The xanthophyll cycle and sustained thermal energy dissipation activity in Vinca minor and Euonymus kiautschovicus in winter

The influence of low temperature on the operation of the xanthophyll cycle and energy dissipation activity, as ascertained through measurements of chlorophyll fluorescence, was examined in two broad-leaved evergreen species, Vinca minor L. and Euonymus kiautschovicus Loessner. In leaves examined under laboratory conditions, energy dissipation activity developed more slowly at lower leaf temperatures, but the final, steady-state level of such activity was greater at lower temperatures where the rate of energy utilization (through photosynthetic electron transport) was much lower. The rate at which energy dissipation activity increased was similar to that of the de-epoxidation of violaxanthin to antheraxanthin and zea-xanthin at different temperatures. However, leaves in the field examined prior to sunrise on mornings following cold days and nights exhibited a retention of antheraxanthin and zeaxanthin that was associated with sustained decreases in photosystem II efficiency. We therefore suggest that this phenomenon of ‘photoinhibition’ in response to light and cold temperatures during the winter results from sustained photoprotective thermal energy dissipation associated with the xanthophyll cycle. Such retention of the de-epoxidized components of the xanthophyll cycle responded to day-to-day changes in temperature, being greatest on the coldest mornings (when photoprotective energy dissipation might be most required) and less on warmer mornings when photosynthesis could presumably proceed at higher rates.     

Exposure of Gambian children to Anopheles gambiae malaria vectors in an irrigated rice production area

Abstract. Variation in exposure of children to malaria vectors of the Anopheles gambiae complex was recorded in a Gambian village situated near an irrigated area of rice cultivation. Observations were made in 1987 and 1988 during two dry seasons, when pumped water was used to grow rice, and two rainy seasons, when rice was produced using a combination of irrigated and rainfed paddies. Routine collections of mosquitoes were made from under bednets. Most of these specimens were assumed to have fed on the occupants of the net and thus represented a crude measure of exposure to malaria. Most nets in the village were in good condition, but even these were a poor defence against blood-seeking mosquitoes. Two annual peaks in the numbers of An. gambiae s.l. corresponded with the irrigation of rice paddies in the dry and wet seasons. When there were few vectors in the village the frequency distribution of mosquitoes caught under nets was described best by a Poisson process. When high numbers were present the daily distributions were over-dispersed and fitted a negative binomial model. The spatial distribution of mosquitoes varied between dry and wet seasons and was related to the predominant wind direction at night, suggesting that wind assisted the dispersal of mosquitoes from their breeding sites. For individual children in the rainy season, increased exposure to malaria vectors was associated with living adjacent to a mosquito breeding site, being resident in larger compounds, having open eaves in the house, a store-room adjacent to the bedroom, the absence of a ceiling in the bedroom, the absence of wood smoke indoors and leaving the bednet untucked at night. In the dry season a high level of exposure was associated with living close to a mosquito breeding site, having an unfenced compound, sleeping in a room without a ceiling and using insecticide aerosols. These observations demonstrate that within a village there are systematic and persistent differences in the level of exposure to malaria parasites experienced by individual children.     

The neutralization of acidic coal mine lakes by additions of natural organic matter: a mesocosm test

Cylindrical polyethylene enclosures 3 m in length and 1 m in diameter reaching from the surface to the bottom were constructed in an acid (pH=3.1) lake on a coal surface mine in southern Illinois. Wheat straw was added to the enclosures to test the effects of dissimilatory sulfate reduction on water chemistry. Added straw increased sulfide concentrations, raised pH to 6.5, reduced O₂ and increased acid neutralizing capacity of the enclosed water columns when compared with a control enclosure and with the open lake. Generation of acid neutralizing capacity exceeded the standing stock of sulfide indicating that sulfide was removed either by precipitation of FeS or outgassing of H₂S. The pH and acid neutralizing capacity within the enclosures eventually returned to the level of the surrounding lake because of water exchange around the enclosure walls. Our results show that additions of organic matter to acid surface mine lakes result in the generation of acid neutralizing capacity.     

Homolytic carbon to phosphorus bond scission of some phosphonates catalyzed by bacterial carbon-phosphorus lyase

The production of volatile degradation products of phosphonates was monitored to investigate the mechanism involved in the biodegradation of propylphosphonic acid and phenylphosphonic acid byRhizobium sp MMM101a. The biodegradation of propylphosphonic acid gave rise to the production, in decreasing order, of propane, methane, ethane, 1-butene, propene, isobutene, butane and ethene. The formation of these degradation products was strongly reduced by adding catalase to the growing cultures indicating the involvement of peroxides in the biodegradation mechanism. OH⁰ radical scavengers did not reduce the rate of biodegradation, and therefore these radicals appear not to be involved. Addition of ascorbate, a known hydroxylating agent in biological systems, increased the amount of biodegradation products. The involvement of iron in the degradation was indicated and was optimal at a concentration of 950 µM. This suggests the involvement of a metalloenzyme involving iron and peroxide. The decomposition of phenylphosphonic acid yielded benzene and biphenyl. No phenol could be detected, again suggesting that OH⁰ radicals were not involved in the biodegradation. The presence of deuterated benzene did not result in the occurrence of biphenyl consisting of one nondeuterated and a deuterated ring, which is chemically more likely. It therefore appears that the degradation of the phosphonates occurs on a multicentered enzyme. The diversity of the products generated by this bacterium from phosphonates, many of them due to rearrangement of the carbon moiety of the substrate molecule, suggests an overall involvement of superoxide radicals in the homolytic carbon to phosphorus bond scission.