Tire rubber leaves a chemical trace on roads, and rain carries it into streams. That trace, a car tire transformation product called 6PPD-quinone, is highly toxic to juvenile and adult coho salmon, and urban stormwater runoff frequently contains it. Large-scale die-offs of coho in urbanized streams have been tied to the pollutant.
What nobody had pinned down is what it does to the youngest fish. A 2023 study in Environmental Science & Technology set out to answer that for embryos.
How the experiment worked
Researchers took developing coho salmon embryos at the eyed stage and exposed them to three concentrations of 6PPD-quinone. The dosing came twice a week and continued until hatch. They then tracked survival and growth. Recently hatched alevin also went through whole-transcriptome sequencing, a way to look at which genes switch on or off, in an effort to find the mechanism behind the toxicity.
What they found
Embryos did not die at the concentrations measured in the environment, even though those same levels kill juveniles and adults. Their growth was stunted, though.
The protection ended at hatching. Right after they emerged, the fish became vulnerable to the chemical’s lethal effects. The authors say this points to a long window of risk for young coho before smoltification, the stage when juvenile salmon get ready for life in saltwater.
A clue in the genes
At the molecular level, 6PPD-quinone caused dose-dependent changes. Pathways that govern contacts between cells and the permeability of the endothelium, the lining of blood vessels, were broadly disrupted.
That fits earlier observations that large molecules build up in the brains of coho exposed to the chemical. The authors suggest that breakdown of the blood-brain barrier may be one route by which the toxin does harm.
Their overall conclusion is that coho exposed to 6PPD-quinone while developing face a risk of health problems after hatching.
