THE EFFECT OF PB, ZN, CU AND NI ON THE EMBRYONIC AND LARVAL STAGES OF S. PURPURATUS

Abstract

<p>In light of the paucity of information on the toxicity of lead (Pb), zinc (Zn), copper (Cu) and nickel (Ni) in the marine environment, the aim of this thesis was to generate data on the mechanisms of toxicity of Pb, Zn, Cu and Ni in the early life stages of a very sensitive marine organism, the purple sea urchin (<em>Strongylocentrotus purpuratus</em>). Previous studies in other systems indicated that these metals can impact ionoregulation, especially calcium homeostasis, so an initial study focused on ionoregulatory changes during the first 96h of development under control conditions in embryonic and larval stages of <em>S. purpuratus</em>. Control tests showed that patterns of Ca, K, Na and Mg had an interesting pattern of accumulation over 96 h development and that the most pronounced changes were observed during the gastrulation stage. A variety of biological endpoints were subsequently utilized to examine potential mechanisms of toxicity. Toxicity tests were performed to determine median toxicity threshold values (i.e. EC50: median effective concentration and LA50: median lethal accumulation) for Zn and Pb. Growth, unidirectional Ca uptake rates, whole body ion concentrations (Na, K, Ca, Mg), and Ca ATPase activity were also monitored every 12 h over the first 84 or 96 h of early development to investigate the mechanisms of toxicity during acute and chronic exposures to lethal and sublethal concentrations of Pb, Zn, Cu and Ni in 100% sea water.</p> <p>Sea urchin embryos were very sensitive to Zn with an EC50 of 2.3 µmol/L (95% C.I. = 1.97–2.71 μmol/L) and LA50 of 4.8 (2.16-11.33) μmol/kg. Embryos displayed even higher sensitivity to Pb with an EC50 of 0.36 (0.25-0.49) µmol/L and LA50 of 1.92 (1.67-2.78) μmol/kg. The toxic effects of these metals were increased when tested in combination with extracts of dissolved organic carbon (DOC) from marine and freshwater sources.</p> <p>From studying larvae chronically exposed to Pb (60 µg/L), Zn (139 µg/L), Cu (6 µg/L) and Ni (47 µg/L) it was apparent that these metals rendered their toxic effects, at least in part, through disruption of Ca homeostasis. Unidirectional Ca uptake rates as well as Ca ATPase activity were significantly inhibited at various time points over development, in larvae in these metal exposures. This resulted in significantly lower levels of Ca accumulated in the larvae. Interestingly, larvae showed some capacity for recovery as Ca uptake rates and internal Ca levels returned to control values periodically over development. Also metal initially accumulated in the larvae often returned to control levels at 72 h of development with the exception of Pb, which was the only non essential metal out of the four tested. Metal effects on the whole body levels of three other ions (Na, K, Mg) as well as larval weight were modest. Surprisingly, acute exposure to much higher levels of these same metals at various stages during development had negligible inhibitory effects on unidirectional Ca uptake rate, suggesting a mechanism other than direct competition for the Ca uptake sites. We propose studying the toxicity of contaminants periodically over development as an effective way to detect sub-lethal effects, which may not be displayed at the traditional endpoint of 72 h.</p>