Enlarge /. Yuki Morono of the Japanese agency for marine earth research and technology deals with bacteria that may be 100 million years old. Attention!
You know the videos where people open (or even eat?) WWII military rations? It is shocking to see how well these "foods" can be preserved after all these decades. In a sense, Yuki Morono and his team of researchers from the Japanese Agency for Earth Science and Technology turned this experience around by giving modern foods to some ancient organisms. But her case was to take ancient mud from the ocean floor and add some food to see if there was anything alive.
There were actually bacteria in the mud, which probably doesn't sound surprising. But considering the environment and the age of this material – 100 million years – it's actually quite remarkable.
In the deep
Life deep underground or under the seabed is not studied as well as the easily accessible surface world. Sampling has shown that seabed sludge in different parts of the ocean differs greatly in terms of the type and frequency of microbial life present. In this case, however, the researchers examined deep sediments in the middle of the South Pacific, where extremely little organic material is available for life.
Enlarge /. Bacteria from the oldest sample (right) and accompanying isotope measurements (left). The white scale is 5 microns – significantly less than the width of a human hair. They reached for sediment plugs up to about 70 meters below the sea floor. Very little sediment accumulates here, so that a 70 meter thick clay pile represents about 100 million years. Muds on the bottom of lakes or swamps often have no oxygen because the respiration of bacteria that break down organic matter consumes everything. But the food here is so sparse that oxygen, nitrate and phosphate were present even in the deepest mud.
The researchers took these small plugs of sediment and injected substances that allow bacteria to grow, such as sugar and ammonia. In fact, bacteria grew and swallowed them up – they even analyzed isotopes of carbon and nitrogen in individual cells to check whether they had absorbed these substances. The initial frequency of microbial cells was far less than in more productive areas of the ocean, but they were present and viable.
The researchers do not believe that these are just modern bacteria that have penetrated deep into the mud. In fact, they shouldn't be able to move in this mud at all. The average distance between the particles in the clay should be significantly smaller than the size of a bacterium. The presence of microbes in the oldest sediments represents communities that are about as old as the sediment itself, the researchers conclude.
Scientists in the team describe what they found in this video.
DNA analysis shows slightly different combinations of bacterial species that are present at different depths. However, it was almost all oxygen-consuming aerobic bacteria. Some experiments did not add additional oxygen beyond what was already in the sludge, and the bacterial activity of the feed added quickly consumed all of the oxygen. In these experiments there was very little growth after the oxygen disappeared, indicating that there are few anaerobic bacteria. This is in contrast to food-rich seabed sites where anaerobic bacteria dominate.
Out of the ordinary
This leads to an extraordinary claim: "Our results suggest that microbial communities that are widespread in organically poor abyss sediments consist mainly of aerobes that maintain their metabolic potential under extremely low-energy conditions for up to 101.5 (million years)."
There are a few links in the chain where this could obviously go wrong. When microbes have a certain mobility in the sediment, the age goes out the window. However, the argument based on the pore space diameter and the existence of hard, impermeable layers is reasonable. The other potential hazard is contamination where bacteria can enter the sediment sample from another location. However, the team took a number of precautions here, including DNA samples taken at the time of sampling. If villain bacteria invaded during sampling, they should appear in later DNA samples, but not in the first – and that didn't happen.
That doesn't mean the data doesn't contain anything special. Cyanobacteria – photosynthetic microbes, better known as "blue-green algae" – appear, which is certainly strange given the total lack of sunlight on (and below) the ocean floor. The specific genus of cyanobacteria thrives at least under extreme conditions. And their growth during the experiment also took place in the absence of light, so microbes may only have to reveal some secrets.
So if the researchers are right about what they found, this is proof that life is nothing if not stubborn. By slowing down life in extremely limited circumstances, these bacterial communities may simply have survived incredibly long.
Nature Communications, 2020. DOI: 10.1038 / s41467-020-17330-1 (About DOIs).