Recent paleomagnetic studies on the Martian meteorite ALH84001 have shown that this rock traveled from Mars to Earth with an internal temperature entirely below 40°C. Dynamical studies indicate that the transfer of rocks from Mars to Earth (and to a limited extent, vice versa) can proceed on a biologically-short time scale, making it likely that organic hitchhikers have traveled between these planets many times during the history of the Solar system. These results demand a re-evaluation of the long-held assumption that terrestrial life. first evolved on Earth, as it could just as easily have evolved on Mars and traveled here. We argue here that the chemical environment on early Mars would have been better for the evolution of early biochemical reactions than that of early Earth.
The results of an experimental test for verifying the hypothesis that the common ancestor of all living organisms (universal ancestor, commonote) was a hyperthermophile (Miyazaki, et al., 2001), are explained. In the experiment, mutant enzymes with ancestral aminoacids were made using a gene engineering technique. The mutant enzymes were purified and tested for thermostability. The mutant enzymes with ancestral aminoacids showed higher thermostability than the contemporary hyperthermophile enzyme. The results suggest that the common ancestor of all living organisms was a hyperthermophile. The argument related to the hyperthermophilic common ancestor hypothesis was reviewed with respect to the experimental test.
Abundant organic matter (kerogen) was identified in 3.5 Ga hydrothermal silica dikes from the North Pole area in the Pilbara craton, Western Australia. The silica dikes developed in the uppermost 1000 m of the ancient oceanic crust. Thus, they would have been deposited in the 3.5 Ga sub-seafloor hydrothermal system. The carbon and nitrogen isotopic compositions of the kerogen were analyzed in this study. Their highly 13C-depleted isotopic compositions (δ13C=-38 to-33%0) strongly suggest that they are originally derived from biologically produced organic matter. The remarkable similarity of the δ13C values between the kerogen and modern hydrothermal vent organisms may suggest that the kerogen was derived from chemoautotrophic organisms. This idea is also consistent with their nitrogen isotopic compositions (δ15N =-4 to + 4‰). The silica dikes consist mainly of fine-grained silica with minor pyrite and sphalerite. These mineral assemblages indicate that the silica dike was deposited from relatively low-temperature (probably less than 150 °C) reducing hydrothermal fluid. Thus, anaerobic thermophilic/hyperthermophilic organisms could have survived in the hydrothermal fluid, which formed the silica dikes. Therefore, it is plausible that a chemoautotrophic-based biosphere (possibly methanogenesis) probably existed in the Early Archean sub-seafloor hydrothermal system.
Sulfate-reducing bacteria (SRB) are considered to be one of the oldest micro-organisms in the Earth history. Sulfur isotope data of sedimentary pyrite indicate that SRB have been active at least since 3.5 Ga. However, living environments of SRB, such as concentration level of sulfate in the Archean oceans and sulfur flux into Archean oceans, have been debated vigorously. Detailed sulfur isotope studies were performed on individual pyrite crystals in black shale and chert samples collected from several Archean greenstone belts. Sulfur isotope compositions of pyrite in all samples are fractionated, suggesting their biogenic origin. It is found that some representative sample sets from 2.7 Ga greenstone belts show a good correlation between organic carbon and pyrite sulfur concentrations. These results suggest that Archean oceans have been sulfate-rich at least since 2.7 Ga. This sulfate-rich ocean model conflicts with the sulfate-poor ocean model based on the mass independent fractionation of sulfur isotopes.
The search for life on the edges (frontiers) of the global biosphere bridges earth-bound biology and exobiology. This communication reviews recent microbiological studies on selected “frontiers”, i.e., deep-sea, deep subsurface, and Antarctica. Deep-sea is characterized as the aphotic (non-photosynthetic) habitat, and the primary production is mostly due to the chemosynthetic autotrophy at the hydrothermal vents and methane-rich seeps. Formation of the chemosynthesis-dependent animal communities in the deep leads to the idea that such communities may be found in the “ocean” of the Jovian satellite, Europa. An anoxic (no-O2), as well as aphotic, condition is characteristic of the deep subsurface biosphere. Microorganisms in the deep subsurface biosphere exploit every available oxidant for anaerobic respiration. Sulfate, nitrate, iron (III) and CO2 are the representative oxidants in the deep subsurface. Below the 3000 m-thick glacier on Antarctica, >70 lakes having liquid water are entombed. One of such sub-glacial lakes, Lake Vostok, has been a target of “life in extreme environments” and is about to be drill-penetrated for microbiological studies. These biospheric frontiers will provide new knowledge about the diversity and the potential of life on Earth and facilitate the capability of astrobiologial exploration.
Seafloor hydrothermal system and deep subsurface are of great interest for microbiologists as paradise of unusual lives so-called “Extremophiles” in this planet. Such peculiar microorganisms have been believed to be minority in the earth throughout the long history after the early evolution of life. Recent investigations for microorganisms present in the active hydrothermal seafloor and subsurface have revolutionized the concept. Ubiquity, predominance and diversity of extremophiles in the present and past global environments signify the unresolved, but significant role in the co-evolution of earth and life. In this article, we summarize the expeditions for the microbial world in the seafloor hydrothermal system and deep subsurface and shed light on the foci of the future investigation.
Occurrence and distribution of microorganisms in the subsurface of deep-sea hydrothermal vents (sub-vents) were investigated using sub-vent rock samples (cores, approximately 6.5 cm in diameter) collected by the Ocean Drilling Program (ODP). The central parts of the cores (subcores, approximately 1.5 cm in diameter) were extracted in an anaerobic chamber. The degree of contamination by drilling fluid (surface seawater in ODP) was tested using perfluorocarbon (PFC) tracers. The test showed that PFC-traced contamination was limited to core surfaces and was not detected in subcores. Therefore, subcores were used in microbiological analyses for direct counts, adenosine 5-triphoshate (ATP) measurements, thermophilic incubations, and 16S ribosomal RNA gene (16S rDNA) sequences. Microbial cells in the subcores were observed from depths shallower than 97.9 meters below the seafloor (mbsf) by 4' 6'-diamidino-2-phenylindole (DAPI) -epifluorescence microscopy. Similarly, ATP was detected only from depths shallower than 44.8 mbsf by the luciferin-luciferase method. Portions of subcores from various depths, 9.7-301.5 mbsf, were directly incubated anaerobically with a heterotrophic medium at temperatures of 60°C and 90°C. After two weeks, an increase in cell numbers was observed for 60 °C-cultures from 59.8-99.4 mbsf samples, and for 90°C-cultures from 69.1-128.9 mbsf samples. The 16S rDNA sequences suggest that microorganisms from the 60°C-and 90°C-cultures are closely related to the thermophilic species belonging to the genera Geobacillus and Deinococcus, respectively. These results indicate 1) existence of microbial habitats in the sub-vent region of subseafloor, and 2) habitat segregation of thermophilic bacteria over the sub-vent thermal gradient.
Extreme halophiles (halobacteria) are microorganisms that require a high concentration of NaCl for their optimal growth. They belong to the domain Archaea, together with methanogens and some thermophiles. It has been shown that halobacterial cells are entrapped within the fluid inclusions when NaCl crystallizes, and viable cells could be recovered after many months. Many halobacteria have been isolated from Permian halite, however, few studies are generally accepted due to questions about sample quality and contamination. A recent report that a halophilic bacterium was isolated from a fluid inclusion within a primary salt crystal of the Permian (250 Ma) suggested again that previously reported isolations of halobacteria from Permian halite might be true. The requirement for elevated salt concentration, the probable ability to survive within low water activity environments for long period, and the presence of concentrated KCl in the cells, which is shown to protect DNA and proteins from harmful irradiation, makes halobacteria likely candidates for life on early Mars.
The abundance and diversity of groundwater microorganisms were studied in a geochemically defined borehole (TH-6, DH-3) in the Tono area, central Japan. Total cell counts by epifluorescence microscopy were estimated to be approximately 1.6 × 105-5.7 × 106 cells ml -1 and showed little decrease with depth. Cell viability, based on cell membrane integrity, respiration-based metabolism, and esterase activity was estimated to be 0.001% to approximately 100% of the total counts. The distribution of microbial abundance seems to be related to a variety of environmental factors, including fracture numbers, hydrological, and geochemical conditions of the groundwater. The geochemistry of the groundwater suggests that sulfate reducing bacteria play an important role in the sedimentary rock environment. It appears that microbial sulfate reduction occurs at the highest level in the upper part of the Toki Lignite-bearing Formation. This microbial reaction may play an important role in maintaining anaerobic conditions for uranium fixation, that could in turn scavenge soluble and oxidized uranium. On the other hand, in the granite groundwater, iron-oxidizing/reducing bacteria seem to play an important role in iron redox cycling. Iron-oxidizing bacteria may contribute to the formation and deposition of iron colloids in the upper part of Toki granite.
The bacterial endosymbionts of the solemyid clam Acharax johnsoni and the thyasirid clam Parathyasira kaireiae, collected from the Japan Trench, were characterized. Transmission electron microscopic observations showed numerous bacteria in the epithelial cells of the gill tissues of A. johnsoni. Numerous bacteria were also visible in the gill tissues of P. kaireiae, but were not located within the epithelial cells. Phylogenetic analysis based on the 16S ribosomal RNA gene sequences from the gill tissues of both clams indicated that the bacteria were related to sulfur-oxidizing endosymbionts from deep-sea chemosynthetic environments. The symbiont of A. johnsoni formed a monophyletic group with the thioautotrophic symbiont of Solemya reidi, which lives relatively deeper than other solemyid clams. The symbiont of P. kaireiae formed a monophyletic group with symbiont II in Maorithyas hadalis, which lives in the hadal zone of the Japan Trench. In addition, four vesicomyid species living relatively deeper than other vesicomyid clams also have a specific clade of thioautotrophic symbionts. Bacterial chemotrophic endosymbionts as well as the mode of symbiosis might influence host distributions in deep-sea chemosynthetic ecosystems.
To improve our understanding on the unknown sub-vent biosphere, which is the biosphere under seafloor hydrothermal sites, new ideas and techniques for this extreme environment are proposed. Modifications were made to the Benthic Multi-coring System (BMS) which is a tethered submarine rock drill to use its great advantages in approaching and getting uncontaminated samples from the sub-vent biosphere. Besides, drilled boreholes are used for in situ filtering, trapping, and incubation of microbes not only to get valuable information about sub-vent microbial diversity and its functions, but also to acquire novel bio/gene resources. New techniques, which are evaluated and applied to the multidisciplinary science project “Archaean Park”, have proved to be essential for studies of the deep biosphere. A technical assessment indicates that there is no perfect method for getting uncontaminated samples from the deep biosphere, and care should be taken during laboratory handling to minimize the shortcomings of the method. On the other hand, a proper combination of subseafloor sampling and analytical methods will provide us with fascinating clues to the origins of life on the Earth.