William C. Ghiorse
The field of geomicrobiology is growing at a healthy rate. Judging from
the increased number of news articles and reports in Science and
during the past year, geomicrobiology-related research is receiving more
attention now than was the case a few years ago. The number of research
papers submitted to other peer-reviewed publications like GeomicrobiologyJournal
and the Geomicrobiology Section of Applied and Environmental Microbiology
is also on the upswing. Research funding programs such as Life in Extreme
Environments (LEXEN) at the National Science Foundation and Astrobiology
at NASA have brought new scientists to the field and have stimulated more
interest in geomicrobiological research. In addition, various subsurface
and groundwater research efforts at the Department of Energy and Environmental
Protection Agency continue to provide support for geomicrobiological projects.
A high level of popular interest surrounding the search for microbial life
on Mars and a heightened awareness of the benefits of working together
among geoscientists and biologists bode well for continued growth. In this
article, I have selected a few examples from the past year that highlight
research and other events of
particular significance to the future of this growing field.
Magnetite in ALH84001
Reports related to past or present life on Mars provide good lessons for students of geomicrobiology. One such report last year showed that 27 percent of magnetite crystals found in the martian meteorite, ALH84001, were the same size, shape and chemical purity as those formed by a marine magnetotactic bacterium, strain MV-1 (Summary by R. Kerr, Science, v. 290, p. 2242, of a report by K. Thomas-Keprta and others, Geochimica et Cosmochimica Acta, December 2000).
Several experts interviewed for the Science article agreed that the magnetite crystals in question were indistinguishable from those produced by MV-1. This provides evidence for a similar mechanism of formation. It supports the idea that magnetite crystals can be used as a biomarker. It also keeps alive a piece of the original theory that ALH84001 contained remnants of microbial life that once lived on ancient Mars. But most researchers were unwilling to conclude that magnetotactic bacteria once existed on Mars.
Archaea in acid mine drainage
Many Archaea stretch the limits of life. This was illustrated last year by a report on Archaea in the extremely acidic mine drainage waters at Iron Mountain, California. Negative pH values as low as -3.6 have been measured in these waters, putting them among the most acidic environments on Earth (Nordstrom and others, Environmental Science and Technology, v. 34, 2000, p. 254-258). Toxic metal concentrations are very high (0.1-10 grams per liter). An acidophilic, iron-oxidizing archaeon, isolated from biofilms in the drainage, was capable of growing at extreme acidity in culture media at pH 0 and at moderately high temperatures of 45 Celsius (Edwards and others, Science, v. 287, 2000, p. 1796-1799). The closest relative to the isolate, as determined by sequencing the 16S ribosomal RNA gene and comparing the sequence with sequences in the 16S rRNA database, was Ferroplasma acidophilus. These iron-oxidizing Archaea belong to a group of mostly chemolithoautotrophic, thermophilic acidophiles — the Thermo-plasmatales — all of which possess a specialized cell envelope consisting of a single peripheral plasma membrane that presumably allows them to cope with the low pH and high-metal content of their environment. Phylogenetic probes were used to determine that the F. acidophilum-like isolate represented up to 85 percent of the microbial community of the biofilms and sediments of the mine drainage, indicating that it was a very important contributor to acidification and to the element cycling in the mine drainage. The high proportion of this organism in the biofilms and sediments also emphasizes a common property of microbial communities in all extreme habitats. The number of species will be limited to a few that can tolerate the extreme conditions of that habitat.
250 million-year-old bacteria in salt
Longevity is an important life-limiting property of living things, especially for the plants and animals we know so well. Nature constantly reminds us that all life has a beginning and an end. Or does it? This question was raised again last year by a report of the isolation of a spore-forming Bacillus species from 250-million-year-old primary salt crystals (Vreeland and others, Nature, v. 407, 2000, p. 897-900). The immediate concern was whether or not the Bacillus species that grew in cultures was derived from propagules, presumably dormant endospores, which actually survived for 250 million years in the brine inclusions. Extreme care with sterilization and biological containment assured the authenticity of the isolates (Rosenzweig and others, Geomicrobiology Journal, v. 17, 2000, p.185-194). The methods for sampling and preventing contamination by ambient bacteria were exemplary. This is a compelling story of bacterial longevity, but 250 million years is an extremely long time to endure, even for a Bacillus endospore! Obviously, this work is highly relevant to issues of microbial longevity, a topic that geomicrobiologists should pursue with vigor in the future.
Genomics, geosciences and geobiology
Correlations between the ever-growing genomics database and the geological record pose some significant challenges. In an article on genomics and the geosciences, Banfield and Marshall (Science, v. 287, 2000, p. 605-606) summarized some of the key issues. Genomics allows understanding of how complex organisms are related, how they operate in natural settings, and how they have evolved to meet new environmental challenges.
Geosciences can provide the temporal and environmental context for understanding when and how key biogeochemical thresholds were crossed. The two disciplines are still far apart, both in vocabulary and tradition, but there is new hope that they can come together to accelerate our understanding of current biogeochemical processes and the longer-term coevolution of the biosphere and the geosphere.
These issues were among the topics discussed at an American Academy of Microbiology (AAM) Colloquium (www.asmusa.org), convened Dec. 1 to 3, 2000, in Tucson, Ariz. The result will be an AAM report, to be published in 2001, defining a new field of geobiology, which is positioned at the intersections of the biological sciences, geosciences and a range of other applied and environmental disciplines. This is a very exciting prospect for geomicrobiologists, who are likely to make up the scientific core of the new field of geobiology!
Ghiorse is a professor of microbiology at Cornell University. He is
Co-Editor-in-Chief of Geomicrobiology Journal). E-mail: email@example.com