Sat. Oct 1st, 2022
Yuki Morono of the Japan Agency for Marine-Earth Science and Technology treats bacteria that may be 100 million years old.  Be careful!
enlarge Yuki Morono of the Japan Agency for Marine-Earth Science and Technology treats bacteria that may be 100 million years old. Be careful!

You know those videos where people open (or even eat?) World War II military rations? It’s shocking to see how well-preserved these “foods” can be after all these decades. In a way, Yuki Morono and his team of researchers at the Japan Agency for Marine-Earth Science and Technology have turned that experience around by giving modern nourishment to some ancient organisms. But their case involved bringing up old mud from the seabed and adding some food to see if anything lived in it.

There was in fact bacteria in the mud, which probably doesn’t sound surprising. But given the environment and the age of this stuff – 100 million years – it’s actually quite remarkable.

in deep

Life deep underground or under the seabed is not as well studied as the easily accessible surface world. Sampling has shown that seafloor mud in different parts of the ocean differs widely in terms of the species and abundance of microbial life present. But in this case, the researchers sampled deep sediments in the center of the South Pacific, where extremely little organic matter is available for life to grow on.

Bacteria from the oldest sample (right) and associated isotope measurements (left).  The white scale bar is 5 microns - significantly less than the width of a human hair.
enlarge Bacteria from the oldest sample (right) and associated isotope measurements (left). The white scale bar is 5 microns – significantly less than the width of a human hair.

They picked up plugs containing sediment to about 70 meters below the sea floor. Very little sediment accumulates here, so that a 70-meter thick pile of clay represents approximately 100 million years of time. Mud on the bottom of lakes or swamps often lacks oxygen, as the respiration of bacteria that break down organic matter swallows it all up. But food is so scarce here that oxygen, nitrate and phosphate were present even in the deepest mud.

The researchers took these tiny plugs of sediment and injected substances that bacteria can use to grow, such as sugar and ammonia. And indeed, bacteria grew and gobbled them up — they even analyzed isotopes of carbon and nitrogen in individual cells to verify that they had taken up those substances. The initial abundance of microbial cells was much lower than found in more productive areas of the ocean, but they were present and viable.

The point is, the researchers don’t think these are just modern bacteria that got deep into the mud. They really shouldn’t be able to move not at all in that mud. The average space between particles in the clay should be significantly smaller than the size of a bacterium. The presence of microbes in the oldest sediments represents communities roughly as old as the sediment itself, the researchers conclude.

Scientists from the team describe what they found in this video.

DNA analysis shows slightly different combinations of species of bacteria present at different depths. However, they were almost all oxygen-devouring aerobic bacteria. Some experiments did not add extra oxygen beyond what was already present in the mud, and the bacterial activity of the added food quickly consumed all the oxygen. In those experiments, there was very little growth after the oxygen disappeared, suggesting that few anaerobic bacteria are present. This is in contrast to nutrient-rich seabed sites, where anaerobic bacteria dominate.

Exceptional

This leads to an extraordinary claim: “Our results suggest that microbial communities widely distributed in organically poor abyssal sediments consist mainly of aerobes that maintain their metabolic potential under extremely low energy conditions up to 101.5 [million years]†

There are, of course, a few links in the chain where this can go wrong. If microbes have any mobility in the sediment, the ages go out the window. But the argument against that, based on the diameter of the pore space and the existence of hard, impermeable layers, is reasonable. The other potential pitfall is contamination, where bacteria enter 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 rogue bacteria had invaded during the sampling, they should show up in later DNA samples, but not in the first — and they didn’t.

That’s not to say there’s nothing special in the data. Cyanobacteria – photosynthetic microbes more commonly known as “blue-green algae” – are appearing, which is certainly strange given the total lack of sunlight on (and below) the seafloor. In any case, the specific genus of cyanobacteria thrives in extreme conditions. And their growth during the experiment also took place in the absence of light, so the microbe may be giving up a few secrets.

So if the researchers are right about what they found, that’s proof that life is nothing but persistent. By slowing down to live with extremely limited resources, these bacterial communities may have survived an incredibly long time.

Nature Communications, 2020. DOI: 10.1038/s41467-020-17330-1 (About DOIs).

List image by Morono et al./Nature Communications

By akfire1

Leave a Reply

Your email address will not be published.