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November, 2007

Methane-making plants in the Inner Mongolian steppe

Science News - Although a new study confirms previous findings that plants make methane, this ability may be limited to shrubs.

When, nearly 2 years ago, a study first suggested that plants emit methane, scientists received the news with a flurry of excitement, a dash of skepticism, and hasty speculations on plants' contribution to global warming. A new study is the first to confirm that plants do indeed make this potent greenhouse gas. But it also finds that the methane-making ability varies among types of plants and, at least in the grasslands of Inner Mongolia, is limited to woody shrubs.

In the original study, published in January 2006, Frank Keppler of the Max Planck Institute for Nuclear Physics (Germany) tested 30 temperate and tropical trees and grasses. He found them burping tiny amounts of methane, which added up to global emissions of 62–236 million metric tons per year (Mt/yr), larger than emissions from wetlands—the biggest known source of methane. Subsequent studies by other groups reduced the estimates to 20–60 Mt/yr.

In April 2007, Thomas Dueck of Wageningen University (The Netherlands) cast a shadow of doubt on the validity of Keppler's findings. Unlike Keppler, who used detached leaves, Dueck used intact plants and failed to find biologically significant levels of emissions. Using detached leaves is faulty methodology, says Dueck, because it doesn't rule out the possibility of background methane diffusing out of leaves.

In the current ES&T study, lead author Zhi-Ping Wang of the Chinese Academy of Sciences and his team took a "landscape approach" instead of randomly picking plants as done in previous studies. The team wanted to know whether methane emissions from plants could be important across a vast landscape such as that of Inner Mongolia, says coauthor Jay Gulledge of the Pew Center on Global Climate Change.

The researchers tested 34 upland plants—25 herbs and 9 shrubs—from the Inner Mongolian steppe. None of the herbs showed any sign of methane production, whereas seven of the shrubs emitted methane at levels that varied among the species. And unlike Keppler's work, this study found a carbon isotope fingerprint that explicitly marked the methane as of plant and not microbial origin. However, the total amount of the gas released was insufficient to make a dent in the grasslands' huge carbon-absorbing ability. The findings suggest that methane is likely to be "very important in some ecosystems and not in all others," says Gulledge.

"This is a nice piece of work," says Keppler, referring to the ES&T study. "It also shows how complex plants are—that is, release rates of methane from plants are varying tremendously between species."

And even the skeptical Dueck is impressed. He admits that he may have failed to detect methane in his experiments because he tested only herbs. And he is more convinced by Wang's methods than by Keppler's, because chopping up the leaves, as Wang did, would have released any background methane from them.

Although the study provides "some support" for Keppler's discovery, "the possibility of methane diffusion from plant tissues has not been entirely ruled out by this work," says Michael Keller, chief of sciences at the National Earth Observatory Network (NEON). The results "provide interesting circumstantial evidence but not absolute proof" that the source of the methane is indeed a chemical process in the plant.

Gulledge understands the skepticism. He says the biggest problem with his and Wang's findings, and with those of Keppler, is that the potential physiological mechanism behind their observations remains a mystery. "Until you explain the pathway, you can't be entirely sure what's going on here," he says. Keppler's group is trying to solve this mechanistic mystery. "We have found specific carbon moieties in plants as precursors that are able to form methane under aerobic conditions," Keppler adds, referring to some of his unpublished results.

As for estimating global methane contributions from plants, it is still too early to extrapolate. For now, the fact that plants can indeed synthesize methane in the presence of oxygen is "just very, very huge, from a biological perspective," says Gulledge. "From an environmental perspective, it's probably a less major discovery."

1 Z-P Wang, X-G Han, GG Wang, Y Song & J Gulledge. 2008. Aerobic Methane Emission from Plants in the Inner Mongolia Steppe. Environmental Science Technology 42: 62–68.

Rhitu Chatterjee

 

July, 2007

Methane Mystery

Scientific American - No methane from plants?

German reserachers upturned scientific dogma last year when they reported that plants release the potent greenhouse gas methane [see "Methane, Plants and Climate Change", SciAm, February 2007]. In the April 27 online issue of New Phytolgist, a Dutch group contradicts the findings. Tom A. Dueck, a botanist at Wageningen University in the Netherlands, and his colleagues monitored methane emissions from six species of plants (basil, evening primrose, maize, sage, tomato and wheat) under controlled aerobic conditions. The researchers found no evidence that plants produce methane - not even those that the German group tested.

But methane emission from plants vay tremendously and are dependent on many environmental parameters, asserts Frank Keppler of the Max Planck Institute for Chemistry in Mainz, Germany, who led the team that made the original discovery. "In our opinion, the Dueck et al. study did not cover these parameters sufficiently to allow them to conclude that plants do not produce methane," Keppler says.

The disparate findings should not necessarily come as a surprise, suggests plant biologist Kevin L. Griffin of Columbia University's Lamont-Doherty Earth Observatory. "Conclusions are not always 180 degrees out of phase like this, but they can be, especially when the thought is new, "he says. A step toward solving this discrepancy is to figure out how plants produce methane - if, that is, they actually do.

Thania Benios

 

June 1, 2007

Are Plants Really Villains in Climate Change?

Scientific American - New research disputes the widely publicized claim that they emit loads of methane .

Tropical forests spew methane into the air, unwittingly abetting human-produced climate change. So say geochemist Frank Keppler of the Max Planck Institute for Chemistry in Mainz , Germany , and atmospheric physicist Thomas Röckmann of the Institute for Marine and Atmospheric Research Utrecht in the Netherlands . But new research, using more sensitive measurement techniques and better growing conditions, failed to turn up any evidence of significant methane emission.

The new study, conducted by botanist Tom Dueck and his colleagues at Plant Research International in Wageningen, the Netherlands, found only negligible methane emissions coming from plants. "[The concentrations] were so low and variable that they did not significantly differ from zero," Dueck says. "We thought that if Keppler's claim was true, we could affirm his findings with better methods. If not, we'd have a scientific dispute."

In Keppler's study, the background levels of methane in the atmosphere, nearly 2000 parts per billion (ppb), dwarfed the 37 ppb of methane emitted by vernal grass, the plant species he found to produce the most methane. To clearly delineate what methane came from where, Dueck's team grew plants in an atmosphere with carbon 13, an isotope of carbon with an extra neutron at the center of each atom. Because the carbon was thus labeled, any methane (a compound of carbon and hydrogen—CH 4 ) that the plants produced would also carry this unique marker. Measuring small amounts of methane containing carbon 13 in a controlled setting is less difficult because background atmospheric levels of the isotope are much lower, 22 ppb.

Keppler says that using carbon 13 to grow plants could have significant and unknown impacts on the normal function of the plants. He also argues that Dueck's studies lack "sufficient scientific rigor" to prove that normal plants, which contain the more common isotope carbon 12, do not emit methane.

Scientists, however, have been cultivating simple carbon 13–labeled plants like algae since the late 1980s as well as more complex plants like kale for the past few years without evidence of significant physiological changes. Botanist Thomas Sharkey of the University of Wisconsin–Madison found that carbon 13 plants he grew in his lab "behave exactly the same" as normal plants.

Dueck believes that Keppler's experiment, and thus his findings, may have been flawed. The reason, he says: the sealed jars Keppler used to grow the plants did not allow enough air to circulate around the leaves, which "might result in abnormal reactions from the plants." Keppler also applied stresses like light and heat to the plants, which could further interact with lack of air circulation and possibly cause the vegetation to "cook," a process that could plausibly produce methane due to the organic breakdown of the plant matter, according to Sharkey.

Dueck and his colleagues grew their plants under steady-state conditions with enough air circulation in the chambers for gas exchange to occur between the vegetation and the surrounding environment. Sharkey considers these flow-through methods, which have been used by botanists for the past decade or so, to be "far superior and very convincing" and much more similar to plants growing in a natural environment.

The lack of a known physiological process or metabolic pathway that would cause plants to create methane also raises questions about Keppler's results. But such a biological basis would help explain the high methane concentrations that are often found above tropical forests. Keppler's findings, if confirmed, would indicate that nearly 10 to 40 percent of yearly methane emissions stem from plants.

Dueck says that he and his team are scheduled to meet with Thomas Röckmann, Keppler's co-author, in a few weeks to discuss the findings and to try to "clear up these differences." In the meantime, the question of whether plants emit copious amounts of methane remains a mystery.

Meredith Knight

 

May 2, 2007

Methane News: Not Quite So Missing

Carl Zimmer - A bit of journalistic irony. Last week I groused that a new paper on methane from plants was getting very little attention in the press, despite the fact that it refutes a 2006 paper published in Nature that got lots of press. I wished aloud that the situation would be set right. Well, five days later, a few more sites have published the press release, but I've only seen one new piece of original reporting.

It appears in the news section of today's issue of Nature. Hats off to Nature for making room for some uncomfortable news (http://www.scienceblogs.com/loom/).

 

May 1, 2007

Missing gas saps plant theory

Nature - Michael Hopkin published today a comment in Nature (nature.com/news for subscribers)on our paper in New Phytologist saying that “A team of plant scientists has cast doubt on one of the most startling research results the field has seen in recent years - the finding that green plants emit methane”. He explained that Keppler’s paper “was immediately controversial, not least because Keppler's team could not come up with a metabolic mechanism for their lab-based observations”.

He continues with “Both groups have criticized the other's choice of experimental method. Dueck says that Keppler's group kept plants in sealed plastic containers instead of flow chambers, and exposed them to sources of stress such as bright sunlight and high temperature, which could have produced methane as an artefact. Keppler retorts that the use of 13C is an artificial piece of chemical trickery with unknown effects on plant metabolism, and also argues that methane production can vary by up to three orders of magnitude between species.”

This latter argument by Keppler is a not very fair. Even some of his colleagues from the Max Planck Institute just published a paper1 on metabolite pools using 13C labeling stating that “CO2 tracing yielded ground-breaking biological insights into photosynthetic carbon assimilation, photorespiration and metabolism and, thus, into essential life-sustaining physiological mechanisms on earth”. Apparently, Keppler couldn't find a better defence.

Hopkins ends his comment with “Nevertheless, without a mechanism, Keppler's claim remains open to attack: "We are aware that while some scientific groups are having difficulties in repeating our work, several others have been able to do so and their results agree with our findings," Keppler says. These confirmatory results have yet to be published. Meanwhile, another plant scientist, David Beerling of the University of Sheffield, UK, says that he has not seen any methane using a method that is similar to Dueck's, but without relying on 13C . His research has not yet been published, but Beerling thinks that if it joins Dueck's in the peer-reviewed literature, "the two together could kill off the theory".”

1 Huege J, R Sulpice, Y Gibon, J Lisec, K Koehl & J Kopka. 2007. GC-EI-TOF-MS analysis of in vivo carbon-partitioning into soluble metabolite pools of higher plants by monitoring isotope dilution after 13CO2 labelling. Phytochemistry (doi:10.1016/j.phytochem.2007.03.026).

 

April 30, 2007

La végétation n'émettrait pas de méthane, puissant gaz à effet de serre

Le Monde - Début 2006, la revue Nature publiait une étude surprenante: une équipe de l'Institut Max-Planck d'Heidelberg (Allemagne) venait de constater que les plantes émettaient de grandes quantités de méthane (CH4), un gaz à effet de serre dont le pouvoir de réchauffement de l'atmosphère est vingt fois plus puissant que celui du CO2. Cela chamboulait la répartition des sources de carbone adoptée par les experts, qui considéraient jusqu'alors le méthane comme un sous-produit de l'activité microbienne en l'absence d'oxygène. Et remettait en cause la stratégie de reforestation envisagée pour lutter contre le réchauffement climatique.

"Pas si vite!", préviennent en substance des chercheurs néerlandais dans le dernier numéro de la revue New Phytologist. Tom Dueck (Plant Research International, Wageningen) et ses collègues ont reproduit l'expérience présentée dans la revue Nature et abouti à des conclusions totalement opposées. "Nos données indiquent que la contribution des plantes terrestres aux émissions globales de méthane est au mieux très faible", concluent-ils.

PLUS PROCHE DES CONDITIONS RÉELLES

Tom Dueck a testé l'activité biologique de six plantes - dont le maïs, le basilic, la sauge et le blé - après les avoir fait pousser dans un environnement enrichi en carbone radioactif (13C). Ce marqueur lui a ensuite permis de distinguer le méthane naturellement présent dans l'atmosphère de celui, marqué au 13C, que la plante était censée émettre. Or "la teneur de celui-ci n'était pas statistiquement différente de zéro", conclut-t-il.

Pour Philippe Bousquet, du Laboratoire des sciences du climat et de l'environnement (LSCE), ces nouveaux résultats sont d'autant plus intéressants que "Dueck a travaillé dans des conditions plus proches des conditions réelles que Keppler", avec des chambres de mesure plus grandes et des conditions mieux contrôlées. Il note que plusieurs articles récents évaluant les sources de méthane avaient déjà révisé à la baisse la contribution de la végétation, par rapport aux évaluations de Keppler. "C'est une étape mais pour moi la question n'est pas encore définitivement tranchée", indique-t-il.

Hervé Morin

 

April 27, 2007

Scientists clash over methane mystery

Chemistry World - A major row has broken out about whether plants emit methane, a potent greenhouse gas. 

The startling claim that trees could be responsible for putting millions of tonnes of methane into the atmosphere every year was published last year in the prestigious journal Nature. But that has now been rubbished by rival researchers who report that plants emit virtually no methane whatsoever1. 

Tom Dueck, of Wageningen University in the Netherlands, says his team's independent investigations are the first published results to show that plants' methane emissions are negligible or zero. That means their contribution to the global methane budget, and potentially to climate change, simply isn't worth worrying about. 

But Frank Keppler, now at the Max Planck Institute for Chemistry, in Mainz, Germany - whose team announced in January 2006 they had detected methane exhaled from living plants2 - is sticking to his guns. 'I am one hundred per cent confident that plants emit methane', he told Chemistry World, insisting that as yet unpublished research would confirm his findings once and for all. 

Global questions

Keppler's unexpected discovery has stirred up heated debate among biologists and atmospheric chemists over the past year. Though bacteria in soil or decaying matter produce methane in anaerobic conditions, there seems to be no reason - or mechanism - for living plants to emit the gas in an oxygen-rich environment. The implications of the findings are worrying: on a global scale, Keppler estimated, methane emissions from plants and trees could amount to hundreds of millions of tonnes a year, throwing scientists' understanding of the greenhouse gas's sources and sinks way off kilter. 

Since last year, many researchers have questioned that global impact, suggesting that Keppler's scale-up calculations, based on methane emission per unit of metabolically active mass of plant, were a gross overestimate. Yet until now, no published research has questioned Keppler's central discovery that plants can emit methane in the first place.

Now Dueck's team suggests that claim was also suspect. The Dutch group didn't repeat Keppler's experiment exactly, Dueck said, 'because it was not methodologically sound'. Instead, his team boosted the sensitivity of their methane measurements by growing plants in an atmosphere saturated with a heavy carbon isotope, 13C. If the plants subsequently emitted methane (13CH4), it would be easy to spot above the background of light (13CH4) methane in the air. Photo-acoustic spectroscopy (a laser-based measuring technique) was used to detect the gas; it showed that even a large mass of plants produced negligible methane emissions - at most 0.3 per cent of Keppler's values.

Six of one ...

So if Dueck's team is correct, where did Keppler go wrong? Dueck suggests that Keppler's team might have forgotten that gas trapped in plants' intercellular spaces, and in air pockets in the soil, could diffuse out and be counted as 'emitted' by the plant in their experiment. 

But Keppler denies this, saying that this sort of methane desorption could only be partially responsible for the amounts of gas he found. Indeed, he suggests that Dueck's use of heavy isotopes may have actually changed the plants' metabolic preferences, killing off their methane emissions. Dueck counters that no literature reports suggest this might be a problem.

Keppler's own work has stalled since last year, as a consequence of moving labs and changing team members, he said. But he told Chemistry World that other researchers had backed up his claims in work yet to be published, and that he was investigating a biological pathway by which plants could produce methane - something that would really convince the sceptics. Yet other scientists contacted by Chemistry World said they too were about to publish work which showed plants emitted no methane at all. 'Some research groups say nothing is going on, and an equal number say Keppler is absolutely right,' summarised one researcher, who declined to be named as their own   research publication on the topic is currently under review.

Technical challenge

The main problem with answering what seems to be such a simple question - do plants emit methane? - is that measuring methane emissions at such tiny rates is a huge technical challenge, said David Beerling, of Sheffield University, UK. The debate looks likely to rumble on for some years, Keppler conceded. 

Two scientists contacted by Chemistry World both compared the current situation to the atmosphere of the 1989 cold fusion debate. As they point out, one group has announced a sensational and unexpected finding, which hasn't yet been independently replicated. Keppler admits his discovery was hardly 'in the textbook', and understands that people have difficulty accepting it.

Even if the final consensus is that plants do exhale the greenhouse gas, its implications for climate change are still uncertain. The latest study to estimate the overall impact on the Earth tones down the global plant methane budget to 20-60 million tonnes a year3 , far less that the 60-240 million tonnes originally predicted by Keppler.

One rare point of agreement is that the story is far from over. 'I don't think anyone will accept [our study] as conclusive - and I hope other groups will continue to do more work,' said Dueck. 

Richard Van Noorden

References

1 T A Dueck  et al ,  New Phytologist , 2007, DOI:10.1111/j.1469-8137.2007.02103.x 
2 F Kepler  et al, Nature , 2006,  439 , 187
3 C L Butenhoff and M A K Khalil,  Environ. Sci. & Tech., 2007, DOI:10.1021/es062404i 

 

April 27, 2007

Don't Blame Climate Change On Plants, Methane Emission Study Says

Science Daily - A recent study in Nature1 suggested that terrestrial plants may be a global source of the potent greenhouse gas methane, making plants substantial contributors to the annual global methane budget. This controversial finding and the resulting commotion triggered a consortium2 of Dutch scientists to re-examine this in an independent study. Reporting in New Phytologist, Tom Dueck and colleagues present their results and conclude that methane emissions from plants are negligible and do not contribute to global climate change.

The consortium brings together a unique combination of expertise and facilities enabling the design and execution of a novel experiment. Plants were grown in a facility containing atmospheric carbon dioxide almost exclusively with a heavy form of carbon (13C). This makes the carbon released from the plants relatively easy to detect. Thus, if plants are able to emit methane, it will contain the heavy carbon isotope and can be detected against the background of lighter carbon molecules in the air.

Six plant species were grown in a 13C-carbon dioxide atmosphere, saturating the plants with heavy carbon. 13C-Methane emission was measured under controlled, but natural conditions with a photo-acoustic laser technique. This technique is so sensitive that the scientists are able to measure the carbon dioxide in the breath of small insects like ants. Even with this state-of-the-art technique, the measured emission rates were so close to the detection limit that they did not statistically differ from zero. To our knowledge this is the first independent test which has been published since the controversy last year.

Conscious of the fact that a small amount of plant material might only result in small amounts of methane, the researchers sampled the ‘heavy' methane in the air in which a large amount of plants were growing. Again, the measured methane emissions were neglible. Thus these plant specialists conclude that there is no reason to reassess the mitigation potential of plants. The researchers stress that questions still remain and that the gap in the global methane budget needs to be properly addressed.

1 'Methane emissons from terrestrial plants under aerobic conditions' by Keppler F, Hamilton JTG, Braß M, Rockmann T. Nature 439: 187–191

2 The Dutch consortium includes scientists from Plant Research International, IsoLife and Plant Dynamics in Wageningen, Utrecht University , and the Radboud University in Nijmegen .

Reference: 'No evidence for substantial aerobic methane emission by terrestrial plants: A 13C-labelling approach' Tom A. Dueck, Ries de Visser, Hendrik Poorter, Stefan Persijn, Antonie Gorissen, Willem de Visser, Ad Schapendonk, Jan Verhagen, Jan Snel, Frans J. M. Harren, Anthony K. Y. Ngai, Francel Verstappen, Harro Bouwmeester, Laurentius A. C. J. Voesenek and Adrie van der Werf, New Phytologist.

 

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