Looking at the scientific headlines during the past year, one has to conclude that the greenhouse gas methane has become a major issue. A major research effort has gone into understanding the global distribution of methane, the second most important greenhouse gas worldwide. Despite this research, the uncertainties about methane sinks and sources have not been diminished. Frankenberg et al. (2005) and Miller et al. (2007) observed a number of discrepancies between measured and modelled global methane emissions, and the article by Keppler et al., in the current issue of New Phytologist (pp. 808–814), might shed some light on these findings.
Just 2 yr ago, Keppler et al. (2006) shook up the plant physiology world by claiming that plants produce methane under aerobic conditions. Plants are indeed able to emit various volatile organic compounds such as propanes and isoprenes, but, until this report, no mention had been made of emissions of methane from plants. Keppler et al. (2006) measured methane from fresh and dried plant material in air, as well as from intact plants enclosed in static chambers. The thought that plants might be responsible for 10–40% of the atmospheric methane budget without proposing a mechanism for production under aerobic conditions sparked a great deal of discussion among plant physiologists. The paper caused even more commotion when estimating the contributions of various sources to the global methane budget, which included a possible plant source for methane (Lelieveld, 2006; Lowe, 2006; Schiermeier, 2006). The first experimental rebuttal came a year later, when Dueck et al. (2007) reported being unable to measure 13C-methane from plants grown in an atmosphere containing 99%13CO2.
In their most recent paper, reported in this issue of New Phytologist, Keppler et al. come up with an explanation for a first step in the process of methane production in plants under aerobic conditions. They were able to show, through an elegant isotope-labelling study, that precursors found in plants may result in methane production.
‘The fact that methane production originates in plant (structural components), but is not actively produced by plants, will be met with some degree of relief by plant physiologists.’
Mechanism remains unknown
Previous research by Keppler et al. (2004) suggested that methoxyl groups in structural plant components might play a key role in the formation of methane in plants. In their 2006 paper, Keppler et al. (2006) found indirect evidence for the role of pectin in methane formation, by measuring similar amounts of methane emitted from pectin and from dead and detached leaves. But just because these emission rates were similar was not good enough proof for them.
Thus, in the paper presented in this issue of New Phytologist, Keppler et al. employed isotope analysis, a powerful tool, to determine the origin of methane production. They used this isotope technique together with pectin and polygalacturonic acid, both of which contain trideuterium-labelled methyl groups, to demonstrate that it is indeed methoxyl groups in plant pectin that are precursors of methane. Via this mechanism, plants could feasibly act as a source of methane under aerobic conditions.
In their first study, Keppler et al. (2006) observed high methane-emission rates influenced by elevated temperatures and direct sunlight. This led them to concentrate their present work on the influence of temperature and ultraviolet (UV) radiation on methane emissions from pectin and polygalacturaonic acid. They first labelled the methoxyl groups in pectin and polygalacturaonic acid with various delta values of trideuterium. Then, the vials containing samples of both compounds were either heated (from 40 to 80°C) or exposed to UV radiation (3.7 and 42 W m−2 for unweighted UVA and UVB, respectively). Gas samples were extracted from the vials and measured using an isotope ratio mass spectrometer. By use of this technique it was possible to demonstrate that heat and light (especially in the UV range) are important factors stimulating methane production from the precursors pectin and polygalacturaonic acid.
Status of methane emissions
At this point in time, experimental evidence has reported high levels of methane emissions (Keppler et al., 2006), moderate levels of methane (Wang et al., 2008) and no methane emissions (Dueck et al., 2007) from living and dead plants. However, the conditions under which these measurements were made have not been standardized. The emission rates measured by Wang et al. and Dueck et al. were not measured under the influence of elevated temperature or UV radiation, and these factors can indeed influence the emission of volatile organic compounds, as has been recently shown for methane by Vigano et al. (2008). For example, a linear relationship between isoprene emissions and light (including UV) has been shown to be typical for tropical deciduous forests (Lerdau et al., 1997). And now it has been clearly demonstrated by Keppler et al. that compounds like pectin or polygalacturaonic acid, which contain methoxyl groups, can indeed function as precursors for the production of methane. The fact that methane production originates in plants (structural components), but is not actively produced by plants, will be met with some degree of relief by plant physiologists.
And yet the mystery behind the mechanism still remains to be unravelled, as is so well stated in this paper. So, the suggestion is put forward to concentrate research on the mechanism and whether or not these precursors of methane in plant organic matter are stimulated to produce methane also under ‘normal’ conditions.
And the global methane budget?
The annual global emission of methane is estimated to amount to c. 600 Tg yr−1, of which c. 40% is thought to originate from natural sources. In a comparison of the global methane distribution, analyzed by space-borne near-infrared absorption spectroscopy and by model studies, Frankenberger et al. (2005) and Miller et al. (2007) concluded that huge amounts of methane in the atmosphere above tropical regions are unaccounted for. The difference between modelling results and scanning imaging absorption spectrometer for atmospheric chartography (SCIAMACHY) measurements (Frankenberg et al., 2005) are illustrated in Fig. 1, indicating where a number of regional gaps occur in the methane budget. The suggestion that terrestrial plants may also emit methane under aerobic conditions by an, as yet, unknown physiological process (Keppler et al., 2006) might at least partly explain these gaps (Bousquet et al., 2006). Scaling up from individual plants to global vegetation resulted in estimated values for methane emission by terrestrial plants that varied from 10 to 260 Tg yr−1 (Houweling et al., 2006; Keppler et al., 2006; Kirschbaum et al., 2006). These values are impressive and might partly account for the plumes of methane observed through satellite observations above tropical forests (Frankenberg et al., 2005).
To address this issue in greater detail we first need to perform specific experiments focussed on measuring emission rates of methane from living plants or plant parts (stems). This requires the use of novel techniques like those used by Keppler et al. and Dueck et al. (2007), avoiding the criticisms addressed by Kirschbaum et al. (2006) and Dueck et al. (2007).
A second point is the emission of methane from dead plant material (Keppler et al., 2006). Can the amount of dead plant material in the canopy of tropical rainforest (partly) account for the observed plumes? In light of the major sources of methane emissions (wetlands, animals, rice paddies), what might be the relative contribution of the dead and living plant sources (Kirschbaum et al., 2007)? Measurements and modelling exercises are required to estimate the amounts of dead and living plant matter in regions in which discrepancies in the methane budget occur and relate them to (upscaled) emission rates from plants.
A third aspect to consider is that terrestrial plants emit huge amounts of volatile compounds (Lerdau et al., 1997). Can these volatiles play a role in the emission of methane under conditions of high-UV radiation? We need to conduct more research into the specific role of volatiles emitted by plants, how they are influenced by factors like UV radiation and heat, and how they might relate to or influence atmospheric methane concentrations.
By addressing these issues we should be able to take a step forward in filling the gap in the global methane budget. And only then can we place the role of plant emissions into proper perspective.