Understanding microbial communities can give clues to how life shaped the Earth billions of years ago - and help find signs of life on distant planets.
Life has shaped the chemical and geological nature of Earth. What living things eat and excrete impacts the composition of atmosphere, rocks, and oceans.
Life began soon after the formation of the planet, initiating over three billion years of intertwined evolution between living systems and the Earth. Without life's influence, Earth would be a different planet.
Astrobiologists want to understand this co-evolution that created today's Earth, both to understand how life evolved and to discover ways to find life on other planets. To do this, they study Earth's oldest and most successful inhabitants: the microbes.
Microbes cluster in communities called microbial mats. These are cities of cells, living in symbiosis. Microbial mats can be found anywhere. They inhabit dry places, salty places, hot places and cold places. They can be found in the cracks and puddles uninhabitable by multicellular organisms. They are familiar to many of us as the film on our teeth. Their history traces back at least 3.4 billion years - more than 10 million times the amount of time humans have existed and 3/4 of the time since the Earth's formation.
Microbial mats may look like a brownish or greenish scum layer, but they are complex ecosystems that contain a great diversity of living things. A cooperative network of give and take makes the communities sustainable. One microbe will use another's waste to produce the food of another.
NAI researcher Kelly Decker of the California State University - Monterey studies the microbes in the salt evaporation ponds of a salt company in Baja California . Here, the microbes grow in dense layers. When sliced through, the mat shows differently hued stripes descending for several inches. The microbes settle to their ecosystem's niche: The ones that produce energy from light are at the top - closest to the sun, while those that don't like oxygen are nearer to the bottom. Energy in the form of chemicals is passed between the layers, as are biologically important elements such as iron and sulfur.
Ecosystem simulations: From micro to global
Decker and her collaborators are developing a computer program that simulates microbial mat ecosystems. Their program MBGC (Microbial BioGeoChemistry) models the growth of four major groups of microbial mat inhabitants and takes into account environmental influences to calculate the movement and cycling of oxygen, sulfide, and carbon.
This is not the first simulation of microbial mat behavior, but it is a significant advance in the field. It is the first to model mats in equilibrium, creating predictions that the authors can compare to a real mat.
In an article published in FEMS, Decker and colleagues compare the results of their simulations to the behavior of microbial mat communities in Mexico . The mats, located in Guerrero Negro, Baja California Sur, are submerged under a few feet of very salty water (70-98% salinity). The mats are about 2 inches deep and composed of thin layers of microbe communities, like compact layers of filo dough. The greenish tops of the mat are dominated by cyanobacteria, which capture sunlight as energy. Other members of the complex ecosystem use the chemicals produced by the cyanobacteria to make chemicals that the cyanobacteria and other bacteria use. The diverse community includes bacteria that make or use hydrogen sulfide, make methane, and ferment sugar.
The microbial mats have a separate day-life and night-life. During the day, the cyanobacteria photosynthesize, supersaturating the top layers with oxygen. At night, the oxygen is absorbed for respiration, depleting the mat of oxygen. Sulfide shows the opposite pattern. The top layers of the mat are low in sulfide during the day as the cyanobacteria use it for photosynthetic processes. At night the sulfide levels climb through the work of the sulfur-reducing bacteria, living in layers under the cyanobacteria.
Colorless sulfur bacteria and purple sulfur bacteria also use the sulfide made by the sulfur-reducing bacteria. The purple layers of purple sulfur bacteria reside underneath green layers of cyanobacteria and the colorless sulfur bacteria, because they use a wavelength of light that penetrates deeper into the mat than the light used by their neighbors.
The MBGC program models the behavior of all four of these major groups of bacteria and follows the flow of oxygen, sulfur, and carbon through the layers. To test the model's predictions, Decker and her colleagues visited the Mexican mats to make measurements of key properties, including oxygen and sulfide gradients and depth of light penetration. They sampled at various times in a day to compare to the daily fluctuations predicted by their model.
The researchers generally found good agreement between their model and the mat on small distance and time scales. They will now start scaling up the model. Ultimately, the size scale will be the entire Earth. The time scale will be the period of 2.5 billion years ago to a few million years ago, recreating the history of how life affected the chemical composition of the Earth.
This macro-model will track the release of gases like oxygen, carbon dioxide, methane, and hydrogen sulfide into the atmosphere. The model will be adapted to determine what atmospheric conditions on other planets may signal that life is present. It's then up to other researchers to study the atmosphere on other worlds, in hope of finding these biosignatures.
Life at a distance
Nearly 150 planets have been detected beyond our solar system, and the rate of discovery is accelerating. Any evidence for life on these planets must be found from a distance, since it would take too long to travel there to collect samples. In fact, it would take over 400,000 years to reach just the closest star at Space Shuttle speed.
NAI investigator Victoria Meadows of CalTech doesn't want to wait this long. She and her colleagues at NAI's Virtual Planetary Laboratory (VPL) are finding ways to detect life on distant planets. They are determining which signs are most likely to signify planetary life and how to measure them.
The most accessible feature for studying far away planets is their atmospheres. The gases that surround a planet can be identified by the spectrum of light they give off. For example, oxygen gives off different wavelengths of light than methane. Scientists hook up a spectrometer to a telescope to read which wavelengths of light the atmosphere of a planet is giving off. This experiment is challenging because the light from the star greatly outshines light given by the planet.
Before atmospheric readings can be interpreted, scientists need to figure out which gases are good indicators of life. To determine this, VPL researchers begin with Earth - our only known example of a living planet. They ask: Which atmospheric gases would have tipped off an alien population that Earth has life? And: How will we recognize life-environments when we see them?
One potential sign of life, or biosignature, is oxygen. Because oxygen is highly reactive, there would be little oxygen in Earth's atmosphere if plants and bacteria weren't constantly making it. The presence of large amounts of oxygen, especially coexisting with water vapor and carbon dioxide, would be good evidence for photosynthetic or other life on a planet.
Lack of oxygen doesn't necessarily mean lack of life. Life existed on Earth for at least a billion years before its atmosphere acquired oxygen. There are other potential biosignatures. Nitrous oxide is only produced by life, but would be hard to detect if it's as rare in other planets' atmospheres as it is in Earth's. Methane is another possibility, but its presence is not necessarily due to life. A mixture of gases that aren't usually found together is evidence of some active processes - perhaps life.
The methods being developed to model the chemistry of life and measure the atmospheric components on distant planets will be employed on future NASA missions, including the Terestrial Planet Finder scheduled to launch after 2014. With the potential for so many possible planets, this will just be the beginning.