Sometimes I see fiddleheads for sale in the grocery store. I wonder about the fiddleheads in my field of ferns, succulent and… loaded with arsenic. If somebody harvested ferns from an arsenic phytoextraction remediation project, how much fern would they have to eat to harm themselves, or somebody else?
Let’s use me as an example. The lethal dose of arsenic is 0.6 mg arsenic per kilogram body weight per day. This is estimated for humans but based on rat studies, since it is illegal and highly unethical to do toxicity studies on humans. I weigh about 68 kilograms, so my lethal dose is 41 mg arsenic a day (likely 1-4 days, until I succumb to the effects). The ferns in my studies contain about 1000 mg arsenic per kilogram dried plant tissue, so I would have to eat 41 grams of dried fern. That’s well over half of one of the huge ferns in my field study! I don’t think I could eat that much lettuce or collards, let along toxic plant tissue, in one day, let alone days on end. It would be very hard for somebody to acutely poison themselves (or somebody else) eating P. vittata from a phytoextraction project. What a relief!
However, say a 1,200 lb (545 kg) cow developed a taste for ferns. Perhaps the gate was left open and the local cow herd wandered in (urban agriculture, anyone?). If that cow grazed to its hearts content in a phytoextraction field, it would consume the equivalent of 24 lbs (11 kg) per day of dried plant tissue. (That’s the amount of dry matter cows consume as fodder, according to the University of Nebraska.) If they were eating the ferns with 1000 mg arsenic/dry kilogram, they would quickly consume the lethal dose of arsenic, which would be 327 mg per day. In fact, in one day they would consume about three times that much!
Even the common bracken fern is poisonous to cattle, however. Pteridium aquilinum, common in the western US, contains toxins which can kill cattle and horses and harm goats after several weeks of grazing or eating infested hay. A chemical called ptaquiloside causes bleeding and damages bone marrow in cattle, while bracken fern causes thiamin deficiency in horses. Bracken fern could be carcinogenic, and milk from cows grazing bracken fern could be hazardous to humans.
Fortunately, it seems cattle or horses avoid grazing on ferns if given enough other options, regardless of how much arsenic they contain. Deer also dislike ferns, and rabbits will avoid ferns except maidenhair. Our fern field is safe from these critters, who in turn are safe from arsenic poisoning!
Sarick Matzen completed his PhD in Environmental Science, Policy, and Management department at University of California, Berkeley in 2020. He is now a postdoc in the Soil, Water, and Climate Department at the University of Minnesota working on iron cycling in marine systems.