Elevated Temperatures Increase Toxicity of Copper but Decrease that of Oxytetracycline in a Marine Protozoan

by Emil Morhardt

One aspect of increased ocean temperatures is that they may alter the resistance of marine organisms to pollutants. In a paper just published, such was found to be case for the marine protozoan, Euplotes crassus, that lives on the ocean floor where particulate pollutants get deposited. The protozoans were exposed to two common pollutants—the organic antibiotic oxytetracycline, and the potentially toxic metal, copper—over a range of temperatures. The scientists looked at their effects on survival rate, replication rate, feeding rate (endocytosis) and general of toxic stress (measured as lysosomal membrane stability) all interrelated. Increasing the concentrations of both these toxicants decreased all four measures of protozoan well-being, but in almost all cases the organisms exposed to temperatures as high as 33°C did better than the controls at 25°C when they were exposed to the antibiotic, but less well when exposed to the copper. In other words, while both of these are toxic, and there was a clear dose-response relationship, the antibiotic was less toxic to the warmer protozoans, but the copper was more toxic. Or put another way, increasing seawater temperature mitigated the adverse effects of oxytetracycline, but exacerbated that of copper. The one exception is that in the endocytosis test copper was just barely protective at 31°C, and unprotective at most concentrations at 33°C.

What if they were mixed together? This is important, because there certainly is likely to be more than one pollutant present in contaminated marine environments. Are the effects additive? Did the increased antibiotic offset the negative effect of the copper? There’s no particular reason to think that this might be the case since their physiological mechanisms of toxicity are entirely different. In fact, though, they are additive for the three physiological tests. The authors don’t give this much credence though; it’s their opinion that “…the occurrence of OTC in an enriched Cu aqueous solution acted as a supplementary stress source for the organisms that were already experiencing a loss of health status.” I suspect they don’t much like the idea that any pollution can be a good thing. In any case, the bottom line is that these substances are toxic whether global warming exacerbates it or not.

Gomiero, A., Viarengo, A., 2014. Effects of elevated temperature on the toxicity of copper and oxytetracycline in the marine model, Euplotes crassus: A climate change perspective. Environmental pollution (Barking, Essex: 1987) 194, 262-271. Abstract at http://europepmc.org/abstract/med/25163430

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