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Effect of the interaction between two species of marine diatoms on their individual copper tolerance

University of British Columbia

The importance of species interactions in studies assessing the trace metal tolerance of individual species, has been largely overlooked. Most toxicity assessments involve single-species tests. Although relevant in some cases, this approach does not incorporate the biological enviroment of an organism as a factor that might influence its survival. On the other hand, this factor is incorporated in studies at the community level. However, the latter, not examining the mechanisms that drive the interactions in the community, do not allow for prediction of the outcome after the addition of a stress inducer, such as trace metals. The purpose of this study was to examine the effect of the interaction of two species of marine diatoms (Skeletonema costatum (Cleve) Greville and Nitzschia thermalis (Ehrenberg) Auerswald) on their individual copper tolerances. The two species were grown in unialgal cultures in order to determine their copper tolerance. 5. costatum did not exhibit growth above 5 x 10⁻⁷ M (pCu = 8.46) and N. thermalis above 6 x 10⁻⁷M added total copper (pCu = 8.36). Skeletonema exhibited increased growth rate and lag phase with increasing copper concentration (and decreasing pCu). On the other hand, Nitzschia demonstrated decreased growth rate. No effect on lag phase was observed for this species. No difference between the tolerances of two strains of S. costatum was observed. Both strains demonstrated unaffected growth at 1 x 10⁻⁷M (pCu = 9.16) and no growth at 1 x 10⁻⁵ M added total copper (pCu = 7.16). Subsequently, Skeletonema costatum and Nitzschia thermalis were grown together at three copper concentrations (1 x 10⁻⁹ M, 4 x 10⁻⁷M and 5 x 10⁻⁷M added total copper). In the unialgal cultures that were used as controls, the two species grew as predicted from their tolerance tests. However, in the mixed cultures, Nitzschia was the only species that exhibited growth, regardless of the copper concentration in the medium. This inhibition of Skeletonema in the presence of the second diatom, was attributed to a toxic Nitzschia exudate. The effect of the exudate appeared to be temporary, as demonstrated by the extended lag phase and subsequent satisfactory exponential growth rate of Skeletonema. It is suggested that exponential growth rate was resumed because the exudate degrades within a period of five days (= lag phase). It is shown that the interaction between the two diatom species is more important in determining the survival of S. costatwn than its individual copper tolerance. This is not the case for N. thermalis. Such interactions would be unaccounted for in single-species toxicity tests. On the other hand, if they are known, prediction of how a community that includes these two species would respond to copper additions becomes possible.

Text

2010-08-22

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http://hdl.handle.net/2429/27597

Oceanography

University of British Columbia

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