Chapter 30 Microbial Ecology

PERSPECTIVE 30.1 Life on Mars?

The world listened with extreme interest in 1996 when NASA scientists reported finding what could be life forms in a meteorite from Mars.The meteorite, called ALH 84001, is a 4-pound potato-shaped rock found in Antarctica in 1984. It is estimated to have formed on the surface of the red planet 4.5 billion years ago and to have been launched into space by an asteroid collision with Mars about 15 million years ago, finally falling on Antarctica 13,000 years before it was found there. Chemical analysis has established the Martian origin of the rock.The evidence for living forms within the rock is not so conclusive.

Four types of evidence were put forward to support the thesis that life once existed within the rock. Carbonate globules, mixtures of calcium and magnesium carbonates, were present along fractures and pores in the rock.These could have been deposited by water flowing along the rock, water being a prerequisite for life. Magnetite and iron sulfide particles were present.These are often formed by bacteria and other living organisms. Polycyclic aromatic hydrocarbons (PAHs) were present.These are compounds left behind when living organisms decay.Very tiny microscopic fossils, resembling bacteria but smaller, were found inside the carbonate globules. It is proposed that these were tiny bacteria or bacteria-like organisms that lived on Mars long ago, died, and left their remains within the rock.

In 1997, at the next large gathering of planetary scientists, those who made the first report were more confident that they were right, but other researchers doubted their conclusions.The latter suggested that the carbonate globules were formed by high-temperature impacts on the surface of Mars, rather than by water. Some thought the magnetite and iron sulfide particles looked like the trail left by gases from volcanic eruptions. Still others felt that the PAHs were contaminants from the Antarctic ice and snow.The supposed bacterial fossils were about 100 times smaller than any known bacteria on earth, bolstering the skeptics.

The opinions and data accumulate. One reason for disbelief has been the extreme conditions under which these purported organisms must have survived.The recent discoveries of extremophiles thriving under previously unbelievable conditions on earth has made the reports of Martian microbes easier to accept or at least to imagine. It was hoped that the explorer sent to Mars in late 1999 would at least settle the question whether there is water on the red planet.That mission, however, was lost, setting the program back a bit.The series of spacecraft being sent to Mars during the coming years may bring back more evidence to enable scientists to answer the question,"Life on Mars?"

and microbial mats are microbial communities. Molecular techniques are enabling researchers to better understand microbial communities.

■ What are the roles of primary producers, consumers, and decomposers?

■ Why do antibiotic-resistant bacteria become common in a hospital environment?

■ How could FISH (fluorescence in situ hybridization) be used to determine the relative proportions of Archaea and Bacteria in a population?

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