Oddly, the post we’ve gotten the most comments on here is my atrazine article awhile back. So I figure now would be a good time to see if there have been any new research in support of or in opposition to atrazine’s (ATZ) toxicity to amphibians.
And in last month’s EHP ( I tend to stick to this journal for a lot of my posts since its readily available and deals with the “big” environmental issues that are easier to blog about) there was another paper linking atrazine exposure to sexual disruption in frogs. Interesting. So what makes this paper stand out? Well since a lot of the conflicting reports on ATZ toxicity are between laboratory studies this new paper analyzes a commercially prepared ATZ mixture and with exposure at environmentally relevant concentrations using outdoor mesocosms. And in addition, the study analyzed a native frog, R. pipiens, to the region where the study was performed. So from all that, looks like whatever the results might be, they will better mimic the potential toxic effects to amphibians compared to the laboratory results which have been so controversial. There were further lab exposures as part of this study but I’ll just focus on the mesocosms since I find that portion then most novel.
Using 100gallon containers furnished with leaves and other organics and filled with groundwater, tadpoles reared from locally collected egg mats were added to each mesocosm. ATZ was tested at a both 0.1ug/L and 1.8ug/L concentration and against 1.5ug/L 17α-ethinylestradiol (EE2) which serves as a positive control. ATZ concentrations tested were selected to mimic the concentration measured in the local river and streams (and it was detected in every stream tested) which ranged from 0.01-1.6ug/L. And even at these very low concentrations, tadpoles were affected by the presence of the herbicide. In the high concentration ATZ group, survival rate was significantly less compared to the control, 66% and 79% respectively. So that shows ATZ is toxic, but is it the notorious endocrine disruptor its made out to be?
There are some indications, that yes, ATZ exposure induces sexual disruption. The first being metamorphosis. Whereas 76% of control tadpoles achieved metamorphic success, only 45% of the low ATZ group and 50% of high ATZ group were successful. Surprisingly in this measurement, ATZ seems to be a more potent estrogen mimic compared to the positive control, in which 55% of EE2 treated tadpoles completed metamorphosis. Furthermore, the high ATZ group shifted the sex ratio to a female bias (1:1.4 male: female) while the control embryos exhibited a male bias population (1:0.6) which is comparable to sex ratios of adult frogs populations at the collection sites.
Now, here is where things get a little tricky and authors are sure to hedge their results. Unlike my last post on atrazine, no frogs from either ATZ treatment showed signs of intersex while 22% of EE2 treated frogs did. But differences between gonadal studies the authors suggest, may come down to differences in experimental design and choice of frog species tested. The authors also tested for a variety of gene expression and enzyme levels but many showed no difference from the control. Chronic low level ATZ exposure did however alter brain estrogen receptor activity and altered activity of liver enzyme that is thought to be related to feminization. These two changes may be responsible for the metamorphic interference observed, but it is still unclear.So looks like atrazine will remain somewhat controversial. There have been some definite observations of function responses to environmentally relevant concentrations of ATZ but the mechanisms which induce endocrine disruption by way of ATZ cannot yet be confirmed. But, this study does offer a picture of real world chronic exposures which are experienced by amphibians. Atrazine, once again, has been shown to be toxic and should be of serious environmental concern.