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151.
TIM J. CLOUGH LAURA E. BUCKTHOUGHT FRANCIS M. KELLIHER† ROBERT R. SHERLOCK 《Global Change Biology》2007,13(5):1016-1027
There is uncertainty in the estimates of indirect nitrous oxide (N2O) emissions as defined by the Intergovernmental Panel on Climate Change (IPCC). The uncertainty is due to the challenge and dearth of in situ measurements. Recent work in a subtropical stream system has shown the potential for diurnal variability to influence the downstream N transfer, N form, and estimates of in‐stream N2O production. Studies in temperate stream systems have also shown diurnal changes in stream chemistry. The objectives of this study were to measure N2O fluxes and dissolved N2O concentrations from a spring‐fed temperate river to determine if diurnal cycles were occurring. The study was performed during a 72 h period, over a 180 m reach, using headspace chamber methodology. Significant diurnal cycles were observed in radiation, river temperature and chemistry including dissolved N2O‐N concentrations. These data were used to further assess the IPCC methodology and experimental methodology used. River NO3‐N and N2O‐N concentrations averaged 3.0 mg L−1 and 1.6 μg L−1, respectively, with N2O saturation reaching a maximum of 664%. The N2O‐N fluxes, measured using chamber methodology, ranged from 52 to 140 μg m−2 h−1 while fluxes predicted using the dissolved N2O concentration ranged from 13 to 25 μg m−2 h−1. The headspace chamber methodology may have enhanced the measured N2O flux and this is discussed. Diurnal cycles in N2O% saturation were not large enough to influence downstream N transfer or N form with variability in measured N2O fluxes greater and more significant than diurnal variability in N2O% saturation. The measured N2O fluxes, extrapolated over the study reach area, represented only 6 × 10−4% of the NO3‐N that passed through the study reach over a 72 h period. This is only 0.1% of the IPCC calculated flux. 相似文献
152.
NIGEL D. SWARTS ELIZABETH A. SINCLAIR ANTHONY FRANCIS KINGSLEY W. DIXON 《Molecular ecology》2010,19(15):3226-3242
Terrestrial orchid germination, growth and development are closely linked to the establishment and maintenance of a relationship with a mycorrhizal fungus. Mycorrhizal dependency and specificity varies considerably between orchid taxa but the degree to which this underpins rarity in orchids is unknown. In the context of examining orchid rarity, large scale in vitro and in situ germination trials complemented by DNA sequencing were used to investigate ecological specialization in the mycorrhizal interaction of the rare terrestrial orchid Caladenia huegelii. Common and widespread sympatric orchid congeners were used for comparative purposes. Germination trials revealed an absolute requirement for mycorrhisation with compatibility barriers to germination limiting C. huegelii to a highly specific and range limited, efficacious mycorrhizal fungus. DNA sequencing confirmed fidelity between orchid and fungus across the distribution range of C. huegelii and at key life history stages within its life cycle. It was also revealed that common congeners could swap or share fungal partners including the fungus associated with the rare orchid but not vice versa. Data from this study provides evidence for orchid rarity as a cause and consequence of high mycorrhizal specialization. This interaction must be taken into account in efforts to mitigate the significant extinction risk for this species from anthropogenically induced habitat change and illustrates the importance of understanding fungal specificity in orchid ecology and conservation. 相似文献
153.
SYNOPSIS. Conjugation in Stentor coeruleus was investigated, using a standardized culture technic which yielded large numbers of mating pairs within a single culture. Spontaneous bursts of selfing occurred during a definite interval in the development of a culture. Structurally distinct preconjugator cells appeared immediately before as well as during the initial stages of a burst of conjugation. Mating pairs were formed by the union of 2 preconjugators.
Mixing 8 stocks in all possible combinations of pairs and observing their subsequent response revealed they were separable into 2 complementary mating types. The majority of mating pairs formed in mixtures of stocks consisted of individuals of different mating types.
It is suggested that control of mating types in the ciliate order Heterotrichida may be of a somewhat different nature from that found in other ciliates. 相似文献
Mixing 8 stocks in all possible combinations of pairs and observing their subsequent response revealed they were separable into 2 complementary mating types. The majority of mating pairs formed in mixtures of stocks consisted of individuals of different mating types.
It is suggested that control of mating types in the ciliate order Heterotrichida may be of a somewhat different nature from that found in other ciliates. 相似文献
154.
SYNOPSIS. When amoeboid organisms of Naegleria gruberi are treated with acriflavin at concentrations under 70 μg/ml, they still can form flagellates. Acriflavin between 70 μg/ml and 110 μg/ml increasingly inhibits morphogenesis. The effect of acriflavin is specific for the morphogenetic system since amoeboid organisms treated with acriflavin between 110 and 200 μg/ml are not otherwise inhibited and behave like the controls. 相似文献
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157.
DAVID W. FRANCIS 《Differentiation; research in biological diversity》1979,15(1-3):187-192
Three abstract models of divergent cell differentiation to multiple cell types are presented. These differ primarily in the proportion of developmental events common to the pathways leading to each cell type. Two experimental approaches are outlined to determine which type best describes divergent differentiation occurring in a particular organism. The first technique is to describe and compare changes in labeling of specific polypeptides which characterize development to the several cell types. The second is to observe the ability of mutants which are blocked in one pathway to develop along alternate pathways. These approaches are applied to the case of Polysphondylium pallidum , where amebae develop into stalk cells, spores, or microcysts. It is concluded that cell differentiation in P. pallidum is of the truly divergent type, in which developing cells show identical sequences of events until a branch point, and thereafter very different sequences of events. 相似文献
158.
FRANCIS LETHIERS 《Lethaia: An International Journal of Palaeontology and Stratigraphy》1983,16(1):39-49
About one hundred papers and studies have been utilized for a synthesis of ostracode palaeobiogeography during the Frasnian and Famennian. The pelagic entomozoid species have worldwide distributions: they are not used for distinguishing the palaeobiogeographic areas. Five provinces probably existed during the Frasnian and five during the Famennian. The palaeobiogeographic area of the Palaeotethys is constant during this time. There are important faunistic relations between the Western Canadian Platform and the Russian Platform, that is to say, along the Northern margin of the North-Atlantic Continent, during Late Devonian time. The province of Southeastern Asia remains isolated. The number and limits of the palaeobiogeographic areas have been controlled by climate and some geographical barriers. 相似文献
159.
160.