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RICARDO BARRAGAN CELESTINA GONZALEZ-ARREOLA ANA BERTHA VILLASEÑOR 《Lethaia: An International Journal of Palaeontology and Stratigraphy》2004,37(3):223-234
Analyses of ammonite shell forms of two Barremian stratigraphic sections from Southwest Mexico consist of two well-defined morphotypes: (1) Small uncoiled, mostly leptoceratoid ancyloconic shells of the families Ancyloceratidae and Hamulinidae, and (2) middle-sized involute to moderately evolute oxycone to discocone shells of the family Pulchelliidae. Index taxa allow the recognition of standard ammonite biozones for the Barremian, which permit the relative dating of different processes that occurred through the water column in the environment of deposition. The vertical distribution of ammonite morphotypes and facies suggests changes of the palaeoceanographic and sedimentological conditions that prevailed in the area during Barremian time. Petrologic data, analyses of the organic carbon and carbonate contents of the rocks support the idea that oxygen-deficient bottom waters existed within a shallow marine, tectonically active area with little carbonate deposition during the early early Barremian (upper part of the Taveraidiscus hugii Zone through the base of the Nicklesia pulchella Zone). These conditions in the basin caused a proliferation of middle-water depth ammonites of Morphotype 1 but prevented the abundance of nektobenthic forms of Morphotype 2. Oxic conditions on a more calcareous and open normal marine environment seem to have been reestablished progressively during a transgressive episode from late early-early late Barremian (upper part of the Nicklesia pulchella Zone through the Gerhardtia sartousiana Zone). This environmental setting supported more facies dependent nektobenthic ammonites of Morphotype 2 to flourish within the basin. 相似文献
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The relation between the effects of auxin on growth, pH and potassium transport in hypocotyl segments of Helianthus annuus was studied. In a solution containing 20 mM Na2SO4 auxin-induced growth was accompanied by an auxin-induced pH drop in the medium. (NH4)2SO4, at the same concentration, brought about an almost complete abolishment of the effect of auxin on the pH. The magnitude of auxin-induced growth was, however, only slightly reduced. This result does not confirm the hypothesis according to which the action of auxin on growth is a result of its effect on the pH. In a solution containing 2 mM sodium phosphate buffer an inhibitory action of IAA on the release of potassium from the tissue was observed. Addition of 20 mM Na2SO4 to the medium brought about a complete abolishment of this effect. The magnitude of auxin induced pH drop was, however, similar in the two treatments. It was concluded that, although under suitable experimental conditions, a close relationship may exist between the effects of auxin on pH on K+ transport, the coupling between the two phenomena is not obligatory. 相似文献
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Abstract
The phylogenetic diversity of bacteria and cyanobacteria colonizing sediment particles in the permanent ice cover of an Antarctic
lake was characterized by analyses of 16S rRNA genes amplified from environmental DNA. Samples of mineral particles were collected
from a depth of 2.5 m in the 4-m-thick ice cover of Lake Bonney, McMurdo Dry Valleys, Antarctica. A rRNA gene clone library
of 198 clones was made and characterized by sequencing and oligonucleotide probe hybridization. The library was dominated
by representatives of the cyanobacteria, proteobacteria, and Planctomycetales, but also contained diverse clones representing
many other microbial groups, including the Acidobacterium/Holophaga division, the Green Non-Sulfur division, and the Actinobacteria. Six oligonucleotide probes were made for the most abundant
clades recovered in the library. To determine whether the ice microbial community might originate from wind dispersal of the
algal mats found elsewhere in Taylor Valley, the probes were hybridized to 16S rDNAs amplified from three samples of terrestrial
cyanobacterial mats collected at nearby sites, as well as to bacterial 16S rDNAs from the lake ice community. The results
demonstrate the presence of a diverse microbial community dominated by cyanobacteria in the lake ice, and also show that the
dominant members of the lake ice microbial community are found in terrestrial mats elsewhere in the area. The lake ice microbial
community appears to be dominated by organisms that are not uniquely adapted to the lake ice ecosystem, but instead are species
that originate elsewhere in the surrounding region and opportunistically colonize the unusual habitat provided by the sediments
suspended in lake ice.
Received: 16 August 1999; Accepted: 28 December 1999; Online Publication: 28 April 2000 相似文献
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SHMUEL RAZ JOHN H. GRAHAM HAGIT HEL‐OR TOMÁŠ PAVLÍČEK EVIATAR NEVO 《Biological journal of the Linnean Society. Linnean Society of London》2011,102(4):786-797
Rocellaria dubia bores into subtidal rocks of karsted limestone in the Adriatic Sea and elsewhere. It also bores into the shells of various bivalve species. The mechanism of boring has hitherto been debated, but examination of occupied shells suggest that this is achieved by mechanical (the shell) abrasion and chemical etching using secretions produced from glands in the anterior mantle. Fast‐growing bivalves such as Ostrea edulis and Pinna nobilis carry heavy R. dubia burdens, and encapsulate the borer in secreted calluses. Slow‐growing bivalves such as the burrowing Venus verrucosa and Glycymeris violacescens carry low R. dubia burdens, are less able to encapsulate the borers, and probably incur enhanced mortalities as a result. Individuals of R. dubia removed from their limestone boreholes re‐secreted adventitious tubes around their siphons, probably from glands in the posterior mantle. The lifestyle of R. dubia is now better understood, and its ability to bore bivalve shells in particular suggests how the more advanced tropical gastrochaenids Cucurbitula and Eufistulana have evolved from initial (as juveniles) bivalve shell borers into occupants of adventitious crypts and tubes, respectively. It is further argued that the Gastrochaenidae show convergent similarities with the similar crypt‐ and tube‐building representatives of the Clavagelloidea. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 104 , 786–804. 相似文献
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