Genetically modified organisms (GMOs) are back in the news. A few days ago, NPR featured a couple of blog posts (here and here) considering whether the new GMO “supersized” salmon will be harmful to aquatic ecosystems.
A concern with GMOs is that—like the early adoption of pesticides—potential risks are being borne by the environment and consumers as we experiment with new species. There’s a lot of potential for GMOs, and I hope that they all end up being harmless. But there are potential downsides too that we are not able to assess very well at this point. And we may be creating problems that we are not even aware of yet.
As more data come in, it’s not always an encouraging outlook. A couple of recent examples:
Case #1: We saw a few months ago how weeds that were supposed to be eliminated by the agricultural herbicide, Roundup, are now evolving resistance to the chemical, meaning that Roundup-ready soybeans and other crops no longer work as designed.
Case #2: In this week’s Early Edition of the Proceedings of the National Academy of Sciences, Jennifer Tank and colleagues examined what happens to transgenic corn residue (old crop parts left on fields that are not harvested). One of the main transgenic varieties of corn is known as “Bt corn.” Bt stands for the name of a microbe—Bacillus thuringiensis—that makes a protein toxin that destroys the functioning of guts in some insects. Scientists have figured out how to move the Bt gene, and hence Bt toxin manufacturing capacity, from the bacteria to corn plants, thereby conferring general insect herbivore resistance to this crop (the main pest being the European corn borer).
This team asked: What happens when corn stalks, cobs, and leaves end up in streams and rivers throughout the Midwest? Their answer is eye-opening:
Widespread planting of maize throughout the agricultural Midwest may result in detritus entering adjacent stream ecosystems, and 63% of the 2009 US maize crop was genetically modified to express insecticidal Cry proteins derived from Bacillus thuringiensis. Six months after harvest, we conducted a synoptic survey of 217 stream sites in Indiana to determine the extent of maize detritus and presence of Cry1Ab protein in the stream network. We found that 86% of stream sites contained maize leaves, cobs, husks, and/or stalks in the active stream channel. We also detected Cry1Ab protein in stream-channel maize at 13% of sites and in the water column at 23% of sites. We found that 82% of stream sites were adjacent to maize fields, and Geographical Information Systems analyses indicated that 100% of sites containing Cry1Ab-positive detritus in the active stream channel had maize planted within 500 m during the previous crop year. Maize detritus likely enters streams throughout the Corn Belt; using US Department of Agriculture land cover data, we estimate that 91% of the 256,446 km of streams/rivers in Iowa, Illinois, and Indiana are located within 500 m of a maize field. Maize detritus is common in low-gradient stream channels in northwestern Indiana, and Cry1Ab proteins persist in maize leaves and can be measured in the water column even 6 mo after harvest. Hence, maize detritus, and associated Cry1Ab proteins, are widely distributed and persistent in the headwater streams of a Corn Belt landscape.
Who cares? Streams and rivers are the breeding grounds to many insect species, including dragonflies, mayflies, and damselflies. If there are toxins floating in these aquatic ecosystems that are good at killing insects, there is risk of disrupting food webs, including potential changes to bird species as well as many important recreational and sport fish that dine on insects:
Once maize detritus enters stream channels, this carbon source degrades rapidly via a combination of microbial decomposition, physical breakdown, and invertebrate consumption, and that energy may fuel stream food webs. Maize detritus in agricultural streams decomposes in ∼66 d …. Therefore, the material that we found during our synoptic survey had entered these streams relatively recently. Maize detritus is rapidly colonized by stream-dwelling invertebrates, and growth rates of invertebrates feeding on nontransgenic decomposing maize are comparable to those feeding on the deciduous leaf litter commonly found in forested streams
Perhaps this means that the Bt toxins might break down quickly and pose less harm? Doesn’t look like it:
Our data demonstrate that long after harvest, Cry1Ab is present in submerged Bt maize detritus; thus, stream organisms may be exposed to Cry1Ab for several months.
It’s also interesting to learn that low or no-till conservation tillage practices may exacerbate the corn residue inputs because greater material left on fields is susceptible to washing away:
The dried detritus left on fields after harvest, as part of conservation tillage, enters headwater streams as a result of surface runoff and/or wind events occurring throughout the year. During heavy precipitation, overland flow is the likely mechanism transporting this material to stream channels.
It may not even be a matter of leaving less residue; the toxins also appear to be draining through the soils:
Our results from tile drains indicate that tiles may be a mechanism by which Cry1Ab leached from detritus on fields or from soils can be transported to streams.
Cry1Ab released from root exudates or decaying maize detritus moves vertically through soils and can be detected at the base of 15-cm-long soil profiles for up to 9 h.
Their conclusion? An illustration of how little we know at this point:
The question of whether the concentrations of Cry1Ab protein we report in this study have any effects on nontarget organisms merits further study.
Jennifer L. Tank, Emma J. Rosi-Marshall, Todd V. Royer, Matt R. Whiles, Natalie A. Griffiths, Therese C. Frauendorf, and David J. Treering (2010). Occurrence of maize detritus and a transgenic insecticidal protein (Cry1Ab) within the stream network of an agricultural landscape Proceedings of the National Academy of Sciences : 10.1073/pnas.1006925107
Photo credit: snake.eyes
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