Extreme Ecosystems - A Tough Bunch of Microbes!

Microbiologists have found an ecosystem of extreme-loving microbes working together to survive at the bottom of California's strange Mono Lake.

Humans don't like being alone. So when Richard Hoover, a microbiologist at the NASA Marshall Space Flight Center, travels, he looks to see where the locals hang out. Not in hotels, though, or in restaurants or nightclubs. The places he looks are more exotic: Deep mines under the permafrost of Alaska and Siberia, the high mountains of Antarctica, and the salty, alkaline bottom of California's Mono Lake.

Richard Hoover is an extremophile hunter. He searches the most inhospitable places for lifeforms that love extremes: scalding heat, freezing cold, salt, lye, darkness. And like other researchers exploring the limits of life on our planet, he's found a surprising variety of species ranging from simple bacteria to plants and animals.

He also finds that species of extremophiles depend on each other to make a living-much like ordinary lifeforms do.

"The diversity of life on Earth boggles the mind," marvels Hoover. There are hundreds of thousands of green plant species. Green algae alone comes in thousands of different varieties, he says. There are millions of species of animals. More than 6,000 species of bacteria and 3,600 viruses have been named. Researchers suspect there are more than a million species of fungus, although only 70,000 or so have been identified.

Few (and perhaps none) of these species live in splendid isolation. They depend on others. Plants use sunlight, carbon dioxide, and minerals to create organic compounds (sugars, proteins, fats, etc.). Animals take these compounds for their own needs. When animals die, they return to minerals and carbon dioxide and the cycle renews.

Cooperation between species is common. For example, the tropical African Gray Parrot eats fruit from trees. For reasons no one understands, these birds sling bits of fruit containing seeds far from the tree. This helps the trees spread their seeds and reproduce. Many instances are well documented of other animals helping spread plants through their droppings. At the complex plant/animal level in a biologically rich environment, interdependence of species seems to be conducive to life.

A false colour photomicrograph of the new extreme-loving microbe, Desulfonatronum thiodismutans, recently discovered in Mono Lake by Richard Hoover and colleagues. Note the flagellum (lower left), which it uses for movement.

But what about simpler lifeforms found in extreme environments? Do they too exhibit such interdependent life styles? NASA is interested because the agency is tasked to explore for life in the Universe. Many scientists expect the first signs of life confirmed off Earth - on Mars, within comets, or in the suspected oceans of Europa - will be unicellular lifeforms such as bacteria, archaea, or diatoms rather than complex technological species. Understanding how these species live in extreme conditions is vital to NASA's mission.

Hoover and microbiologist Elena Pikuta of the University of Alabama in Huntsville are working to answer some of these questions by studying lifeforms in California's Mono Lake. They recently announced the discovery of a third new species of bacteria, Desulfonatronum thiodismutans, living in the lake in the International Journal of Systematic and Evolutionary Microbiology. All three of Pikuta and Hoover's new species are extremophiles. The bacteria thrive in the dark mud of Mono Lake, devoid of oxygen with 3 times higher salinity than sea water and alkalinity that approaches lye.

This third new species is particularly interesting because of its niche in the extreme ecology of the lake. This bacterium obtains its energy from sulfur and other inorganic compounds. It does not require sunlight or other organic materials to thrive and is a type of organism known as a chemolithotroph. Hoover and Pikuta's two previous new species, Tindallia californiensis and Spirochaeta americana are also extremophiles from Mono Lake, but ingest organic materials. These organisms are known as organotrophs. Together they paint a picture of interlinked and interdependent life, even under extreme conditions.

Richard Hoover collects samples from the mud of Mono Lake.

For example, D. thiodismutans gets its energy from hydrogen and sulfur compounds in the minerals of the lake mud. From these it creates sugars and other organic materials. T. californiensis can consume simple amino acids and other chemicals and also produces complex organic compounds such as sugars, fats, proteins, etc. S. americana ingests the complex organic compounds and excretes hydrogen and other gases. When it dies, it returns to minerals and the cycle is complete.

Unlike the plant/animal cycle in our "normal" environment, this bacterial cycle does not necessarily need visible energy from sunlight to drive photosynthesis. It can be driven completely by the chemical energy of the reactions. So in a dark, extreme environment, life appears to develop the same interdependent strands, with different species finding the niche that allows each to thrive.

One day, perhaps, lifeforms like these will be found on other worlds. The work of Hoover and Pikuta is telling us that if we find one species, we should look for more. Extremophiles, like "ordinary" lifeforms, don't like being alone.