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111.
Most prebiotic syntheses depend on the reaction of concentrated precursor compounds to produce bio-organic molecules. It is now believed that the early Earth’s atmosphere was not reducing enough to have permitted copious synthesis of precursor molecules. Freezing allows reaction to occur even from dilute solution. This reaction has been demonstrated for the purines but not for the pyrimidines. It is shown here that dilute solutions of simple prebiotic molecules produce the biological pyrimidines cytosine and uracil upon freezing. Cold environments may have allowed synthesis of all of the RNA bases even from low organic yielding atmospheres, such as those of the early Earth, Mars, Titan and Europa. 相似文献
112.
Roulier S Baran N Mouvet C Stenemo F Morvan X Albrechtsen HJ Clausen L Jarvis N 《Journal of contaminant hydrology》2006,84(1-2):81-105
The objective of this study was to identify the main controls on atrazine leaching through luvisols and calcisols overlying fissured limestone using the dual-permeability model MACRO. The model parameterisation was based on a combination of direct measurements (e.g. hydraulic properties, adsorption and degradation), literature data and calibration against bromide leaching experiments in field plots. A Monte Carlo sensitivity analysis was carried out for a typical application pattern, considering two different depths of unsaturated limestone (15 and 30 m). MACRO calibrations to the field experiments demonstrated the occurrence of strong macropore flow in the luvisol, while transport in the calcisol could be described by the advection-dispersion equation. MACRO simulations of tritium and atrazine leaching qualitatively matched tritium concentration profiles measured in the limestone and atrazine concentrations measured in piezometers and in aquifer discharge via a spring. The sensitivity analysis suggested that the thickness of the limestone, as well as the transport properties and processes occurring in the unsaturated rock (e.g. matrix vs. fissure flow) will have little significant long-term effect on atrazine leaching, mainly because degradation is very slow in the limestone. No mineralization of atrazine was detected in one-year incubations and a mean half-life of 10 years was assumed in the simulations. Instead, processes occurring in the soil exerted the main control on predicted atrazine leaching, especially variations in the degradation rate and the strength of sorption and macropore flow. However, fissure flow in unsaturated rock is expected to exert a much more significant control on groundwater contamination for compounds that degrade more readily in the deep vadose zone. 相似文献
113.
Risk-based regulation assumes that risk assessment can evaluate risks against policy objectives. However, policy goals are often ambiguous and require risk assessors to interpret them for use in risk assessment. This risk assessment-policy gap stems partly from normative and imprecise policy language but is rooted more fundamentally in society's uncertain expectations for the environment. Until this uncertainty is resolved, the democratic and regulatory effectiveness of risk regulation will be undermined by ad hoc policy decisions abdicated to risk assessors. 相似文献
114.
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116.
The morphologically variable reef coral previously known as Montastraea annularis (Ellis and Solander, 1786) has recently been separated into three species based on differences in morphology, behavior, allele
frequencies and some life-history traits of Panamanian specimens. To further investigate the proposed reclassification and
its conformity to the biological species concept we conducted reciprocal intra- and inter- specific fertilization experiments
with gametes from each of the three species on Florida reefs. With one exception, self-fertilization rates were very low or
zero. Within-species crosses resulted in production of planulae, as did all inter-species (hybrid) crosses, but there was
much variation in fertilization success within each type of cross. In an experiment with separated gametes, hybrid crosses
between M. annularis and M. franksi produced more larvae than within-species crosses for each species. Hybridization crosses between M. faveolata and the other two species produced fewer larvae than did within- M. faveolata crosses in the experiment with separated gametes, but many larvae resulted when the hybridizations were performed by mixing
entire gamete bundles. Additional observations showed that M. franksi had 20% larger eggs and fewer eggs per gamete bundle than did the other two species and that it consistently spawned 1 to
1.5 h before the others, a potential temporal barrier to hybridization. These results indicate that there is no inherent pre-zygotic
barrier to cross-fertilization among the three morphological species, although post-zygotic survival and fertility remain
to be determined. The adherence of the proposed reclassification to the biological species concept requires further examination.
Received: 16 April 1997 / Accepted: 17 June 1997 相似文献
117.
Michael J. Miller Sam Wouthuyzen Gen Minagawa Jun Aoyama Katsumi Tsukamoto 《Marine Biology》2006,148(5):1101-1111
A survey for leptocephali around Sulawesi Island in the central Indonesian Seas during May 2001 found that the leptocephali
of the congrid eel, Ariosoma scheelei, were present in all seven areas that were sampled. A total of 551 leptocephali (22–166 mm TL) were collected, and A. scheelei was by far the most abundant species of leptocephali collected during the survey. The wide range of sizes in most areas indicated
that spawning had occurred during a period of several months in many different areas, although the exact spawning locations
were not determined. The larger size classes were more abundant in all areas except in Tomini Bay on the northeast side of
Sulawesi Island. The highest catch rates were observed at the eastern edge of the Java Sea and to the north in the Celebes
Sea near Makassar Strait. Premetamorphic leptocephali were also collected in surface samples at 11 stations (N=62), but metamorphosing leptocephali (N=86) were only caught in IKMT tows that fished from the surface to about 200 m. Metamorphosing leptocephali were collected
primarily at two stations in the Java Sea and Makassar Strait where a surface layer of lower-salinity water was detected.
Their total lengths (105.3–153.3 mm) and the largest premetamorphic individuals suggested that this species can reach maximum
sizes of about 165 mm before beginning to metamorphose. It is hypothesized that this species may be abundant in the Indonesian
Seas region and that it has ecological traits such as large size at recruitment and a small size at reproduction that have
made it successful in many regions of the Indo-Pacific. 相似文献
118.
T.?WernbergEmail author M.?Coleman A.?Fairhead S.?Miller M.?Thomsen 《Marine Biology》2003,143(1):47-55
Ecklonia radiata (C. Ag.) J. Agardh is a common macroalga on reefs in the warm-temperate parts of the southern hemisphere. It is a dominant habitat-former and as such has a strong structuring effect on associated algal assemblages. Morphological variation in E. radiata potentially affects its interactions with the surroundings and contributes to confusion about its taxonomy. We quantified the magnitude of morphological variation in fully developed E. radiata sporophytes across Australasia and tested the hypotheses that E. radiata has different morphology at different locations and that the degree of morphological difference depends on spatial distances among locations. A total of 11 morphological characters were sampled from 11 locations along the Australian coastline from Kalbarri in Western Australia to Sydney in New South Wales as well as from Doubtful Sound, New Zealand. Most morphological characters varied considerably from one location to another. For example, the average (±SE) thallus length was 135.2±12.5 cm in Kalbarri and only 69.7±5.5 cm in Sydney. There were no consistent spatial patterns of variation among individual morphological characters, and, generally, variations among individual characters were poorly correlated (-0.5< R<0.5). This suggests that individual morphological characters develop independently of each other in response to processes operating at different spatial scales. Multivariate measures of morphology were found to be different among some locations and similar among others (-0.37=Clarke's R=1), but there was no correlation (Spearman's R=0.08) between morphological similarity and distance between locations. Consequently, our results do not support clinal variation in E. radiata morphology. Rather, they suggest the presence of discrete morphologically different populations, in which the morphology at any one location reflects multiple forcing factors operating on different morphological characters at different spatial scales. 相似文献
119.
During the spring and summer of 1979, 48 species of geofungi and several sterile, unidentified fungi were isolated from two off-shore stations on the Bay of Fundy, Canada. Isolations were made from subsurface water and surface slicks on 6 agar media including one containing hexadecane, and from subsurface water baited with hemp seeds. Numbers of fungi were 10 to 100 times higher than those reported for other offshore stations and similar to those reported for a saltmarsh outflow. Species isolated during a high freshwater flow largely corresponded to a typical soil flora. Species isolated during a reduced freshwater flow were either marine in character or noted for association with area littoral seaweeds. Three fungi (Cladosporium cladosporioides, Trichoderma viride, and Gliocladium rosium) were tested for growth on media of increasing salinity. Growth was measured by dry weight production, pH drop and glucose used in liquid culture. C. cladosporioides showed responses typical of a marine fungus, T. viride of a terrestrial fungus and G. roseum was intermediate. The results are discussed in relation to the role of geofungi in the sea. 相似文献
120.