Life at One Hundred Million Years Old

Citation: Morono, Y., Ito, M., Hoshino, T., Terada, T., Hori, T., Ikehara, M., D’Hondt, S., & Inagaki, F. (2020). Aerobic microbial life persists in oxic marine sediment as old as 101.5 million years. Nature Communications, 11(3625). https://doi.org/10.1038/s41467-020-17330-1

Ever wondered how old the oldest life on Earth is? Recently, a case was made for microorganisms in the South Pacific Gyre. A team led by scientists from the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) found that microorganisms lying dormant below the seafloor for 100 million years can be coaxed back to life in the lab.

Small, Unlikely Heroes

Out at the bottom of the middle of the ocean, there are microorganisms living on the seafloor. (A microorganism, or microbe, consists of a single cell, rather than multiple.) In the absence of most sources of food (no light, no coral reefs, no kelp forests, no anything, really), the microorganisms survive by consuming marine snow – a term for the small particles of carbon and other nutrients that drift down from the surface, thousands of meters overhead. Over time, as these tiny amounts of marine snow accumulate, the microbes living at the seafloor are slowly buried deeper and deeper in the sediment. At some point, they must run out of food and energy, and die. Right?

Not so fast. In any Survivor competition, microbes would be the clear favorites. Time and again, they’ve proven they can survive (and even thrive) in some of the most extreme places on Earth. The scientists from JAMSTEC were curious how long energy-starved microorganisms could survive below the seafloor, given their limited food supply.

Collecting and Growing Marine Microorganisms

Figure 2: The R/V JOIDES Resolution, the scientific drilling vessel used to collect samples for this study. The ship is operated by the International Ocean Discovery Program. (Image Source: Wikimedia Commons)
Figure 1: The five main ocean gyres, with the Southern Pacific Gyre in color (Image Source: Wikimedia Commons).

To find out, the researchers set out for the South Pacific Gyre (Figure 1), the region of the ocean with the lowest amount of primary productivity, and the greatest chance of finding old sediments and old microorganisms. The reason is that this part of the ocean is far away from any coasts, and thus doesn’t get much carbon or nutrient input from terrestrial erosion, agriculture, rivers, or cities. It’s the equivalent of the Sahara Desert to a microbe. Using a specialized ship with an attached drill rig (Figure 2), the team pulled up several 100-meter long “cores” of subseafloor rocks and sediment, which were drilled from the ocean floor 6 kilometers below the ship. From bottom to top, layer by layer, these cores served as a record of the ocean floor through time (Figure 3).

The researchers brought the cores back to the laboratory, and by chemically age-dating the rocks and sediments within them, they found that the oldest (the bottommost) layers had been deposited on the ocean floor 100 million years ago. Back then, dinosaurs still walked the Earth! In these sediment layers, the researchers also observed cells. But they needed to determine whether the cells were alive, and not just fossilized.

Figure 3: A sediment core brought to the surface by the R/V JOIDES Resolution. (Image Source: International Ocean Discovery Program)

To do this, the researchers set up “incubations” of the sediment in the 100 million year old layers. They mixed the sediment with plenty of oxygen, and far more nutrients than were available at the bottom of the South Pacific Gyre. The researchers wanted to see whether they could stimulate growth if they kept the microbes fed and happy, rather than leaving them in their energy-starved state. The catch? Some of the nutrients they added were isotopically labeled.

Figure 4: The two main isotopes of carbon.

Isotopes, remember, are just different forms of an element, which each contain a different number of neutrons. Carbon-12, for example, has 6 neutrons, while carbon-13 has 7 neutrons (Figure 4). In nature, there’s a constant ratio of one isotope to another for every element. The ratio of carbon-13 to carbon-12 is roughly 1 to 89.9, so for every atom of carbon-13, there are 89.9 atoms of carbon-12. By adding nutrients with 100% carbon-13 to the incubation, and later examining the cells under a specialized mass spectrometer, the researchers could determine whether the microorganisms used any of the added carbon-13 to build new cells. If so, the ratio of carbon-13 to carbon-12 in their cells would be higher than 1 to 89.9.

Life Finds a Way

The researchers found, to their surprise, that over 99.1% of the cells they recovered had incorporated the carbon-13 and other isotopic labels, and were thus capable of growing and dividing once subjected to the nutrient-rich conditions provided in the lab. Furthermore, the speed of the response was breathtaking. Within about 2 months, cell numbers increased by about 4 orders of magnitude! This was a stunning demonstration of just how long microorganisms in an oxygenated environment could remain dormant, and still survive.

By showing that life persists in ancient, oxygenated ocean sediments, this finding shows us that the limits of life are far less restrictive than we thought. The study also provides a unique opportunity to examine the evolutionary arc of energy-starved organisms. Due to the lack of growth and cell division in the subseafloor, mutations are slower to accumulate, and the rate of evolution is much slower. Examining these microbes may help us understand what microbial life was like in the geologic past, and may help answer questions about the ancient ocean.

In any case, the existence of 100 million year old microorganisms is awe-inspiring.

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