Extreme Life Adaptation
Life in extreme environments — hot acids and heavy metals exposure are particularly nasty — can apparently make very similar organisms deal with stress in very different ways, according to new research from North Carolina State University. One single-celled organism from a hot spring near Mount Vesuvius in Italy fights uranium toxicity directly — by eating the heavy metal and acquiring energy from it. Another single-celled organism that lives on a smoldering heap near an abandoned uranium mine in Germany overcomes uranium toxicity indirectly — essentially shutting down its cellular processes to induce a type of cellular coma when toxic levels of uranium are too high in its environment. Interestingly, these very different responses to environmental stress come from two organisms that are 99.99 percent genetically identical.
In a paper published this week online in Proceedings of the National Academy of Sciences, NC State researchers show that these extreme organisms — basic life forms called Archaea that have no nucleus and that are so tiny they can only be seen under a microscope — can teach us a lot about how living things use different mechanisms to adapt to their surroundings.
Archaea were first classified as a separate group of prokaryotes in 1977 based on the sequences of ribosomal RNA (rRNA) genes. These two groups were originally named the Archaebacteria and Eubacteria and treated as kingdoms or subkingdoms. It was argued that this group of prokaryotes is a fundamentally different sort of life. To emphasize this difference, these two domains were later renamed Archaea and Bacteria
The researchers, led by Dr. Robert Kelly, Alcoa Professor of Chemical and Biomolecular Engineering at NC State, exposed two very close relatives of thermoacidophilic Archaea — they live in highly acidic environments with temperatures of more than 70 degrees Celsius, or about 160 degrees Fahrenheit — to pure uranium. One, Metallosphaera sedula, metabolized the uranium as a way to support its energy needs.
That in itself was surprising to Kelly and his fellow researchers, as it was the first report that an organism can directly use uranium as an energy source.
Archaea exhibit a great variety of chemical reactions in their metabolism and use many sources of energy. These reactions are classified into nutritional groups, depending on energy and carbon sources. Some archaea obtain energy from inorganic compounds such as sulfur or ammonia (they are lithotrophs). Now we have one using Uranium for energy.
"This could be a new way to mine uranium using microorganisms to release the metal from ores — a process referred to as bioleaching," Kelly says of M. sedula.
Its genetic twin, Metallosphaera prunae, reacted very differently. When faced with pure uranium, it went into a dormant state, shutting down critical cellular processes that enable it to grow. When the toxic threat was removed, M. prunae rebooted its cellular processes and returned to its normal state.
Kelly says the findings could also have implications for understanding how antibiotic resistance develops and operates in pathogens.
"We have come across a new model for how organisms learn how to live in an environment that would otherwise be deadly for them," he says.
Kelly adds that the study calls into question the ways that scientists classified living things before the rise of the genomic era.
"How do we classify microorganisms now that we can compare genomes so easily?" Kelly asks. "These are not different species by the classical definition because their genomes are virtually identical, but they have very different phenotypes, or lifestyles, when faced with stress."
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