Diatom ecology in the phyllosphere of the common duckweed (Lemna minor L.)

Epiphyton associated with thick, floating mats of the common duckweed (Lemna minor L.) was studied at four sites in western Canada between 1985 and 1988. Maximum epiphyton abundance generally occurred in spring as biomass of the duckweed mat was increasing. Epiphytic biomass was low during summer and increased at some sites in autumn with mat decomposition. The community was composed mostly of diatoms and, during summer, photosynthetic bacteria. Species richness of the diatom flora was low, suggesting that duckweed mats are environments to which few species are adapted. Photosynthesis - irradiance curves indicated that duckweed epiphyton was not adapted to low light levels that occurred in the mat (< 1 % of ambient), suggesting they may survive via other means of nutrition. The mat phyllosphere was also characterized by wide spatial and temporal variation in temperature, and sharp vertical profiles of dissolved oxygen and nutrients.     

Waterfowl feces as a source of nutrients to a prairie wetland: responses of microinvertebrates to experimental additions

We tested the hypothesis that inorganic nutrients released fromwaterfowl feces would stimulate primary production, therebyaffecting microinvertebrate grazers, by making controlledadditions of waterfowl feces to fishless wetland enclosures andmeasuring the response of planktonic and phytophilouscladocerans, copepods, and rotifers. Feces were added in twopulses, four weeks apart, to duplicate enclosures at a ‘high’level (115 g m⁻² wet feces), simulating the total P load(1.6 g m⁻²) applied in an earlier fertilizationexperiment, and a ‘low’ level (11.5 g m⁻²). Density ofmicrocrustacean grazers in the water column increased inresponse to both feces additions, although the response wasmore noticeable after the second feces addition. After eachaddition, cladocerans (predominantly Ceriodaphnia dubia)and copepodites in the water column (and associated withperiphyton on acrylic rods in the water column) were mostabundant in enclosures with high loading. In contrast, densityof microcrustacean grazers associated with macrophytes(predominantly Chydorus spp. and copepodites) increasedin response to the second feces addition only.Microinvertebrate density increased only slightly with lowfeces loading. Community composition showed similar changesover the season in all enclosures, and differences in relativeabundance were not attributable to treatment effects. Given thesmall effects produced by nutrient additions that greatlyexceed natural loadings, nutrients leaching from waterfowlfeces do not appear to have a significant impact on the foodwebof this wetland. This revised version was published online in July 2006 with corrections to the Cover Date.     

EXAMINATION OF TWO DIMENSIONAL SPATIAL PATTERN OF PERIPHYTIC DIATOMS USING AN ADHESIVE SURFICIAL PEEL TECHNIQUE1

A simple surficial peel technique using adhesive tape was developed for quantitative removal of haptobenthic diatom communities from topographically simple substrata. The method combines high removal efficiency with low peel distortion, permitting the use of spatial statistics to test whether populations are distributed in the peel randomly or form aggregated or uniform patterns. Using this technique, the microdistribution of Cocconeis placentula Ehr. on a smooth acrylic rod was examined. Using conventional nearest neighbor analyses, a clonal population of C. placentula. characterized by an indentation of the value margin, was significantly aggregated, whereas the overall C. placentula population was uniform or aggregated depending on whether the method of analysis allowed for cell size. Using refined nearest neighbor analysis, the indented population was aggregated, and the overall population was random at distances greater than cell size. The results suggest that the indented clone was weakly motile following cell division and that its directional bearing was random.     

Effect of an aquatic herbicide on sediment nutrient flux in a freshwater marsh

Three concentrations of the herbicide simazine were added to in situ macrophyte-free enclosures with and without sediment contact. Changes in the concentrations of total ammonia, total reactive phosphorus, and silicon were monitored, and net sediment flux was calculated from the difference in nutrient concentration between bottomed and unbottomed enclosures. Rates of sediment release for all three nutrients were unaltered by 0.1 mg · l^–1 simazine in relation to a control, whereas rates were increased proportionally at 1.0 and 5.0 mg · l^–1. These results suggest that increases in dissolved nutrients commonly observed following herbicide treatment of shallow waters may not be attributable solely to macrophyte decay, byt may also involve a complex interaction of biotic and abiotic sediment nutrient exchange processes.Contribution Number 103 from the University of Manitoba Field Station, Delta Marsh, Canada     

Cloning and partial DNA sequencing of two new human papillomavirus types associated with condylomas and low-grade cervical neoplasia.

Using low-stringency Southern blot analysis and cloning in lambda bacteriophage, two new human papillomavirus types (HPV-43 and HPV-44) were identified and their DNAs were cloned from vulvar tissues. The isolates were characterized by restriction endonuclease mapping and shown to be new HPV types on the basis of their minimal hybridization with all other known HPV types at high stringency. Both HPVs are most closely related to types 6, 11, and 13. HPV-43 did not exhibit any cross-reactivity with these HPV types at high stringency. HPV-44 showed minimal cross-reactivity to HPV-13, which was in the range of 20 to 25% according to liquid hybridization analysis. The deduced genomic organization of each of the two new HPVs was colinear with HPV-6b. Prevalence studies revealed that HPV-43 and HPV-44 together were found in 6 of 439 normal cervical tissues, in 8 of 195 cervical intraepithelial neoplasms, but in none of 56 cervical cancers tested thus far.     

Characterization of murine A-raf, a new oncogene related to the v-raf oncogene.

A 1.6-kilobase cDNA (A-raf) has been isolated from a murine spleen cDNA library which encodes part of a protein related to the raf oncogene. Its amino acid sequence has 85% homology to raf in a central portion of 100 amino acids. In contrast to raf, A-raf shows a highly restricted tissue distribution of expression, with highest levels observed in epididymis, followed by intestine. When incorporated into a retrovirus, the resulting gag-A-raf fusion gene causes transformation in vitro and induces tumors in newborn mice. Thus, A-raf represents a new proto-oncogene. Transformation by A-raf is independent of ras gene function, as is the case for raf and mos but not other oncogenes.     

Inhibition of Glutathione Peroxidase Mediates the Collateral Sensitivity of Multidrug-resistant Cells to Tiopronin

Multidrug resistance (MDR) is a major obstacle to the successful chemotherapy of cancer. MDR is often the result of overexpression of ATP-binding cassette transporters following chemotherapy. A common ATP-binding cassette transporter that is overexpressed in MDR cancer cells is P-glycoprotein, which actively effluxes drugs against a concentration gradient, producing an MDR phenotype. Collateral sensitivity (CS), a phenomenon of drug hypersensitivity, is defined as the ability of certain compounds to selectively target MDR cells, but not the drug-sensitive parent cells from which they were derived. The drug tiopronin has been previously shown to elicit CS. However, unlike other CS agents, the mechanism of action was not dependent on the expression of P-glycoprotein in MDR cells. We have determined that the CS activity of tiopronin is mediated by the generation of reactive oxygen species (ROS) and that CS can be reversed by a variety of ROS-scavenging compounds. Specifically, selective toxicity of tiopronin toward MDR cells is achieved by inhibition of glutathione peroxidase (GPx), and the mode of inhibition of GPx1 by tiopronin is shown in this report. Why MDR cells are particularly sensitive to ROS is discussed, as is the difficulty in exploiting this hypersensitivity to tiopronin in the clinic.