4. Lipid Biomarker Profiling in Modern Stromatolites
In addition to morphological and molecular investigations into modern stromatolites, recent studies examining lipid profiles in these ecosystems have revealed both complementary and novel information. Lipid biomarkers, specifically fatty acids (FAME), hydrocarbons, ether-bound hydrocarbons, wax esters, sterols, and hopanoids, have been investigated in Shark Bay microbial mats and stromatolites (Allen, 2006). Input from cyanobacterial oxygenic photosynthesis was detected in each sample by FAME, hydrocarbon and hopanoid markers, while anoxygenic phototrophs were detected by FAME and wax ester markers. Lipids indicative of heterotrophic metabolism from the FAME, hydrocarbon, wax ester and sterol fractions were observed. Sulphur-cycling microorganisms were tentatively identified by FAME markers, and by a diverse array of branched quaternary carbon alkanes (BAQC) attributed to the sulphur-oxidising bacteria (Kenig et al., 2003). Actinobacteria, some species of which are active in nitrogen-cycling, were tentatively identified by FAME. Contributions from Archaea were indicated by ether-bound phytane, however, markers specific for methanogenic Archaea were not detected in these stromatolites (Allen, 2006). Sterols indicated the presence of bivalves and their dinoflagellate symbionts, and higher plant input, likely due to aerosols, was also identified in the FAME and hydrocarbon fractions. Of particular significance, was the fact that in general the functional groups of organisms detected by signature lipid markers correlated well with the metabolisms inferred for microbial mat and stromatolites from 16 S rDNA gene sequences (Allen, 2006; Burns et al., 2004; Papineau et al., 2005). The lipid biomarkers produced by the microbial mat communities were very similar to those observed from stromatolites in the same environment (Allen, 2006), suggesting that the microbial mats in Shark Bay are excellent analogues for the extant stromatolites in this location. Further, as the lipid profiles of many modern hypersaline or hot spring microbial mats do not contain BAQC such as those detected in Shark Bay (Allen, 2006), the microbial mats are likely to represent better analogues for the Shark Bay stromatolites than many other mat systems.
Not all lipid classes detected in stromatolites can be compared with ancient sediments since fatty acids are relatively quickly degraded, and wax esters may only survive up to 50,000 years (Cranwell, 1986). In addition, many studies do not investigate the presence of ether-bound lipids, indicative of archaea, although they have been detected in 10,400 year old sediments from Ace Lake, Antarctica (Coolen et al., 2004). Hydrocarbons, however, including the multiple-ring backbones of hopanoids and sterols, are very stable, and a number of these lipid biomarkers have been detected in Archean sediments. Steranes and 2-methyl hopanoids have been detected in 2.7 Ga shales from the Pilbara craton, Western Australia (Brocks et al., 1999; Summons et al., 1999; Brocks et al., 2003) suggesting that both eukarya and cyanobacteria were present at this early stage in Earth's history, while BAQCs, tentatively assigned to sulfur-oxidising bacteria, have been detected in 2.2 Ga sediments (Kenig et al., 2003). These same compounds have also been detected in the extant stromatolites of Shark Bay (Allen, 2006), suggesting they are good analogues of ancient sediments. Unfortunately, lipid profiles from Archaean stromatolites are yet to be investigated, so direct comparison with the extant stromatolites of Shark Bay is not possible. On the basis of currently available information, however, Precambrian stromatolite microbial communities appear to have been similar to the diverse microbiological assemblages observed in Shark Bay today.
Further indications that the Shark Bay microbial communities represent reliable analogues of ancient stromatolites come from varied approaches: carbon and sulphur isotope data indicating the presence of oxygenic phototrophy and sulphate-reducing metabolism in 2.7 Ga stromatolites (Kakegawa and Nanri, 2006), correlates with the detection of these processes in Shark Bay sediments (Bauld et al., 1979). The detection of cyanobacterial microfossils as outlined earlier, and the similarity of Shark Bay stromatolite microfabrics with ancient stromatolites also supports this conclusion (Reid et al., 2003). The microbial mats and stromatolites of Shark Bay are therefore highly significant resources for understanding life on early Earth.
Stromatolites are excellent natural laboratories for the study of microbial ecosystems that may have shaped the biology of early Earth (Des Marais, 2003). Stromatolites provide us with a glimpse of what life may have been like on the early Earth, the kinds of complex microbial interactions that occur, and what kind of metabolisms/physiologies may have been important in early microbial communities. Indeed evidence suggests Archean life forms may have been relatively advanced (Altermann et al., 2006), and thus metabolic pathways observed in present stromatolites may have already been utilised by their ancient counterparts. Although culture-independent molecular analyses alone do not allow us to absolutely determine whether sequences represent active stromatolite organisms, the studies discussed in this review on extant stromatolites show we can take advantage of phylogenetic affinity with well-studied species to make predictions about the metabolic contributions of the organisms identified. Researchers can now build constructively on this platform of microbial community analyses by targeting specific functional genes and enzyme activities, as well as conducting large-scale functional genomics studies, thereby furthering our knowledge on how individual and combined physiologies contribute to stromatolite systems. The close physical association of microorganisms in this setting may also facilitate horizontal gene transfer of adaptive and evolutionally significant traits such as antibiotic resistance, which has implications both for organism evolution and the field of biotechnology.
Finally, the identification of stable biomarkers in stromatolites that are uniquely produced by microorganisms is an area of increasing interest. Characterising the breadth of biomarkers, including lipids and characteristic pigments, in ancient microbial systems such as stromatolites, has important implications in the field of astrobiology, with the exciting potential for using the knowledge gained in the rational search and detection of possible biosignatures of life outside Earth. Furthermore, communities of microbes in stromatolites are responsible for the production of important trace gases, including photosynthetic oxygen, and the use of remote sensing to interpret infrared spectra may help us identify biological signatures arising from life on distant planetary atmospheres (Des Marais et al., 2002). The quest to understand early life on Earth and the prospects for life elsewhere addresses some of the most profound questions of humankind, and one of the extant analogues of early life, stromatolites, may be key in providing these answers.
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Biodata of Jessica C. Goin and Sherry L. Cady, authors of "Biosedimentological Processes that Produce Hot Spring Sinter Biofabrics: Examples from the Uzon Caldera, Kamchatka Russia"
Dr. Jessica C. Goin is currently a senior staff geomicrobiologist for S. S. Papadopulos and Associates, Inc. in their Portland, Oregon office. She obtained her Ph.D. from Portland State University in 2007. Dr. Goin's scientific interests are in the areas of: development of stromatolitic fabric, modeling biological and geochemical factors in sinter fabric development, mathematical analysis of stro-matolitic fabrics, biogeochemical cycles as a tool in environmental science, and the interaction of geology, hydrology, and contaminant geochemistry.
E-mail: [email protected]
Professor Sherry L. Cady is currently an Associate Professor at the Department of Geology, Portland State University, in Portland, Oregon. She obtained her Ph.D. in Geology from the University of California in Berkeley, and continued her research at the SETI Institute and NASA Ames Research Center before moving to Oregon. Her current scientific interests focus on microbial biosignatures, habitable environments beyond Earth, and early and extreme environments on Earth. Dr. Cady is the founding and current Editor-in-Chief of Astrobiology, the leading peer-reviewed journal that explores the secrets of life's origin, evolution, distribution, and destiny in the universe.
E-mail: [email protected]
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