Estimates of protozoan- and viral-mediated mortality of bacterioplankton in Lake Bourget (France)

1. We performed three, 1‐week in situ experiments in March‐April (expt 1), May (expt 2) and August (expt 3) 2003 in order to assess protozoan and virus‐induced mortality of heterotrophic bacteria in a French lake. Viral and bacterial abundances were obtained using flow cytometry (FCM) while protozoa were counted using epifluorescence microscopy (EFM). 2. A dilution approach, applied to pretreated grazer‐free samples, allowed us to estimate that viral lysis could be responsible for 60% (expt 1), 35% (expt 2) and 52% (expt 3) of daily heterotrophic bacterial mortality. Flagellate (both mixotrophic and heterotrophic) grazing in untreated samples, was responsible for 56% (expt 1), 63% (expt 2) and 18% (expt 3) of daily heterotrophic bacteria removal. 3. These results therefore suggest that both viral lysis and flagellate grazing had a strong impact on bacterial mortality, and this impact varied seasonally. 4. From parallel transmission electron microscopy (TEM) analysis, we found that the burst size (i.e. the number of viruses potentially released per lysed cell) ranged from nine to 25 (expt 1), 10 to 35 (expt 2) and eight to 25 (expt 3). The percentage of infected heterotrophic bacteria was 5.7% (expt 1), 3.4% (expt 2) and 5.7% (expt 3) so that the calculated percentage of bacterial mortality induced by viruses was 6.3% (expt 1), 3.7% (expt 2) and 6.3% (expt 3). 5. It is clear that the dilution‐FCM and TEM methods yielded different estimates of viral impact, although both methods revealed an increased impact of viruses during summer.     

Characterization of Sulphate Uptake in Anacystis nidulans

Sulphate uptake in the blue-green alga Anacystis nidulans appears based upon an active mechanism with a Km of 0.75 μM and V_max of 0.7 pmol/min × 10⁶ cells. Sulphate uptake is competitively inhibited by thiosulphate and sulphite. The sulphate uptake has a pH optimum at 8 and a temperature optimum at 40°C. By increasing the extracellular sulphate concentration from 0.1 to 10 μM the sulphate pool in Anacystis was altered from 8.3, 10^?5M to 5.9, 10^?4M.     

Effect of High-Pressure Gas Atmospheres and Anaesthetics on Chilling Injury of Plants

High pressures and anaesthetics were used to study chillinginjury in plants. Changes in membrane fluidity at low but nonfreezingtemperatures are thought to be involved in chilling injury-aphysiological disorder of many economically important plants,e.g. banana, cotton, cucumber, maize, rice, sorghum, and tomato.High-pressure helium and nitrogen atmospheres of 12 MPa increasedthe severity of chilling injury (i.e. rate of ion leakage) inexcised cucumber cotyledon discs, cucumber hypocotyl segmentsand tomato pericarp discs, and also increased the thresholdtemperature at which chilling occurred by 2° to 6°C.Exposure to vapours of the anaesthetics halothane and methoxyfluranereduced chilling injury in cucumber cotyledon discs, cucumberhypocotyl segments and tomato pericarp discs. The relative effectivenessof the two anaesthetics in reducing chilling injury was similarto their relative effectiveness in inducing anaesthesia in animalsand their relative lipid solubilities. The response of the tissuesto halothane and methoxyflurane, which increase membrane fluidity,and to high pressures, which reduce membrane fluidity, are consistentwith the hypothesis that cold-induced phase transition of membranescould be responsible for chilling injury. However, other cellularcomponents may also be affected, e.g. low temperatures, highpressures and anaesthetics can alter protein conformation, affection channels, depolymerize microtubules and cause the releaseof calcium from membrane lipids. Key words: Cucumber, Cucumis sativus, tomato, Lycopersicon esculentum, halothane, ion leakage, methoxyflurane     

True Metabolizable Energy for Seeds of Common Moist-Soil Plant Species

Abstract: Understanding the true metabolizable energy (TME) value of food is important for constructing bioenergetic models. We estimated gross energy, nutrient composition, and TME values for the seeds of 3 native and 1 invasive exotic wetland plant that occur in the diet of waterfowl. True metabolizable energy values were 0.50 ± 0.080 kcal/g for spike rush (Eleocharis palustris), 0.65 ± 0.080 kcal/g for alkali bulrush (Schoenoplectus maritimus), 1.31 ± 0.090 kcal/g for perennial pepperweed (Lepidium latifolium), and 2.52 ± 0.080 kcal/g for lamb's quarters (Chenopodium album). The TME value for 3 of 4 species, including the invasive species, was low relative to the TME value of other seeds consumed by waterfowl.