UCSD Biologist Marty Yanofsky discuses crops as a threat to the ozone layer on The Australian Broadcasting Corporation's "Science Show"

MAY 1, 2004
Media Contact: Kim McDonald (858) 534-7572

Halides and the Ozone Layer


Now that fluorocarbons have been removed from the bulk of manufactured goods another threat to the ozone layer has been identified - this time from crops.

Program Transcript

Robyn Williams: One of the great successes of environmental prudence in recent years has been controlling the chemicals that eat the ozone layer - the Montreal Protocol saw to that. But while we've been converting our fridges to ozone-friendly chemicals another source of damaging halides has been burgeoning out there on the farm.

This was discovered by putting geneticists together with atmospheric physicists at the University of California, San Diego. And here they are, Lars Ostergaard from Denmark, Martin Yanovsky, Eric Saltzman and Robert Rhew.

Marty Yanofsky: Okay, so methyl bromide, methyl chloride and methyl iodide are environmentally and economically important compounds in the atmosphere. Bromide and methyl chloride are the primary sources of bromine and natural chlorine to the stratosphere where they can break apart and catalyse the destruction of stratospheric ozone, and that of course is a very important issue nowadays. Now since 1950s they've been far superseded by human produced compounds such as chlorofluorocarbons and halides, which are responsible for the current problem in the stratosphere, it goes into pollution.

But methyl bromide and methyl chloride, unlike the CFCs in halides have both human and natural sources. Among the human sources they're used widely and because of their ozone depleting potential, they've been banned according to the Montreal Protocol. Among the natural sources there are oceans and biomass burning, woodrot fungi, and apparently terrestrial plants as well. And the big question is, how much has been produced by nature and how much is being produced by humans?

And so there are two ways to address this. You can either add up all the known sources or you can add up all the known sinks. So we've added up all the known destruction mechanisms and they greatly outweigh the known sources. And so therefore there is some imbalance in our understanding of the budgets of these compounds, and therefore there is a huge missing source that is presumably natural and presumably terrestrial in origin.

Robyn Williams: How long has it been known that it was a plant source really?

Marty Yanofsky: How long has it been known? Probably for about a decade was the first realisation that certain plants can produce some methyl halides. And so there is no real fundamental understanding of why plants produce these compounds, what their production mechanisms are, or an ability to predict what other types of plants will produce it. So we didn't really have a fundamental understanding of the biology of methyl halide production.

Robyn Williams: Well, that's a problem. How did you set about looking in this department into which plants are producing what halides?

Robert Rhew: Well, our favourite plant is a plant called Arabidopsis, which is a close relative of cauliflower and brussel sprouts and broccoli and so forth. And it's used in virtually all laboratories worldwide.

Robyn Williams: It's a sort of fruitfly of botany.

Robert Rhew: That's right, it's the fruitfly of botany or E.coli of botany. This particular study I think is interesting in that it's rather unusual to bring together two very different disciplines: the atmospheric chemist with plant molecular geneticists. Usually they don't talk to each other, and so here we have two people, Robert and Lars, they got together kind of as a friendship and started talking about the possibility of using molecular genetics in Arabidopsis to explore methyl halide production in plants.

Robyn Williams: So far any clues as to why plants use halides?

Lars Ostergaard: One of the speculations is that it is involved in some kind of defence mechanism for the plant, since it is able to convert compounds that are Finvolving defence mechanisms to properly protect the plant against the insect attacks, pathogens. But we don't know anything about that for sure yet.

Robyn Williams: Have you any reason to believe that all plants produce these halides?

Lars Ostergaard: Yes we do. We have done a comparison of genomes between different plants. The entire genome of the Arabidopsis is known, and we have taken the whole gene and compared it to genomes from other plants, and we have found it in virtually all the plants that we have looked at. So that actually ranges from cabbage, barley, maize and rice also.

Robyn Williams: And of course 'halides' means the group including fluorine, chlorine and so on. Is there any indication as to which kind of chemical is produced by plants or do they produce pretty well the entire group?

Marty Yanofsky: Well, we didn't mention methyl fluoride production, but it's very unlikely that plants produce methyl fluoride, but we did measure the other halides. So, we did measure methyl bromide, methyl chloride and methyl iodide and Arabidopsis produces these compounds like gangbusters.

As to whether other plants produce these compounds as well, well the presence of the gene appears to be nearly ubiquitous. But the real question is, will these other plants express this gene as well.

Robyn Williams: Given that so many plants produce the halides, presumably they're having some sort of effect on the ozone layer, whether it's a profound one or whether it's one that in normal circumstances makes no difference. What sort of conclusions have you reached so far on that?

Marty Yanofsky: It appears that crops, Brassica crops in particular are a globally significant source of methyl halides. And already we've been able to link it with rice plants, which are a globally significant source of methyl iodide and methyl chloride, and we're also linking it to other plants that may turn out to be globally significant sources as well. ++And as for the question of how much are terrestrial plants significant in terms of the global methyl halide budget, they are very significant. They may account for upwards of 20 to 50%, I think.

Robyn Williams: 20 to 50% is huge.

Marty Yanofsky: But biomass burning is a major source of methyl chloride, oceanic and biological production is a major source of methyl bromide and methyl chloride. And then there is the industrial use of methyl bromide, which is estimated to be about 20 to 40%.

Robyn Williams: If you look at the population of the world doubling, say, presumably agriculture has got to increase. If you double these crops, the brassicas, the rice and so forth, isn't that likely to have an effect as well on the ozone layer?

Marty Yanofsky: Absolutely, that's one of the questions that we're trying to address is, how much are agricultural crops increasing, the amount of methyl halides in the atmosphere, and well, that's one of the reasons why we need to figure out how much of it is due to crops versus natural sources versus industrial and agricultural fumigation sources.

Robyn Williams: Would it be out of the question to imagine a genetically manipulated crop that reduces halides, and therefore makes the problem to some extent go away?

Lars Ostergaard: It's difficult, but it would not be impossible, but we would have to know what the effects are, for instance, on resistance in these plants against pathogens and insects as talked about before.

Guests on this program:

Martin Yanofsky
Professor of Biology
Division of Biological Sciences
University of California
San Diego

Lars Ostergaard
University of California

Robert Rhew
University of California

Further information:

Research - Rhew Lab

Presenter: Robyn Williams
Producer: Polly Rickard