How it all started

We all have heard that photosynthesis is the source of energy that powers the biosphere—have you thanked a green plant today? But, where did the earliest life evolve on Earth about 3500 million years ago? The land surface is unlikely, as it would have been subject to a withering flux of ultraviolet light from the Sun, until the ozone layer developed about 500 million years ago. A couple of years ago, I blogged about life under translucent rocks as a potential early habitat for microbial activity (https://blogs.nicholas.duke.edu/citizenscientist/where-the-sun-shines-dimly/). Other habitats are also possible.

Microbial life is found in the Earth’s crust beneath the abyssal depths of the sea—an environment where the sun never shines. Pores and cracks in the rocks host microbial activity that feeds on hydrogen generated by volcanic activity, especially along the mid-ocean ridges that are found in all the world’s oceans. These submarine mountain ranges total about 37,000 miles in length. Along these ridges the Earth’s tectonic plates are moving apart, allowing magma from the deep Earth to bubble upward. This volcanic activity is pretty much invisible to us, unless a mid-ocean ridge happens to be tall enough to reach the surface, such as in Iceland, which is along the mid-ocean ridge of the North Atlantic.

There are a lot of processes that generate hydrogen at mid-ocean ridge volcanoes. For instance, hydrogen is generated when iron minerals react with the oxygen in seawater. For her PhD dissertation, Stacey Worman, working with Lincoln Pratson and others at Duke University, has estimated the total amount of hydrogen that is likely generated each year at Mid-Ocean Ridges—72 x 1012 g. A lot of the hydrogen is held in rocks of the marine crust, but about 30% may escape and fuel microbial metabolism in the deep sea and its sediments.
Similar to the green plants that we see at the Earth’s surface, which generate organic matter by photosynthesis, these microbes use hydrogen in reactions known as chemosynthesis to generate organic carbon in complete darkness. The rate of metabolism is slow and in some cases limited by high temperatures from the ongoing volcanic activity. (Life is not known to occur above about 121 C or 250 F). Nevertheless, adding up the amount of hydrogen available, the total production of organic matter could amount to 2 x 1012 g of carbon each year. This is much less than the total rate of photosynthesis in the oceans, that today is more than 10,000 times greater—5 x 1016 g C/yr. The subsurface microbes are metabolizing at less than 0.1% of the rate we find in the surface waters.

Normally, we might overlook this low-level of activity by a few subterranean microbes living in cracks in rocks at the seafloor, but life in this environment may have been a likely source of the earliest life on Earth. Microbial activity in rocks may characterize life on other planets and their moons. In a very real sense, it’s where to look for life on Mars.

Meanwhile, it gives us some pause to see that the marvelous, productive biosphere that we enjoy today had such very humble beginnings.

References

Bradley, J.A., S. Arndt, J.P. Amend, E. Burwicz, A.W. Dale, M. Egger, and D.E. LaRowe 2020. Widespread energy limitation to life in global subseafloor sediments Science Advances 8: doi 10.1126/sciadv.aba0697

Kashefi, K and D.R. Lovley. 2003. Extending the upper temperature limit for life. Science 301: 934.

McDermott, J.M., S.P. Sylva, S. Ono, C.R. German, and J.S. Seewald. 2020. Abiotic redox reactions in hydrothermal mixing zones: Decreased energy available for the subsurface biosphere. Proceedings of the National Academy of Sciences 117: 20453-20461.

Worman, S.L., L.F. Pratson, J.A. Karson, and W.H. Schlesinger. 2020. Abiotic Hydrogen (H2) Sources and Sinks near the Mid-Ocean Ridge (MOR) with Implications for the Subseafloor Biosphere. Proceedings of the National Academy of Sciences, US, 117: 13283-13293.

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