Insights into the Marine Deep Biosphere: Limits of Life
The Center for Dark Energy Biosphere Investigations (C-DEBI) is a multi-year research initiative sponsored by the National Science Foundation. This initiative and the projects it encompasses span many institutions and have four intertwining themes. Here, we present material on the third of these themes, Limits of Life. First, however, what is the center all about?
The world we live in is dominated by microbial life. You can’t see them, but microorganisms are all around us. Everything living thing depends on them in one fashion or another. Many natural processes that we take for granted, such as plants growing from sunlight, and the decomposition of waste, result from microbial activity. Microbes provide essential nutrients, vitamins and other essential molecules for human and animal diets. They even regulate oxygen and carbon dioxide in the air. Microbial organisms both produce and consume huge amounts of greenhouse gases like carbon dioxide and methane. On a global scale, even “small” shifts in microbial activity can change Earth’s entire climate.
Although microbial processes are essential to life on Earth, scientists don’t know some of the most basic biological details about many of these organisms: who, what, and where are they?
In this world of unanswered questions about microbes, an enormous new habitat has recently come to light as scientists’ explore the distant reaches of Earth–the deep biosphere. As it turns out, there are vast numbers of microbes living below the surface of the land, below the seafloor, and into the very crust of the Earth itself.
What new forms of life live there? How do they survive so far from light and under such extreme pressures and temperatures? Will their discovery change the way we look for life elsewhere? What useful species and biological, chemical, and physical processes might we find there, such as new ways to make energy, store carbon, or treat wastewater? C-DEBI addresses these questions and more.
The third theme of deep biosphere research is the Limits of Life: Extremes and Norms of Carbon, Energy, Nutrient, Temperature, Pressure, and pH. There are many different ways that animal and plant life are tested to their limits. The marine realm has long been fertile ground for research into the extremes at which life can thrive and survive. A classic example was the 1977 discovery of life at hydrothermal vents where organisms were found to be thriving in water well above the conventional boiling point[i]. That discovery changed our understanding of the limits of life, especially with regard to temperature.
One of the reasons that scientists are particularly intrigued by life in the deep biosphere is that it tests many different limits to life as we know it: not just temperature, but extremely high-pressure, high and low pH, as well as starvation conditions. By “starvation,” we mean extremely low supplies of the nutrients and compounds required by microbes for their metabolism. (An interesting corollary to this limit is the concept of time as a limiting factor; how long can microbes survive running an extremely slow metabolism? In humans, there are fascinating stories of individuals who have survived many minutes under water without oxygen after plunging into frozen rivers or lakes, on account of their metabolisms slowing down. Some microbes can apparently slow down so much that they persist thousands and even millions of years at a very low metabolic rate, a topic touched upon in the backgrounder for Theme #1.)
Research into the limits of life is of interest to scientists and citizens because of its implications for biotechnology, our fundamental understanding of what it means to be “alive,” and the search for extraterrestrial life. A good example of the application of this research to looking for alien life forms is ongoing investigations of how microbial life can be supported by natural “background” radiation – processes at work not only in the deep biosphere on Earth, but on any wet, rocky body throughout the universe such as a planet or moon[ii].
Background radiation naturally results in the production of molecular hydrogen (H2)[iii] if water is present. Since hydrogen can also be an energy source for microbes, some microbial life can thrive on H2 even in the absence of other resources that are typically needed for life such as sunlight, organic matter, or even other microbes. Although background radiation is found everywhere (with some materials producing more radiation than others), microbes that live on it are nearly impossible to detect on the surface of Earth because of the tremendous amount of other life. In the deep biosphere, however, the tables are turned, and radiogenic life can be a large fraction of the community[iv]. Potentially, these microbes could be equally “happy” under the Martian surface or even on a rogue planet wandering in space without a star to shine on it. However, there are many unknowns about this unique process, including how much hydrogen is produced in different environments; how many deep biosphere organisms could rely on it for sustenance; and how fast this life could grow and reproduce.
One C-DEBI research project is currently looking at some of the poorest, most carbon depleted sediment in the world – that of the South Pacific – in order to focus on these strange hydrogen-guzzling microbes. University of Rhode Island graduate student Justine Sauvage calculated that while these unidentified cells must have extremely slow metabolisms, the process appears nonetheless capable of supporting life even at extremely low hydrogen levels[v]. In other words, this form of life could be just about everywhere on Earth, below the surface. Not quite like the prehistoric forest of Jules Verne’s imagination, but perhaps even more strange.
In other cases, the limits of life aren’t any further removed from our everyday experience than the under-sink cupboard. The disinfecting cleansers stored there may include bleach, a harsh chemical disinfectant that also happens to have a high pH – about 11.9. This is much higher than typical solutions and not very amenable to life[vi], but exceptions do exist.
Naturally occurring springs with pH equal to or greater than that of bleach are found in diverse locations, offering a living laboratory for C-DEBI researchers. Here, scientists such as Dr. Matt Schrenk and his colleagues[vii] have found microbes that specialize in tolerating the high pH in order to get priority access to the microbial “fuel” that is also found in these waters. Here, again, the “fuel” for microbial life could be hydrogen, produced by serpentinization, the same process that generates the high pH waters in rocks originating from the upper mantle[viii]. In some places, this high pH water wells to the surface and carries its microbes with it, giving us a window into deep Earth environments, and potentially the very deepest portions of the biosphere. This process occurs under both the continental surface as well as the seafloor, and researchers are scrambling to understand what, if any, similarities there are between the oceanic communities and the terrestrial ones.
Does it really matter to the microbes living in the deepest reaches of the crust and the upper mantle where they live? According to data released at an American Geophysical Union meeting in late 2012 by C-DEBI researcher Dr. William Brazelton, these microbes appear to be found everywhere there is H2 to be had – though only the hardiest taxonomic groups can take the highest pHs of 11 and 12[ix]. Still, it doesn’t appear that it’s just the high pH weeding out the competition; understanding how multiple stressors, of which pH is one, can combine to select for different organisms is one of the difficulties of real-world science. While researchers
are beginning to identify which organisms can survive at this pH extreme, C-DEBI researchers are still trying to determine how they do so and what lessons they might be able to teach us about chemistry and the adaptability of life.
For more information:
This backgrounder was written by John Kirkpatrick, a post-doctoral fellow at the University of Rhode Island Graduate School of Oceanography, as part of the education and outreach efforts for C-DEBI.
[i] It is worth noting that, even at what we consider “boiling” temperatures, water at hydrothermal vents doesn’t actually boil; the pressure at depth prevents that. Just as the boiling point decreases at altitude (which is why cooking pasta is different in Denver), it increases with depth in and below the ocean.
Life at hydrothermal vents: Lonsdale, P. (1977). Clustering of suspension-feeding macrobenthos near abyssal hydrothermal vents at oceanic spreading centers. Deep Sea Research 24, 857–863.
[ii] At least, for the first 10 or so billion years of their existence; since radioactivity decays over time, very old rocks eventually become radioactively inert.
[iii] “Molecular hydrogen” refers to H2, which can be dissolved in water, or can be a gas in its own right. The term “molecular hydrogen” is used to distinguish H2 from hydrogen ion (H+), which are very different chemically.
[iv] D’Hondt, S., Spivack, A. J., Pockalny, R., Ferdelman, T. G., Fischer, J. P., Kallmeyer, J., Abrams, L. J., et al. (2009). Subseafloor sedimentary life in the South Pacific Gyre. Proceedings of the National Academy of Sciences of the United States of America, 106(28), 11651–6. doi:10.1073/pnas.0811793106
[vi] Life can operate across a very wide range of pH, but the pH inside of cells is typically close to neutral – i.e., pH 7. Many of the foods we enjoy are acidic, i.e. below pH 7 (coffee is approximately pH 5), but it’s neutralized before circulating through the body. Human blood is typically between 7.35 and 7.45 – any lower or higher, and there can be serious consequences. Seawater is more basic, about 8.1.
[viii] Funny enough, the process that produces this basic, hydrogen rich fluid is the formation of serpentinite rocks; serpentinite can be a wonderful green color that some people find pleasing for countertops – such as those in their kitchen. See Müntener (2010) for a primer on serpentinization. http://geology.gsapubs.org/content/38/10/959.full
[ix] AGU fall 2012 meeting: presentation by William Brazelton, “Biogeography of serpentinite-hosted microbial ecosystems”