Stars of the Deep

The star-shaped bacterium was isolated from mine-slime, 1.7 km below the surface. The ruler shown for scale is in centimetres.
Credit: Wanger et al., 2008

Researchers studying life in the deep subsurface of our planet have discovered a unique bacterium living 1.7 km below the Earth’s surface. The tiny bacteria live in a community of subsurface microbes inhabiting a South African platinum mine.

The deep subsurface of Earth harbors many unique microbes that are only accessible through large scale drilling projects or mining. By trekking into the ultra-deep mines of South Africa, researchers are getting a rare glimpse into this unique habitat. In the depths of South Africa’s Northam Platinum mine, scientists from the University of Western Ontario and Princeton University have gained access to many previously undiscovered microbial communities.

While mining and drilling allow scientists to sample the unique environment below the Earth’s soil, these activities obviously disturb the subsurface of the planet. Digging into the ground disrupts the microbial communities that live there. When people enter mines and caves, they bring with them a massive number of non-native microbes. Because of this, it’s difficult to get uncontaminated samples.

The team from the recent study decided to test samples from mines in order to determine just how contaminated they really are. They collected samples from slime, or biofilm, growing on the walls of the Northam mine in South Africa. An explosion of life occurs where subsurface water leaks out of the mine walls and meets with oxygen, leading to films of microscopic organisms.

Previously, researchers overlooked these biofilms because they thought the films would be too heavily contaminated. To test this theory, the team determined whether or not their biofilms were formed by contaminant organisms from the surface, or by unique subsurface organisms.

The study, by Greg Wanger, Tullis Onstott and Gordon Southam, was published in a recent edition of the journal Geobiology. The authors showed that the biofilms contained a number of unique organisms associated with the deep subsurface, and therefore such films might be an excellent place to search for new and unusual species of microbes. In fact, in their study the team came across one particularly strange microbe shaped like a tiny, microscopic star.

Shaping Up Bacteria

The cell membrane of the bacterium twists and turns to provide its unique shape.
Credit: Wanger et al., 2008

Microbes come in a number of shapes and sizes, but most of these shapes are rather uncomplicated. The easiest shape for a microbe to make is a sphere. Like a soap bubble, the cell membranes of microbes tend to naturally form this simple structure due to forces such as surface tension. According to the research team, "the diversity of all bacterial shapes is more difficult to explain". Other shapes often seen in microbes include rods and spirals, but these take a bit of extra work on the part of the microbe. To make more complicated shapes, microbes have to use extra energy to fight against the natural forces that favor the sphere. According to the research team, the biofilms from Northam mine "contained a morphologically diverse assortment of bacteria."

Some rare microbes go beyond the common and form radically unique shapes. The microbe discovered in the depths of the Northam mine is one such microbe. Using high-powered microscopes, the team captured images that show star-shaped cells with four to nine points. It’s a unique structure for a microbe and one that has not been witnessed before.

So why would a microbe want to take the shape of a star? As living organisms, every microbe needs food. When we need food, we can simply pick it up and put it in our mouths. That’s not the case for most microbes. Many microbes simply float about in their environment in the hope that they’ll be able to absorb the nutrients they need to survive.

Many microbes ‘eat’ by letting nutrients diffuse through their cell membrane. A sphere may be easy to form, but it doesn’t provide the largest surface area for a cell. By forming a more complicated shape, with a cell wall that folds and bends, the surface area of the cell is increased in relation to its interior volume. This means there’s more cell wall through with the microbe can absorb its food.

The new microbe discovered by the researchers in South Africa has likely developed its unique shape in response to its unique environment. The deep subsurface of the planet is thought to be quite ‘nutrient poor’ — there’s not a lot of food to go around. Microbes need to develop clever strategies to out-compete their neighbors. The surface-area-to-volume ratio for the star-shaped cells is thought to be as much as ten times better than common bacteria like e. coli. This advantage may help the stars survive amidst a neighborhood of microbes competing for the same food.

Deep Below the Planets

About two miles below the ground in a South African gold mine, researchers discovered an isolated, self-sustaining, bacterial community living under extreme conditions. In this photo, Duane Moser stands next to the fracture zone (white area) where the bacteria were found.
Credit: Li-Hung Lin

Scientists are just beginning to understand the unique types of life that live beneath the surface of our planet. Astrobiologists are particularly interested in the subsurface because it can help them understand how microbes might survive deep beneath the topsoil of other planets.

Upcoming and current missions to search for signs of past or present life on Mars are focusing on life beneath the martian soil. Right now, NASA’s Phoenix lander is using a scoop to dig on Mars. Recent images returned from Phoenix are already revealing clues about subsurface ice on the Red Planet.

The European Space Agency’s ExoMars rover may take the exploration of Mars’ subsurface one step further. Current plans are to place a drill on ExoMars that could allow the rover to dig up to 12 feet.

NASA has also been developing prototype drills for use by human explorers on Mars. Drilling technologies have already been tested by NASA researchers in extreme environments on Earth, including the Canadian high arctic.

Microbes use many methods to survive in the nutrient-poor, oxygen-free, pitch-black world deep beneath our feet. Studying these microbes might provide clues about how organisms could live in harsh environments on other planets like Mars. Because of this, unique microbes like the ‘stars’ of Northam mine may shed a bit of light on the future of planetary exploration.

Related Web Sites

Golden Bacteria
Acid-Washed Pink Genes