Effects of Submerged Vegetation on Water Clarity Across Climates

A positive feedback between submerged vegetation and water clarity forms the backbone of the alternative state theory in shallow lakes. The water clearing effect of aquatic vegetation may be caused by different physical, chemical, and biological mechanisms and has been studied mainly in temperate lakes. Recent work suggests differences in biotic interactions between (sub)tropical and cooler lakes might result in a less pronounced clearing effect in the (sub)tropics. To assess whether the effect of submerged vegetation changes with climate, we sampled 83 lakes over a gradient ranging from the tundra to the tropics in South America. Judged from a comparison of water clarity inside and outside vegetation beds, the vegetation appeared to have a similar positive effect on the water clarity across all climatic regions studied. However, the local clearing effect of vegetation decreased steeply with the contribution of humic substances to the underwater light attenuation. Looking at turbidity on a whole-lake scale, results were more difficult to interpret. Although lakes with abundant vegetation (>30%) were generally clear, sparsely vegetated lakes differed widely in clarity. Overall, the effect of vegetation on water clarity in our lakes appears to be smaller than that found in various Northern hemisphere studies. This might be explained by differences in fish communities and their relation to vegetation. For instance, unlike in Northern hemisphere studies, we find no clear relation between vegetation coverage and fish abundance or their diet preference. High densities of omnivorous fish and coinciding low grazing pressures on phytoplankton in the (sub)tropics may, furthermore, weaken the effect of vegetation on water clarity.     

Effects of suramin on the immune responses to sheep red blood cells in mice. I. In vivo studies

The effect of Suramin on the cell-mediated delayed-type hypersensitivity (DTH) and the humoral immune responses elicited in mice by sheep erythrocytes was studied. The results show that administration of Suramin, at various times before or after antigenic sensitization, results in a profound inhibition of cell-mediated responses but has no adverse effect on antibody production. Suramin was particularly effective when given during the effector phase of DTH: mice which were treated with this drug, 4 days after immunization, at the time of skin testing, exhibit negative or low DTH responses compared to control mice. Evidence is presented that this short-term Suramin-induced suppressive effect on the expression of DTH is related to a defective recruitment, by sensitized T lymphocytes, of phagocytic cells at the site of the inflammatory reaction. In addition, when treatment with Suramin precedes by 8 days (Day -8) or by 1 hr sensitization with sheep erythrocytes for DTH, decreased DTH reactions over controls were observed. The inhibitory effect exerted by Suramin administered on Day -8 can be reversed by increasing the dose, from 10(6) to 10(8) sheep erythrocytes, of the sensitizing antigen. The possibility is discussed that, in this case, Suramin may interfere with the generation of DTH-mediating cells through a rapid degradation of antigen related to the Suramin-induced hyperplasia of the mononuclear phagocyte system. In contrast, DTH anergy in mice treated with Suramin 1 hr before sensitization is maintained regardless of the sensitizing antigen dose. Analysis of the sensitized lymphocyte population in these mice indicates that Suramin does not prevent the induction of DTH-mediating cells and suggests that the expression of these latter is inhibited by suppressive cells which are generated as a result of drug treatment.     

Tail-flagging and other antipredator signals in white-tailed deer: new data and synthesis

We present a series of predictions concerning the costs andbenefits of antipredator behavior in ungulates and then testthem with data on white-tailed deer reacting to a human on foot.Costs of tail-flagging were apparently low and no data supportedthe idea that flagging serves as a warning signal to conspecifics,in either this or in other studies. Flagging deer fled at greaterspeeds than nonflaggers, indicating that flagging could possiblysignal prey's ability to escape. Dropping the tail at the endof the flight may additionally have made deer inconspicuous.Snorting did not appear directed at conspecifics, and comparativedata suggest that it signals that the predator has been detected.In contrast, foot-stamping was effective in alerting other deerto the observer's presence. Deer may have bounded to clear obstaclesalong their flight path. These preliminary data indicate thatseveral aspects of antipredator behavior in white-tailed deermay be pursuit-deterrent signals, and they therefore highlightthe necessity of observing natural predators' reactions to signalsgiven by deer in future studies.     

A Comparison Between Different Methods for the Determination of Reduced and Oxidized Glutathione in Mammalian Tissues

In this study, three rapid assay techniques for the determination of glutathione, one enzymatic, one flu-orometric and one newly patented colorimetric method, were compared by measuring reduced (GSH) and oxidized (GSSG) glutathione in guinea-pig heart and liver. The HPLC technique was used as a standard, since it is considered the most reliable assay method. In heart, all methods measured the same levels of GSH (about 1 µmole/g wet tissue), whereas in liver the fluorometric assay gave GSH levels about half as high as those measured by the other methods (about 3 vs. 7 µmoles/g wet tissue). Conversely, the fluorometric assay grossly overestimated GSSG concentration (by 5 to 8 times) in both heart and liver. These results confirm previous doubts about the use of the fluorometric technique for GSSC determination in mammalian tissues and also raise some questions about its use for the measurement of GSH in liver. In this tissue, the GSH concentration determined by the fluorometric method was shown to be inversely correlated with the size of the sample, suggesting the presence of some quenching material.     

Isolation of Temperature-Sensitive Mutants of Schizosaccharomyces pombe

Seventy-one mutants of the yeast Schizosaccharomyces pombe that were able to grow on complete medium at 25 C but not at 37 C were isolated. Strains selected for further study showed: (i) single gene mutation and (ii) cell lengthening at the restrictive temperature. Preliminary characterization of 13 mutants is reported. Seven of them have a less pronounced synthesis of deoxyribonucleic acid at the restrictive temperature, and four of them seem to be affected in cell division.