In a series of posts of the next few
days months (or weeks years as time permits), I’d like do delve a bit into a topic that has fascinated me for some time: life under and inside the Earth’s crust.
Can life survive in the Earth’s crust, in the tiny pore spaces under miles of granitic rocks? What about under the bottom of the ocean – can eke out a life down there?
The answer to both questions is yes. Microbes – both bacteria and archaea – have been found down there, and trying to understand how they live, and how important and extensive these extreme ecosystems are compared to other parts of the biosphere, is a major goal of modern microbial ecology.
The place to begin our discussion is with an oft-cited 1998 article by William B. Whitman, David C. Coleman, and William J. Wiebe. This masterpiece of the underappreciated art of estimation lays out a rough calculation of the total number and mass of prokaryotic cells on the Earth.
The estimated the population of terrestrial subsurface prokaryotes in three different ways: by extrapolating from the (then) extremely limited measurements of free-floating microbial cell counts observed in deep groundwaters; by assuming reasonable values for average sediment/rock porosities and for the average volume fraction of pore space occupied by microbial cell mass; and by extrapolating from measured microbial cell counts found in “unconsolidated sediments”.
Unconsolidated sediments cover the vast majority of the ocean bottom and about 20% of the terrestrial surface, and so even though the data for microbial cell counts in these ecosystems is fairly good, the 80% of the terrestrial surface covered by other materials (e.g., consolidated sediments or granitic rock) leads to considerable uncertainty.
Back in 1998, these guys estimated that in the oceanic subsurface, there were about 355×1028 microbial cells, comprising about 303 Pg of carbon. They could only give a range of possible values for microbes in the terrestrial subsurface, from 25-250×1028 cells, comprising 22-215 Pg carbon. To put that in perspective, all the plants in world – all the forests, all the trees, all the grass, and all of our crops – contain something like 560 Pg of carbon. So subsurface microbial biomass nearly as much as all the plant-derived biomass on Earth.
And about these microorganisms, their environment, and their way of life, we know next to nothing. But we’re learning more, and over the next few posts I’ll highlight some of the most exciting of the recent discoveries in the field.
UPDATE: I’m still working posts on these topics. Stay tuned but don’t hold your breath.