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

There were, in fact, bacteria in the mud, which likely doesn’t sound surprising. But given the environment and the age of this stuff—100 million years—it’s actually pretty remarkable.

In deep

Life deep below ground or below the seafloor isn’t studied as well as the readily accessible surface world. Sampling has shown that seafloor mud in different parts of the ocean differ a lot in terms of the types and abundance of microbial life that are present. But in this case, the researchers sampled deep sediments in the middle of the South Pacific, where there’s extremely little organic matter available for life to grow on.

They grabbed plugs of sediment up to about 70 meters below the seafloor. Very little sediment accumulates here, so that 70-meter-thick pile of clay represents about 100 million years of time. Muds at the bottom of lakes or marshes often lack oxygen, as the respiration of bacteria breaking down organic matter consumes it all. But food is so sparse here that oxygen, nitrate, and phosphate were present even in the deepest mud.

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

The thing is, the researchers don’t think this is just modern bacteria that have made their way deep into the mud. In fact, they shouldn’t be able to move at all in that mud. The average space between particles in the clay should be considerably smaller than the size of a bacterium. The presence of microbes in the oldest sediments represent communities that are about as old as the sediment itself, the researchers conclude.

DNA analysis shows slightly different combinations of types of bacteria present at different depths. They were almost all oxygen-consuming aerobic bacteria, though. Some experiments didn’t add any extra 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 was gone, suggesting there are few anaerobic bacteria present. That’s in contrast to food-rich seafloor sites, where anaerobic bacteria dominate.

Extraordinary

This leads to an extraordinary claim: “Our results suggest that microbial communities widely distributed in organic-poor abyssal sediment consist mainly of aerobes that retain their metabolic potential under extremely low-energy conditions for up to 101.5 [million years].”

There are a couple links in the chain where this could obviously go wrong. If microbes do have some mobility in the sediment, the ages go out the window. But the argument against that, based on pore-space diameter and the existence of hard, impermeable layers, is reasonable. The other potential pitfall is contamination, with bacteria getting into the sediment sample from somewhere else. But the team took a number of precautions here, including DNA samples taken at the time each sample was collected. If rogue bacteria had gotten in during sampling, they should show up in later DNA samples but not the initial one—and that didn’t happen.

That’s not to say there’s nothing peculiar in the data. Cyanobacteria—photosynthetic microbes better known as “blue-green algae”—make an appearance, which is certainly strange given the total lack of sunlight on (and beneath) the seafloor. 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 that microbe might just have some secrets to give up.

So if the researchers are right about what they’ve found, it’s a testament to the fact that life is nothing if not persistent. By slowing down to live within extremely limited means, these bacterial communities may have survived for a simply incredible length of time.

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

https://arstechnica.com/?p=1695007