Do you know those videos where people open (or even eat?) Military rations from World War II? It’s shocking to see how well preserved these “foods” can be after all those decades. Somehow, Yuki Morono and his team of researchers at the Japan Agency for Marine Science and Technology of the World lit up that experience by giving modern food to some ancient organisms. But their case involved ancient mud from the sea and adding a little food to see if there was anything alive there.
In fact there were bacteria in the mud, which probably didn’t look surprising. But considering the environment and the age of this stuff – 100 million years – it’s actually quite remarkable.
Deep life underground or under the sea is not studied as well as the easily accessible surface world. Sampling has shown that mud in the sea in different parts of the ocean varies greatly in terms of the types and abundance of microbial life that is present. But in this case, the researchers took deep samples in the middle of the South Pacific, where there is extremely organic material available to make life grow.
They caught sediment plugs up to about 70 meters under the sea. Here little sediment accumulates, so much so that a 70-meter-thick clay pile represents about 100 million years. Mud at the bottom of ponds or marshes often lacks oxygen, because the respiration of bacteria that break down organic matter consumes all of this. But food here is so rare that oxygen, nitrate, and phosphate were present even in the deepest mud.
The researchers took these little plaques of sediment and injectable substances that bacteria could use to grow, such as sugar and ammonia. And sure enough, the bacteria grew and emerged – they also analyzed isotopes of carbon and nitrogen in individual cells to verify that they had taken in those substances. The initial abundance of microbial cells was far less than that found in more productive areas of the ocean, but they were present and viable.
The thing is, researchers don’t think these are just modern bacteria that have made their way deep into the mud. In fact, they should not be able to move above all in that mud. The average space between the particles in the clay should be considerably smaller than the size of a bacterium. The researchers conclude that the presence of microbes in the oldest sediments represents communities that are as old as the sediment itself.
DNA analysis shows slightly different combinations of types of bacteria present at different depths. But it was almost all aerobic bacteria that consumed oxygen. Some experiments did not add any excess oxygen beyond what was already present in the mud, and the bacterial activity from the added food quickly used up all the oxygen. In those experiments, there was very little growth after the oxygen had gone, suggesting that there are few anaerobic bacteria present. In contrast to food-rich marine sites, where anaerobic bacteria dominate.
This leads to an extraordinary claim: “Our results suggest that microbial communities widely distributed in poor organic abyssal sediment consist mainly of aerobes that maintain their metabolic potential in extremely low energy conditions up to 101.5. [million years]. “
There are a few links in the chain where this can obviously go wrong. If germs have little mobility in sediment, ages pass through the window. But the argument against this, based on the diameter of the pore space and the existence of hard, impermeable layers, is reasonable. The other potential drawback is contamination, with bacteria entering the sediment sample from elsewhere. But the team took a number of precautions here, including DNA samples taken at the time each sample was collected. If the deceptive bacteria entered during sampling, they should appear in DNA samples later but not at first – and this did not happen.
That’s not to say there’s nothing particular about the data. Cyanobacteria – photosynthetic microbes better known as “blue-green algae” – make an appearance, which is certainly strange due to the total lack of sunlight on (and under) the sea. The specific genus of cyanobacteria is one that thrives in extreme conditions, at least. And their growth during the experiment also occurred in the absence of light, so the microbes may have a few secrets to give up.
So if researchers are right about what they found, it is a testament to the fact that life is nothing if not persistent. With the slowness to live by extremely limited means, these bacterial communities could have survived for a simply incredible length of time.
Nature Communications, 2020. DOI: 10.1038 / s41467-020-17330-1 (On DOIs).