Doctoral Dissertations

Date of Award

5-1992

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Life Sciences

Major Professor

B. Gordon Blaylock

Committee Members

Walter Farkas, Owen Hoffman, Lee Shugart, Richard Strange

Abstract

Selenium is a naturally occuring, essential, trace element in aquatic ecosystems but becomes toxic at concentrations slightly above its nutritional levels. Recent research at various reservoirs and lakes associated with coal­ fired power plants in the United States has implicated selenium bioaccumulation in fish and waterfowl as the cause for lethalities and deformities in larvae and hatchlings and the inhibition of growth and fecundity in adults. The objectives of this research were to: i) quantify and compare the cycling and fate of selenium in an aquatic ecosystem utilizing selenite and selenomethionine; ii) determine the relative toxicities of selenite and selenomethionine using both an acute static toxicity test upon adult Pimephales promelas (fathead minnow) and a partial life cycle study using the eggs and larvae of Cyprinus carpio (common carp); and iii) determine whether chronic sub-lethal concentrations of waterborne selenium altered the 5'-methyl cytosine methylation percentage in fathead minnow DNA.

These objectives were accomplished through three experiments. An acute release of either 75Se-selenite or 75Se-selenomethionine was made into one of two outdoor experimental ponds. Biotic and abiotic compartments were sampled throughout the 318-d experiment. Results from this experiment showed that the uptake rates in the biotic compartments for selenomethionine were at least an order of magnitude greater than for selenite but the elimination rates for both selenium species were similar. The data further suggest that both rooted aquatic plants and sediment may be an important secondary sources of selenium rather than terminal storage sites in some shallow freshwater systems.

The toxicity studies showed that waterborne selenomethionine is more toxic to adult fathead minnows than selenite with LC50-120 h values of 0.14 mg/L and 2.33 mg/L, selenomethionine and selenite, respectively. Results also showed that waterborne selenomethionine and selenite are incorporated and concentrated within developing carp embryos and that selenomethionine is more toxic to developing carp embryos and carp larvae than selenite.

A study investigating a molecular level effect of selenium showed that the fathead minnow DNA methylation percentage of the minor nucleoside 5m-dcyd was hypermethylated after a 60-d exposure to sublethal waterborne concentrations of selenium. This finding suggests that the fathead minnow is responding to the toxicant stress by reducing DNA genome expression. The hypermethylation also suggests that reduced gene expression may increase destabilization of the fish homeostatic response mechanisms.

Selenium is both an essential element and a toxicant to aquatic organisms with slight differences between the two concentration doses. Because of this slight variance between the two concentrations, the examination of various biological response levels (ecosystem to enzymatic activity) to Se exposure can be used to explain the effects of Se on aquatic organisms. The levels used in this study included the measurement of macro­ molecular, organism response to selenium in a variety of aquatic organisms to a specific molecular response in a representative teleost. Each response level was unique and offered important findings which are presented in this dissertation.

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