The Physics Police

The Physics Police

Thursday, October 3, 2013

Glyphosate and Aflatoxin

Misrepresentation of science really pisses me off. I'm tired of talking about GMOs. These days, though, the topic is just too ripe with science abuse for me to ignore. Sayer Ji of Green Med Info is one of the worst offenders.

In a recent blog post titled Study Links Roundup 'Weedkiller' To Overgrowth of Deadly Fungal Toxins, he references a study published by an Argentinian team in the Journal of Environmental Science and Health which concludes:
... that these Aspergillus flavus and A. parasiticus strains are able to grow effectively and produce aflatoxins in high nutrient status media over a range of glyphosate concentrations under different water activity conditions.
It's not surprising that fungus can grow in high nutrient media, with or without the presence of an herbicide. You see, glyphosate isn't simply a poison. It's a chemical that interferes with the production of aromatic amino acids in plants, and only in plants. It is not a fungicide, so it doesn't kill fungus!

The interesting thing about this study is that the growth of these fungi was actually increased by glyphosate. This stimulation of soil biology is a feature of glyphosate, not a bug. The proliferating fungi attack dying weeds, completing the nitrogen cycle.

It's also important to notice that while growth was stimulated, aflatoxin production was not:
Aflatoxin B1 production did not show noticeable differences among different pesticide concentrations assayed at all aW in both strains.
So, why did glyphosate stimulate these fungi to grow, anyway? It's possible that growth stimulation may have been nutritional, not enzymatic. Before administering the glyphosate, fungi were starved overnight to halt their growth. This is standard practice in microbiology to establish an adjacent control colony which experienced identical conditions.

If the growth-limiting nutrients in this study were phosphorus or carbon, then digestion alone could explain the stimulated growth. Yes, fungi can eat glyphosate!

That raises the question, what were the concentrations of glyphosate used in this study?
The glyphosate increased significantly the growth of all Aspergillus section Flavi strains in different percentages with respect to control depending on pesticide concentration. At 5.0 and 10 mM this fact was more evident; however significant differences between both concentrations were not observed in most strains.
That unusual unit mM stands for millimolar. So, is this dose environmentally relevant?
(10 mM) * (169.07 g/mol) = 1,691 mg/L = 1,691 ppm
That's a thousand times larger than the tolerance on corn forage, 13 ppm!

So, we certainly can't conclude anything about human exposure, in food, from this in vitro study.

Nevertheless, Ji concludes that glyphosate is:
... seriously undermining the quality of our global food supply, and may help to explain recent observations that GM corn heavy markets, such as the U.S., have a significant aflatoxin problem.
This is a strong accusation, not at all justified from this one study. Let's take a look at another, older study published in 2007 by the USDA. Like the Argentinian study, this one looked at fungi aflatoxin production in a Petri Dish. They didn't find any.
No aflatoxin production was detected on water agar regardless of strain or glyphosate treatment.
They also tested glyphosate on soil fungi. It didn't effect them, and they explained why.
In soil, no effect of glyphosate was observed on the recovery of A. flavus cfu from soil regardless of rate, formulation or time after treatment. Glyphosate is readily bound by soil colloids and less material would be bioavailable to interfere with growth.
Finally, they looked for significant contamination in real-life corn fields. They didn't find any.
Although mycotoxin contamination was minimal in the 4 years of this study, there was no evidence that rotation of corn with cotton had any affect on reducing inoculum potential in soil and there was no consistent effect of glyphosate on propagule density of A. flavus. 
Clearly, the use of glyphosate on corn crops doesn't put humans at risk from aflatoxin poisoning.

Then, what does cause dangerously high aflatoxin production? According to Cornell:
Water stress, high-temperature stress, and insect damage of the host plant are major determining factors in mold infestation and toxin production. Similarly, specific crop growth stages, poor fertility, high crop densities, and weed competition have been associated with increased mold growth and toxin production
Weed competition! Hey, you know what's good at fighting weeds? Herbicides.

Herbicides kill plants, not fungus.

Or people!

1 comment:

  1. I found another relevant study from 2010.

    "Aflatoxin and fumonisin were relatively low in all plots. Herbicide application or the lack thereof had no negative impact on the incidence of kernel contamination by these two mycotoxins."