Water Pollution: A hazardous cyanobacterium common in lakes turns on estrogen-related genes in fish
By Janet Pelley
Fertilizer runoff from farms can feed blooms of toxic cyanobacteria, which are deadly to pets and livestock. As if that weren’t enough to worry about, new research suggests for the first time that the blooms also could disrupt reproduction in aquatic wildlife through estrogenic effects (Environ. Sci. Technol., DOI: 10.1021/es103538b).
Until now, most studies of Microcystis aeruginosa, the most common bloom-forming cyanobacterium, have focused on the bacterium’s 80 or so microcystin toxins, says Ted Henry, an ecotoxicologist at the University of Plymouth, in the U.K. The toxins can cause massive internal bleeding and liver damage in mammals and fish.
Henry, Emily Rogers of the University of Tennessee, Knoxville, and their colleagues stumbled on the estrogenic actions when they were looking for genes that turn on when fish encounter Microcystis. They hoped that these genes could serve as biomarkers for bloom events in lakes when biologists observe fish die offs. “We wanted to develop a short list of genes that would determine whether a fish had been exposed to Microcystis or not,” Henry says.
To find the genes, the scientists treated one group of larval zebrafish (Danio rerio) with microcystin-LR, the most toxic microcystin toxin, and treated another group with dried and reconstituted Microcystis cells, which are more convenient than live cells to use in experiments. They then measured gene expression levels in the fish.
Not surprisingly, the microcystin-LR boosted the expression of genes that the liver uses to detoxify itself. But the big surprise was that fish exposed to Microcystis cells had 19 to 100 times greater expression levels of the vitellogenin gene than unexposed fish did.
Kick-started by estrogen, vitellogenin is a protein that mature female fish produce for their egg yolks. Usually, when larval fish produce vitellogenin, scientists take it as a sign of estrogen pollution in the environment. Scientists often attribute this pollution to human or animal hormones released by sewage and farm runoff. Henry says that estrogens in the environment can disrupt fish reproduction and possibly lead to population decline.
Henry and his team concluded that the cause of vitellogenin production wasn’t the microcystin toxin, because fish exposed to the toxin alone showed no change in the gene’s expression. Charles Tyler, an ecotoxicologist at the University of Exeter, in the U.K., says that the microbes might release estrogen-like chemicals called phytoestrogens, just as plants and fungi do. But Tyler wonders to what extent Microcystis induces an estrogenic response in fish in the wild, because the study used dried and reconstituted cells, not live ones.
Still, Allen Place, a biochemist of the University of Maryland’s Center for Environmental Science, thinks that the study will make other researchers pause and consider naturally occurring molecules, rather than only manmade pollution, as the source of estrogens in the environment.