by Roberto
Fig. 1. Crew on the Woods Hole Oceanographic Institute research vessel Neil Armstrong prepare to deploy a sediment corer to the seafloor of the Puerto Rico Trench. (Photo by Paul Walczak, Oregon State University). Source
Estimates are that, at the global scale, there are more microbial cells – maybe 5 x 1029 – in the seabed than in the water column above. These cells are not just near the seafloor, many of them they lie buried deep in the sediment. Very deep indeed, as deep as 2.5 kilometers. And they've been there for a long time, even millions of years (see here in STC). How can we know this? This is not your standard microbiology field work; you can't just decide to get up and go sampling. You'll need major equipment, like the corers found aboard the Neil Armstrong research vessel, housed at Woods Hole, MA (Fig. 1). Plus, the specialized equipment and methodologies to analyze the core samples obtained. But with the proper care and controls, what you find is "out of this world" microbiology. Yet, it's how most marine microbes live.
And how is that? What are these inhabitants of the deep biosphere doing? Not unexpectedly, whatever they do, they do it very slowly. Their times scales are literally geologic. Imagine you are a bacterium that finds itself in a microbial community at the seafloor. Sediment will accumulate over your community at about 1 meter per 100,000 years. So, by the time you are 40 meters below the seafloor, you (or your descendants) will have been there 4 million years. Those microbes found kilometers below the seafloor will have been there 100 million years or more. The deeper they are, the less residual organic matter will be available for their growth and maintenance metabolism. Estimates are that a typical doubling time in these environments is in the order of 1000 years.
Why should those microbes still be metabolically active? Why not just form an endospore and go totally dormant? Sounds like a Talmudic Question but it's not. The answer is known. Completely dormant spores do not survive well because they can't repair the spontaneous DNA damage over such long times. Spores might be okay for a few thousand years. But apparently, after that, it's much better to be maintain low metabolic activity. Of course, by laboratory standards, all those deep biosphere cells are dormant. Tells you how far off laboratory conditions can be from what happens down there.
If you now feel motivated to learn more about of microbial life in the very slow lane, I highly recommend you read "Slow Microbial Life in the Seabed," by Bo Barker Jørgensen and Ian Marshall. A thrilling review that I suspect you won't be able to put down until you're done!
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