Bacteria that photosynthesise using compounds of arsenic, rather than water, have been discovered in the warm, bubbling pools of Mono Lake in California.
Background:Modern plants, from the biggest trees down to microscopic algae floating in the ocean, use sunlight, water and carbon dioxide to fuel photosynthesis, and they in turn supply the oxygen that nearly every animal on the planet breathes.
Some bacteria use arsenate - arsenic with four oxygen atoms attached - as an energy source. It was thought that this form of metabolism didn't get going until long after photosynthesis filled the atmosphere with oxygen about 2.7 billion years ago. When this happened, naturally occurring arsenite would be transformed into arsenate.
The new study:U.S. Geological Survey scientists based in California found a new microbe that lives in hot springs - which are alkaline, salty, oxygen-depleted, and contain high concentrations of both arsenites (AsO
33-) and arsenates (AsO
43-). The microbe uses arsenite, a substance that is toxic to most life forms, instead of oxygen for photosynthesis.
The new organisms were found in Mono Lake in California, a 45,000-acre lake near Yosemite National Park in eastern California. The lake was formed a million years ago by subsidence in the Sierra Nevada Mountains and geothermal heat helps arsenic and other metals leach into ground water that flows into the lake.
Photo to the left: Mono Lake in California, USA NASA Landsat 7 image."These lakes are fed by hydrothermal waters that leach out arsenic-containing minerals from the surrounding rocks and the arsenic stays in solution and doesn't precipitate," said senior Geological Survey scientist
Ron Oremland. "And the lake ... doesn't overflow or go anywhere else, except evaporate in the desert. So there's a heck of a lot of arsenic in this lake."
The researchers noticed that the bacteria had colonised small, hot pools, forming colourful "biofilms". Oremlands team isolated and bred these bacteria in the lab. By growing them with arsenite as the only possible food source, the researchers showed that the bacteria can indeed thrive.
"It was quite tricky to culture the critters, because if you give them too much arsenic, its still toxic even to them. But once we had figured out the right dose and gave it to them at regular intervals, they flourished on the stuff", said Oremland. The bacteria were extracting electrons from arsenite by photosynthetic oxidation, in order to help convert CO
2 to biomass.
Duquesne-based scientist
John F. Stolz figured out that the bacteria were able to do this because they
contain certain enzymes, or proteins, that act like a key, allowing the
chemical reaction to occur.
Bacteria that generate energy by metabolising (reducing) arsenate are already known. But Ronald Oremland and colleagues at the US Geological Survey in Menlo Park, California, were puzzled by the great range of arsenic-eating bacteria. If they evolved recently they must have passed the ability to metabolise arsenic to each other by lateral gene transfer, he says.
Alternatively, arsenic metabolism could have evolved much earlier, giving plenty of time for bacteria to diversify. The newly discovered bacteria from oxygen-free hot springs in Mono Lake, California, support this interpretation. It's likely that the newly-discovered arsenite photosynthesis, which produces arsenates, opened up niches for these arsenate reducing microbes, the researchers suggest.
Other bacteria have been found that photosynthesise using sulfur and iron compounds - and these, together with the arsenite microbes, probably evolved before conventional oxygen-producing photosynthesis arose around 2.7 billion years ago.
Because the ingredients required by the bacteria - arsenic and sunlight - could exist in oxygen- and water-starved corners of our solar system, NASA funded the research with the hope of uncovering clues to life beyond our planet.
"When looking for life elsewhere in the solar system, NASA needs to know the diversity of life on Earth," said NASA astrobiologist Michael New. "So we fund research especially into organisms that live under conditions that we normally think of as quite extreme. "There aren't that many places in the solar system that we would consider comfortable," he said. "There's a lot more places that we would consider extreme."
The original study
T.R. Kulp, S.E. Hoeft, M. Asao, M.T. Madigan, J.T. Hollibaugh, J.C. Fisher, J.F. Stolz, C.W. Culbertson, L.G. Miller, R.S. Oremland,
Arsenic(III) Fuels Anoxygenic Photosynthesis in Hot Spring Biofilms from Mono Lake, California, Science, 321 (2008) 967-970.
DOI: 10.1126/science.1160799
Related studies Joanne M. Santini, Lindsay I. Sly, Aimin Wen, Dean Comrie, Pascal De Wulf-Durand, Joan M. Macy,
New Arsenite-Oxidizing Bacteria Isolated from Australian Gold Mining Environments--Phylogenetic Relationships, Geomicrobiol. J., 19/1 (2002) 67-76.
DOI: 10.1080/014904502317246174 T.M. Salmassi, K. Venkateswaren, M. Satomi, K.H. Nealson, D.K. Newman, J.G. Hering,
Oxidation of Arsenite by Agrobacterium albertimagni, AOL15, sp. nov., Isolated from Hot Creek, California, Geomicrobiol. J., 19/1 (2002) 53-66.
DOI: 10.1080/014904502317246165 S.E. Hoeft, F. Lucas, J.T. Hollibaugh, R.S. Oremland,
Characterization of microbial arsenate reduction in the anoxic bottom waters of Mono Lake, California, Geomicrobiol. J., 19/1 (2002) 23-40.
DOI: 10.1080/014904502317246147 Rita Mukhopadhyay, Barry P. Rosen, Le T. Phung, Simon Silver,
Microbial arsenic: from geocycles to genes and enzymes, FEMS Microbiol. Rev., 26/3 (2002) 311-325.
doi:10.1016/S0168-6445(02)00112-2 A.W. Turner, J.W. Legge,
Bacterial oxidation of arsenite, Aust. J. Biol. Sci., 7 (1954) 452-514.
Related Resources Mono Lake Chemistry NASA Planetary Biology Internship Program (PBI) USGS Arsenic studies group
Related EVISA News (newest first) December 4, 2010: Arsenic, an element of archaic life ? October 14, 2010: Scientists solve mystery of the two sides of arsenite March 17, 2009: Researchers Find Alga that Influence Arsenic Cycling in Hot Springs June 18, 2006: Bacteria supposed to remove poisonous arsenic from drinking water June 16, 2005: Arsenic concentration in groundwater may be affected by bacteria last time modified: June 26, 2020
