oxygen or sunlight, dinosaurs walked the Earth! So how are they still alive? The bacteria living in these sediments are breathing oxygen at a much, much slower rate than other microbial communities. Scientists calculated that the respiration rate of these bacteria dropped ten thousand fold at 30 meters compared with at or near the surface of the sediment. Thus, aerobic metabolism can persist in deep marine sediments, although at an extremely low rate. It is amazing how little it takes to sustain life! The scientists measured the oxygen in the core with special sensors; because they know how much oxygen should have diffused down to each mud depth, any "missing" oxygen had to mean that microbes had consumed it. What is special about this North Pacific sediment is that the microbial community was too sparse to consume it all, which is why they detected oxygen all the way down to ~30 meters. In most places, bacteria consume all the oxygen within 10 cm of sediment. The very slow oxygen consumption by the microbes allow them to subsist on very small amounts of long-buried organic materials. This type of research may be useful in the search for life on other planets, given the extreme conditions in these deep clay sediments. Of course, to be absolutely sure these extreme microbes are alive, one would need to see them divide. But while E. coli divides every 1/3 hour, the calculated rate for these extremely slow microbes would be 1,000 years. We may need to wait a bit to see these microbes divide.
Barely Breathing Microbes Still Living in 86-Million-Year-Old Clay
At first glance, there doesn't appear to be much happening in the mud buried 30 meters below the Pacific Ocean sea floor. But this ancient muck, which hasn't had a fresh shot of food or sunlight since the days of the dinosaurs, still harbors life—if just barely. Scientists have discovered that deep-sea microbial communities, buried for 86 million years, are still consuming oxygen, albeit at extraordinarily low rates. These microorganisms eking out an existence in slow motion reveal just how little it takes to sustain life on our own planet, and potentially on others.
Microbes such as bacteria are the most numerous organisms on Earth, and about 90% of them live in sediments buried under the sea floor. To take a peak at this microscopic life in its natural habitat, a team of scientists including Hans Røy, a microbiologist at Aarhus University in Denmark, traveled to the tropical waters of the Pacific Ocean. There, they collected mud from the sea floor, which builds up for millions of years like a giant layer cake as newer sediments pile on top of older ones.
The scientists jammed a large metal pipe 30 meters into the sea bottom and used a piston to suck out a long column of reddish clay. After hauling the sediment onboard, they probed the core with a needlelike sensor to measure the oxygen concentration in each layer. The researchers knew how much oxygen should have diffused down into each section of sediment from the seawater, so any "missing" oxygen meant microbes had consumed it.
Moving deeper through the core is like moving back in time, studying older and older communities of microorganisms. "We can use the Pacific as a natural experiment that has been running for 86 million years," Røy says.
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