By Matt Barr
Our expansive journey into the heart of science begins with the story of the hardy microbe. A single-celled organism that has made its home quite literally everywhere, we are only beginning to discover the fascinating past and potentially revolutionary future that these microscopic organisms share with us.
Microbes can be found everywhere, from the harshest environments to the most sterile. First discovered in the 1600’s, little information was known about how they actually operated until the late 19th century. Even then, most of our detailed observations of microbial activity did not occur until the second half of the 20th century. However, once it was understood that microbes inhabit virtually any space where liquid water exists, a new view of life on Earth emerged and opened the door to exciting discoveries in a field of science that has only recently been explored more deeply.
Before delving into the specifics of our research at Georgia Tech, it’s a good idea to establish a context in which the research is being conducted. That involves explaining where the microbes have been, where they’re going, and how they’re taking us along on their path through history.
A long time ago in a galaxy very, very close by (our own, in fact), the earth looked nothing like it does today. The land masses we’ve come to know and love, the oxygen that our bodies enjoy breathing on a pleasant spring day (and all of the other days), the warmth of a sun whose luminosity and energy output that allow for those pleasant spring days – all were nowhere to be seen. Indeed, there were no eyes as we know them with which to see, even if all those lovely life-sustaining conditions were, in fact, anywhere to be seen.
And yet, to quote those immortal words of Jurassic Park’s Dr. Ian Malcolm, “life, uh, finds a way.”
What life though? Under the harsh conditions that would have burned your lungs, drowned you, choked you to death, or finished you off in any number of other nasty ways, there was a class of life that defied all the odds stacked against it. We’re talking, of course, about our dear, omnipresent friends known as microbes.
For the first billion or so years of Earth’s history after its accretion (that is, after Earth finished forming into a planet by combining bigger and bigger chunks of rocks, debris, gases, and other materials swirling around our sun in its orbital path), no life seems to have existed. Indeed, it seems that there was little more than one big ocean and an atmosphere on our planet. At this magic billion-year mark, however, evidence of the first single-celled organisms begin appearing in the geologic records. The fascinating aspect of this appearance (one of many) is that our understanding of the conditions under which it’s possible for life to form and sustain itself were very much counter to the conditions present on our planet and in our solar system so long, long, long, long ago.
For starters, we’re fairly certain that a phenomenon referred to as the “faint young sun” was in place. The faint young sun was precisely that: a star that wasn’t hot enough to warm our planet to a temperature that supported liquid water. This is a critical piece of information because without liquid water, life as we know it cannot exist.
As the life of a star progresses, it undergoes a process known as the Main Sequence, during which the hydrogen at its core burns at such high temperatures that the hydrogen atoms fuse together to create the next heavier element, helium.
As this process takes place, the star grows hotter and brighter. However, in what we call the Archaean Eon, that period between 2.5 – 4 billion years ago, our sun’s luminosity and its corresponding energy output were about 70% of what they are today – amounts insufficient to support life. Yet, the evidence of life is found in our rocks that date from the very Archaean Eon when life was not supposed to exist. What gives? What GIVES?!
It turns out that, thanks to the relatively consistent violent explosions via those angry mountains that we call volcanoes – spewing their lava, ash, and general mayhem, a fairly reasonable (though not entirely accepted!) explanation neatly presents itself. This is where we need to introduce greenhouse gases like carbon dioxide, methane, water vapor, and nitrous oxide into the conversation. Some of the most important substances expelled into the atmosphere by volcanoes are, in fact, those infamous greenhouse gases we keep hearing about. The thing about greenhouse gases is that in certain amounts, they can be very beneficial. In this case, they appear to have benefited our early earth by keeping it warm enough to host liquid water when the faint young sun wasn’t pulling its weight.
When we take into consideration that faint young sun, the lack of readily available oxygen in the early atmosphere, and the abundance of these greenhouse gases, our ideas about the development of life on Earth are pushed in a new direction. The great Carl Sagan was one of the first scientists to suggest that it was these greenhouse gases which warmed the planet enough to allow water to exist as a liquid. This, in turn, allowed for the earliest life forms to arise much farther back in time than seems plausible, and ultimately develop into the microbes that may have built up our atmosphere into a sort of blanket of habitability through their metabolic processes.
One final component of our microbe picture is that during their metabolism, they can both consume and produce greenhouse gases. For example, in the presence of iron and methane, a microbe can consume methane and transform it into carbon dioxide. The way it does this is by interacting with iron and trace metals through something called redox chemistry. Redox chemistry involves the transfer of electrons between molecules of opposite electrical charges, and can get rather complicated. For a better idea of how redox chemistry works, check out this site from Washington University.
What’s important to take away from this process is that it can occur without oxygen, which essentially excludes the catalogue of complex life (life that is composed of more than one cell, to be exact) from using it as a way to derive energy. A deficiency of oxygen is exactly what characterized the Archaean atmosphere. With liquid water present in abundant amounts due to the warming greenhouse gases being pumped into the atmosphere from volcanic and microbial activity, more and more microbial life could begin to flourish and ultimately begin increasing the oxygen content of the atmosphere. Gaseous oxygen emitted into the atmosphere is another result of microbial metabolism, and it’s in this way that microbes ultimately paved the way for complex life on our planet to develop.
This interaction with and, specifically, consumption of greenhouse gases also means that microbes are promising partners in the future of environmental engineering processes. They can clean up wastewater, help us reduce our currently excessive greenhouse gases, and move us in the direction of a cleaner, greener planet. That is, if we can determine exactly how they work their magic and exactly which microbes are responsible for which chemical processes.
This brings us to the exciting research being conducted in Dr. Jennifer Glass’ lab by her and her students. Stay tuned for more details and check out Dr. Glass’ website and blog for more in-depth information at jenniferglass.com.