Ancient Life in Ancient Water?

All of the amazing riches of biodiversity we see around us exists in just a thin band of life on the surface of this planet. Estimates claim that between a third and half of all the biomass on Earth is hidden from us, made up of microbes living on the oceanic floors or in the pores and cracks in sedimentary and crystalline rocks, and we've recently even discovered microbial communities living and feeding in the clouds. Our daily life tends to reinforce a parochial view of this world, when in fact life thrives kilometers above and below us. In a paper published this week in Nature, scientists from the UK and Canada studied samples of water taken from 2.4km under a mine in Ontario, Canada. Based on the relative abundance of different isotopes in the water, the team determined that it's been completely isolated from the surface for at least 1.5 billion years and perhaps for as long as 2.6 billion years. The water is also rich in both hydrogen and methane, holding out the tantalizing possibility that something may have been living there all this time.

That dating puts this water in the Proterozoic, an eon stretching from 2,500 to just 500 million years ago. Although the earliest known fossils date back to 3.4 billion years ago (well before the Proterozoic), multicellular life would only make its appearance in the late Proterozoic (around 500-600 million years ago). Sometime in the first half of the Proterozoic, complex cells, eukaryotes, evolved from the symbiosis of simpler prokaryotic cells. The first step was the acquisition of a nucleus, the defining feature of eukaryotic cells. A bacterial cell then moved into the eukaryote, shedding its ability to live freely, and became a mitochondrion, the powerhouse of eukaryotic cells. Today, we can use the genome of the mitochondria in our cells to track our female ancestry. Soon afterwards, some eukaryotes engulfed a photosynthetic bacterium, thus gaining another organelle: the chloroplast. These newly photosynthetic cells spewed their waste — oxygen — into the environment. Oxygen was absent from the atmosphere of the earliest Proterozoic, which was rich in methane and carbon dioxide. Once iron and other oxygen sinks became saturated, free oxygen began to accumulate in the atmosphere, poisoning it and, eventually, transforming the face of the planet. Given the uncertainty in both the age of the water sample and the dating of many of these events, it's hard to know for sure what its last glimpse of the surface looked like. Some time after it was isolated, though, the descendants of the earliest photosynthetic cells gave rise to plants, while their cousins, lacking chloroplasts, spawned animals and fungi.

The water sample from Ontario dates to between the third and fifth twist on this picture of geological time.

So is there anything living in that ancient water? The researchers don't know yet. It's certainly possible — life can thrive in the most unexpected places. A few years ago, researchers used metagenomics to study the microbial communities in samples from the Outokumpu Deep Drilling Project, a 2.5km deep borehole in eastern Finland. They found distinct communities living at different depths all the way to the seemingly inhospitable bottom, at temperatures of over 40°C and pressures of over 150 bars. Another inhospitable environment is found in the underwater sinkholes of Lake Huron, where conditions are similar to those found during the Proterozoic. Some researchers have even suggested that studying the communities in those sinkholes may teach us about life on the early Earth.

Microbes can also persist in profound isolation. The bacterium Desulforudis audaxviator was discovered in water samples taken from 2.8km under the surface near a gold mine in South Africa. This amazing species makes up the entire ecosystem of that water, which has been isolated for millions of years. Energy from the radioactive decay of uranium, thorium, and potassium in the surrounding rock releases hydrogen and sulfur compounds which D. audaxviator feeds on, along with extracting carbon from carbon dioxide and fixing nitrogen. It's well adapted to its isolated lifestyle, so far from the nourishing light of the sun.

Whether or not there's anything living in the water samples from Ontario, it's certain that they don't hold a preserved specimen of ancient life. Anything we find will be separated from those ancient microbes by at least 1.5 billion years of evolution, just like we are ourselves. That's an awfully long time. Like D. audaxviator, they will have adapted to meet the challenges of their particular environment. Any microorganisms living in this water haven't tasted the atmosphere or seen sunlight since before multicellular life evolved on the surface of this planet. To the extent that their environment is similar to that of the early Earth, we may learn something about Proterozoic life, but we won't get a frozen snapshot of Proterozoic life. At best, we may get a glimpse of the ancient biosphere thanks to their descendants, refugees from a long lost world.

Discussion prompt:
In a world that's teeming with life which we haven't discovered and don't understand, where species are dying away faster than we can discover them, why should we care about finding life somewhere like this? One answer offered by the team is the question of extraterrestrial life. Anything living in this water would face conditions somewhat similar to those below the surface of Mars, so learning about them would give us a better idea of where else we might find life Out There. I can think of a few other good reasons. What about you? Do you think this is important? Why or why not?

Further reading
Biddanda, B. A., Nold, S. C., Dick, G. J., Kendall, S. T., Vail, J. H., Ruberg, S. A. & Green, C. M. (2012) Rock, Water, Microbes: Underwater Sinkholes in Lake Huron are Habitats for Ancient Microbial Life. Nature Education Knowledge 3(10), 13
Dylan Chivian; Eoin L. Brodie; Eric J. Alm; et al. (2008) Environmental genomics reveals a single-species ecosystem deep within the Earth. Science 322(5899), 275–278. doi:10.1126/science.1155495
Hessler, A. M. (2012) Earth’s Earliest Climate. Nature Education Knowledge 3(10), 24
Holland G, Lollar BS, Li L, Lacrampe-Couloume G, Slater GF, Ballentine CJ. (2013) Deep fracture fluids isolated in the crust since the Precambrian era. Nature. 497(7449), 357-60. doi: 10.1038/nature12127.
Itävaara, M., Nyyssönen, M., Kapanen, A., Nousiainen, A., Ahonen, L. and Kukkonen, I. (2011) Characterization of bacterial diversity to a depth of 1500 m in the Outokumpu deep borehole, Fennoscandian Shield. FEMS Microbiology Ecology 77, 295–309. doi: 10.1111/j.1574-6941.2011.01111.x
William B. Whitman, David C. Coleman, and William J. Wiebe. (1998) Prokaryotes: The unseen majority PNAS 95(12), 6578-6

Image credits
The Geological Time Spiral (by the USGS) is from Wikimedia Commons.
The endosymbiosis model is adapted from an image on Wikimedia Commons by user Kelvinsong and is CC-BY-SA licensed.