From the initial sparks of human agricultural activity 12,000 years ago, to the flames of the industrial revolution in the late 19th century, and now the present conflagration of our globalized society, human activities have fundamentally altered the Earth’s soils, oceans, and atmosphere. By June 1st 2009, our global population will exceed 6.8 billion. As our population increases, the demand for energy and clean water grows in stride. By removing buried fossil fuels to burn for energy, and releasing by-product carbon dioxide and other greenhouse gases into the atmosphere, we stress the Earth’s climate system in a manner it has not seen for millions of years. However, we are not the first organisms to tinker with Earth’s climate system. Microorganisms have been inching forward various forms of environmental change over their 3.5+ billion years on this planet. In so doing, they have evolved a vast array of survival strategies – by understanding how microorganisms have dealt with environmental shifts through Earth history, we may come away with a few strategies for tackling the challenges of human induced climate change.
What Are Microbes?
You may recall the Five Kingdoms of Life from Biology class, way back when. With the advent of DNA sequencing in the late 1980’s, a quiet revolution in the classification of all life on Earth ended with the reorganization of those Kingdoms into Three Domains: Bacteria, Archaea, and Eukaryotes. Archaea and Bacteria are commonly referred to as Prokaryotes – that is, single-celled microorganisms containing no true nucleus (a small compartment in which the DNA is contained). In contrast, Eukaryotes house their DNA in a nucleus and have more complex organelles. Eukaryotes include everything from single celled yeast to multi-cellular plants and humans. The term “microbe” refers to any single cell life form, either Eukaryotic or Prokaryotic. Environmental Microbiologists generally focus on the Bacteria and Archaea, though the importance of Fungi (single-celled Eukaryotes) in soils and some extreme environments is an increasing area of research.
Microbes are rather tiny, as the name implies. Usually around one micrometer (one millionth of meter) in length, height, or width, and yet, they represent a large proportion of the life on Earth: estimates hold the number of individual bacterial cells on Earth around 10^30! There are likely more microbes in a kilogram of soil than there are stars in the universe. The vast majority of microbial biomass is buried in mud below the ocean floor in thick sediments, many kilometers from the sun-lit surface. Microbes have evolved a vast array of survival strategies that allow them to inhabit the most extreme environments in Earth’s biosphere: from below the freezing point of water in Antarctic ice to above boiling in deep-sea volcanoes and hotsprings. Certain microbes will survive blasts of radiation that would instantly kill a human and even the sturdiest fungal spore, while others thrive in roiling springs of sulfuric acid. Microbes are fantastically adapted to their environment, and their short life-cycle makes them able to evolve quickly, even when environmental change occurs rapidly.
Given the large number of microbes on Earth and the diversity of environments in which they live, it is not surprising that humans and other organisms have evolved to be dependent upon them. Many of the vitamins our bodies require to repair and make new tissues we do not make ourselves; rather, they are made only by microbes living in our gut or attached to the roots of plants we eat. Of the antibiotics doctors regularly prescribe, more than 70% were identified initially as natural products from a group of common soil bacteria, the Actinomycetes. Microbes outnumber us even in our own bodies, where roughly ten bacterial cells exist for every one human cell, though the suite of chemical reactions these microbes mediate remains poorly understood. Moreover, the vast majority of bacteria are not harmful to humans, and the chemical reactions they perform in their environment, be it in farm soil or a human colon, are required for the ecosystem to function.
A Slow Poisoning of Evolutionary Consequence
How can something so small effect sweeping environmental change? The answer is that it happens very slowly, and by the concerted efforts of many, many microbes. Prokaryotes have thrived on Earth for at least 3.5 of the planet’s 4.4 billion year history. Sometime before 2.5 billion years ago the ancestors of modern ocean dwelling Cyanobacteria (blue-green algae) evolved photosynthesis (the ability to make energy from sunlight) and began pumping massive quantities of their waste product, oxygen, into Earth’s atmosphere. At this time few if any life forms could use oxygen to live, and were actually poisoned by it-oxygen by itself is a highly reactive molecule. As more and more oxygen was produced by these solar-powered microbes, the oxygen reacted with then abundant iron and later hydrogen sulfide (the “rotten egg” smelling gas) until the iron rained to the bottom of the oceans as rust and the sulfide was converted to sulfate (a component of Epsom salt and abundant ion in seawater today, along with calcium, carbonate, magnesium, sodium, and chloride ions). Eventually, over millions of years, the oxygen accumulated in the atmosphere. Once surface oceans were aerated (initially around 2.5 billion years ago), other microbes evolved to ‘breathe’ that oxygen, and consume the sugars made by the sun-harvesting oxygen-making cyanobacteria. These other microbes had evolved a means of respiration that all the complex non-photosynthetic life forms employ today, from humans and ants, to slime molds, to a wide diversity of microorganisms dubbed ‘aerobes’ (organisms that live only in the presence of oxygen). In contrast to aerobes, ‘anaerobes’ do not require oxygen, and are poisoned by its presence. Instead they use alternative metabolic strategies, which we will explore below. Evidence for the ‘Great Oxidation Event’ has been preserved in sedimentary rocks that were deposited during this period of time, and modern geologists and geochemists are able to interpret these microbial ‘fingerprints’ and build models predicting what occurred many millions of years ago. These models are critical to our understanding of the modern climate and how the Earth’s climate system will behave if we continue to perturb it.
Nature’s ‘Master Chemists’
As we have seen, microbes live in some of the most extreme environments on Earth. How are they able to do this? Microbes are not limited to eating sugar as their energy source, nor to burning oxygen to release the chemical energy stored in such sugars-this is what we humans, and pretty much all Eukaryotes do, with the exception of photosynthetic plants and fermentative yeast (the producers of alcohol in beer). The array of microbial metabolisms that we know to exist in nature is simply stunning (the term “metabolism” here refers to a means of extracting energy from one chemical and spitting out another as waste). Some of this suite includes the cyanobacteria, as noted above, whose ability to harness sunlight to make carbohydrates (simple sugars) from carbon dioxide and water drives a large part of the global carbon cycle. Other microbes ‘breathe’ minerals, and their activities in soils and fresh waters play a key part in the cleanup of radioactive and heavy metal contaminates (bioremediation). In marine mud where all oxygen has been consumed, as is the case in much of the sea floor, highly abundant sulfate breathing bacteria produce the noxious “rotten egg” gas hydrogen sulfide and carbon dioxide, whilst munching on the waste products from dead marine bacteria, algae, and animals. Perhaps the strangest microbes of all are those at the base of the energy food chain, eeking out a living by breathing carbon dioxide and hydrogen gas to generate methane, known as ‘methanogens’. Indeed, if a chemical reaction will yield enough energy for a microbe to live on, then there is probably one-or a few trillion-somewhere, surviving off of that reaction.
As human demand for energy increases we will need to turn to alternative energy sources. One exciting option is to tap into these master chemists of the natural world; to study the chemical processes they have spent millions of years perfecting through evolution. As a prime example, some bacteria produce hydrogen gas as a byproduct from the breakdown of plant cellulose. This may assist us in developing clean energy technologies.
What Can We Learn From the Microbial World?
Microbes have played a significant role in the shaping of Earths modern biosphere by catalyzing different chemical transformations over long time scales-billions of years. Metabolic strategies have evolved with time in response to environment cues, in some cases in direct response to a microbial impact on the environment, as we saw from the ‘Great Oxidation Event’. Similarly, our own activities are stressing the lands, oceans, and atmosphere-the impacts of which we are only beginning to understand. In stark contrast to our Prokaryotic counterparts, we are imposing these stresses not on evolutionary time scales of millions of years, but over mere tens of years. Unfortunately Nature has not had time to adapt. If we as a species are to weather the ensuing centuries of rapid population growth and are to meet heightened demands for clean water and clean energy, all while preserving a habitable biosphere, we might turn our attention to the microbes. They have proven the ability to survive for billions of years before us, and will continue to thrive long after humankind has gone the way of the dodo.
–William D. Leavitt, Harvard University
For More Information:
The World Population Clock:
< http://www.census.gov/ipc/www/popclockworld.html >
Ancient microbes discovered alive beneath Antarctic glacier, from CNN.:
< http://www.cnn.com/2009/TECH/science/04/16/microbes.antarctic.discovery/ >
The Wikipedia entry on Biohydrogen:
< http://en.wikipedia.org/wiki/Biohydrogen >
Primary Literature:
Tiedje, J. and Donohue, T. Microbes in the Energy Grid. Science, 320: 985 (2008)
Ash, C., Foley, J., and Pennisi, E. Lost in Microbial Space. Science, 320: 1027 (2008)