ATMOSPHERIC oxygen is not in equilibrium with sea water with respect to the isotope exchange illustration but has an 18O excess of about 22‰ compared to sea water1. This could be due to isotope fractionation during respiration2. Another large contribution to the effect has been overlooked up to now. Photosynthesis on land takes place in transpiring leaves, where the difference in the vapour pressure of 16OH2 and 18OH2 concentrates the heavy molecules in their stationary water content. Since the free oxygen stems from the water in which photosynthesis takes place3–8 (with only a very small shift in isotopic composition9), photosynthesis on land is an 18O source for atmospheric O2. We have begun to study this effect quantitatively.
We compared the foraging strategies of Common Terns Sterna hirundo in freshwater (Lake Jeziorsko, Brzeg, Poland) and marine environments (Minsener Oldeoog, German Wadden Sea). Body mass changes, nest relief and duration and number of feeding trips per day were studied by automatically weighing the adults, using electronic balances under the nests. At the freshwater site, adults were lighter both before and after feeding and gained less mass during a trip. in the Wadden Sea, single feeding trips lasted longer than at the freshwater site and the terns made fewer trips per day. To achieve the same mass gain per day as in birds in freshwater, trips at sea had to be longer and food intake per trip was higher. The daily duration of absence for feeding and the daily mass gain were about the same in both areas. The limnetic feeders finished foraging earlier in the evening than the terns foraging at sea. These differences are consistent with the hypothesis that limnetic prey availability was consistent, whereas the tides limited the availability of marine prey. In consequence, foraging over freshwater presents several advantages, such as higher colony attendance, better mate coordination and better parental care. 相似文献
Small free-living amebas belonging to the genera Acanthamoeba and Naegleria occur world-wide. They have been isolated from a variety of habitats including fresh water, thermal discharges of power plants, soil, sewage and also from the nose and throats of patients with respiratory illness as well as healthy persons. Although the true incidence of human infections with these amebas is not known, it is believed that as many as 200 cases of central nervous system infections due to these amebas have occurred world-wide. A majority (144) of these cases have been due to Naegleria fowleri which causes an acute, fulminating disease, primary amebic meningoencephalitis. The remaining 56 cases have been reported as due either to Acanthamoeba or some other free-living ameba which causes a subacute and/or chronic infection called granulomatous amebic encephalitis (GAE). Acanthamoeba, in addition to causing GAE, also causes nonfatal, but nevertheless painful, vision-threatening infections of the human cornea, Acanthamoeba keratitis. Infections due to Acanthamoeba have also been reported in a variety of animals. These observations, together with the fact that Acanthamoeba spp., Naegleria fowleri, and Hartmannella sp. can harbor pathogenic microorganisms such as Legionella and or mycobacteria indicate the public health importance of these amebas. 相似文献
The effects of Cd and Zn on cross-colonization by Paxillus involutus of Scots pine seedlings was examined by using pairs of ectomycorrhizal (ECM) and non-mycorrhizal (NM) seedlings grown in the same vessel. This was done to assess, first, the ability of P. involutus to colonize NM Scots pine seedlings by growth from colonized roots of other Scots pine seedlings in the presence of Cd or Zn, and, second whether ECM colonization of Scots pine by P. involutus provided a competitive advantage over NM seedlings. Ectomycorrhizal colonization of Scots pine was shown to be more sensitive than Scots pine itself to Cd and Zn, but prior colonization did provide a competitive advantage with respect to biomass production. This beneficial effect over NM seedlings was, however, equal in the control, Cd and Zn treatments, and was due simply to growth stimulation in the presence of ECM colonization. Cross-colonization from an ECM to a NM seedling was reduced but not prevented by Cd and Zn. Cd had a more negative effect on cross-colonization than on initial colonization of seedlings, whereas Zn had an equally inhibitory effect on both parameters. These results have important implications for plant establishment on metal-contaminated sites. If cross-colonization between plants is reduced by toxic metals, plant establishment on contaminated sites might be retarded. 相似文献
Abstract. Three parameters influencing the capacity for carbon accumulation, i.e. photosynthesis, respiration, and leaf extension growth, were studied in Beta vulgaris L. (sugar beet) cultured in nutrient solution containing 0.5 to 500 mol m−3 NaCl. Leaf extension growth was the parameter most sensitive to salinity: the initial rate of leaf extension and final leaf length each declined linearly with increase in external NaCl concentration. Photosynthetic O2 evolution of thin leaf slices did not decline until salinity levels reached 350 to 500 mol m−3 NaCl, while respiratory O2 consumption was not affected by salinity throughout the range. The results suggest that the influence of salinity on the capacity for carbon accumulation in B. vulgaris occurs primarily through reduction in the area of photosynthetic surface. 相似文献
CO25 cells, a mouse myoblast line, contain multiple centrioles and primary cilia. A most unusual feature has been the finding of large numbers of separate structures in single cells—up to a maximum of nine centrioles, six primary cilia, and 12 of both organelles together. Aberrant multipolar spindles were occasionally seen containing variable numbers of centrioles. This strongly suggests that cells containing supernumerary centrioles and cilia are lost during mitosis, and that additional centriolar structures are generated during each interphase. No change in centriole or primary cilium frequency was detected after inducing the differentiation of myoblasts into myotubes. However, a significant migration of these structures occurred from a perinuclear to a supranuclear position prior to and during the phase of myoblast elongation. This shift was not maintained during cell fusion, when a net migration back to the periphery was observed, suggesting that it may have some function in relation to cell elongation and the change in the pattern of microtubule distribution which occurs as part of the process. 相似文献
During the last decade investigations of heterokont protists utilizing molecular methods have challenged established biosystematic concepts. Most investigations emphasized the chloroplast genome or sequences from nuclear-encoded, ribosomal genes. Refinement of DNA isolation protocols, advent of “universal primers” and the polymerase chain reaction, automated sequencing and increased accessibility of DNA sequence databases have expanded data-gathering efficiency and increased dataset sizes. Because independent datasets have been easier to obtain, the testing of specific phylogenetic hypotheses has been facilitated, altering relationship concepts, primarily at phylum/class levels, and perceptions of cellular evolution. New approaches have emphasized ecological studies and extended studies to genus/species levels and poorly investigated genomes. This paper reviews studies documenting these impacts and identifies some current limitations. Additionally, new DNA sequence data from our laboratory on nuclear-encoded rDNA internal transcribed spacers and the chloroplast-encoded psb A gene suggest that these regions will provide taxonomic resolution for the Synu-rophyceae, at the class/order level and subspecies level, respectively. 相似文献