Do you know those videos where people open (or even eat?) World War II military rations? It is shocking to see how well preserved these “foods” can be after all those decades. In a sense, Yuki Morono and his team of researchers from the Japan Marine and Land Science and Technology Agency spread this experience by providing old organisms with modern food. But their case was that ancient mud was taken out of the seabed and food was added to see if anyone was alive there.
In fact, there were bacteria in the mud that probably didn’t look surprising. But given the environment and the age of these things – 100 million years – it’s actually quite amazing.
Life deep underground or under the seabed is not explored, nor is the surface world easily accessible. Sampling has shown that seabed mud in different parts of the ocean varies greatly in terms of the types and abundance of microbes present. In this case, however, the researchers took deep sediments in the middle of the South Pacific, where there is very little organic matter to grow.
They grabbed the sediment plugs to about 70 meters below the seabed. Very little sediment accumulates here, so a 70-meter-thick pile of clay means about 100 million years of time. Mud at the bottom of lakes or wetlands often lacks oxygen because the bacteria that consume it consume it all by breaking down organic matter. But the food here is so scarce that oxygen, nitrate, and phosphate were even in the deepest mud.
The researchers took these small sediment stoppers and injected substances that bacteria could use for growth, such as sugar and ammonia. And, of course, the bacteria grew and bubbled them – in individual cells, they even analyzed carbon and nitrogen isotopes to check if they had taken up those substances. The initial abundance of microbial cells was much lower than that found in more productive areas of the ocean, but they were also viable.
The thing is, researchers don’t think it’s just modern bacteria trapped deep in the mud. In fact, they should not be able to move altogether tame purve. The average gap between the particles in the clay should be much smaller than the size of the bacterium. Microbes in the oldest sediments reflect communities that are about as old as the sediments themselves, the researchers conclude.
DNA analysis shows slightly different combinations of bacterial types at different depths. However, they were almost all oxygen-consuming aerobic bacteria. Some experiments did not add any extra oxygen other than what was already in the dirt, and the bacteria derived from the added food quickly consumed all the oxygen. In those experiments, the growth was very low with oxygen growth, indicating that anaerobic bacteria are low. This is in contrast to the high-food areas of the seabed where anaerobic bacteria predominate.
This leads to a remarkable statement: “Our results show that the microbial communities prevalent in organic poor bottom sediments consist mainly of aerobes that maintain their metabolic potential under ultra-low energy conditions up to 101.5 [million years]. “
There are several links in the chain where this can obviously go wrong. If the microbes in the sediment move a little, the age goes out the window. But the argument based on the diameter of the pair space and the presence of solid, impermeable layers is valid. Another potential problem is contamination when bacteria enter a sediment sample from somewhere else. However, the team has taken a number of precautions here, including DNA samples taken during each sampling. If rogue bacteria were introduced during sampling, they should have appeared in subsequent DNA samples, but not in the initial one, and this was not the case.
This is not to say that there is nothing special about the data. Blue-green algae – photosynthetic microbes, better known as “blue-green algae” – seem really strange, given that there is no sunlight on (and below) the seabed. A specific species of cyanobacteria is one that thrives at least under extreme conditions. Their growth during the experiment also occurred in the absence of light so that the microbes could simply abandon some of the secrets.
So if researchers are right about what they found, it testifies to the fact that life is something if not permanent. With the slowdown in living to a limited extent, these bacterial communities were able to survive for just an incredibly long time.
Nature Communications, 2020 DOI: 10.1038 / s41467-020-17330-1 (about DOI).