Blog: UV Irradiation of Aquatic Organic Carbon: An Overlooked Source of Methane?

black burn_0

By Amy Pickard. 3rd December 2013.

Amy Pickard1*, Andy McLeod1, Kate Heal1 and Kerry Dinsmore2

1 School of GeoSciences, University of Edinburgh, UK *  2Centre for Ecology and Hydrology, Bush Estate, Penicuik, UK

Wetland environments, including northern peatlands, are a globally significant source of methane, releasing in the order of 100 Tg CH₄ yr-1 (Wuebbles and Hayhoe, 2002). Emissions from these systems have typically been attributed to microbial metabolism of organic carbon into methane in anaerobic conditions.

However a seminal study by Keppler et al. (2006) showed that methane was produced in aerobic conditions when plant matter was subjected to stress. UV irradiation is a known source of plant stress that was later shown to initiate methane production (McLeod et al., 2008). Further follow up studies indicated that plant pectin was a possible source for the emissions and confirmed that methane could be produced from detached leaf components (McLeod et al., 2008; Vigano et al., 2008). The key outcome of this body of research was the incorporation of aerobic methane production from UV irradiation of plant foliage into the global budget (Bloom et al., 2010). Nevertheless, to date, this pathway of aerobic methane production has only been investigated in the terrestrial environment.

The hypothesis of my research is that plant-derived material transported from the terrestrial environment to aquatic systems may release methane when exposed to UV irradiation. The NERC-funded PhD that I am currently undertaking aims to further investigate the aerobic production pathway as a potential source of methane in aquatic systems.

In order to test this hypothesis, water samples rich in dissolved organic carbon (DOC) were collected from a stream draining Auchencorth Moss (Fig. 1), an ombotrophic peatland area in south east Scotland which is one of the Centre for Ecology & Hydrology’s Carbon Catchments. Upon returning to the laboratory samples were filtered and decanted into quartz vials. They were then exposed to an ambient dose of UV irradiation for 4 hours. After irradiation, vial headspace methane concentrations were measured using gas chromatography.

These initial experiments have demonstrated that aquatic systems contain sufficient levels of plant derived material to stimulate aerobic methane production.  UV irradiation resulted in increased CH₄ production rates of 63.2 ± 16.4 nmol L-1 (mean ± standard deviation for 4 replicates) relative to unirradiated control samples. The increase in gaseous production in the headspace of irradiated vials was coupled with a decrease in DOC concentration in the water sample. This finding is in agreement with the hypothesis that dissolved plant matter acts as the aerobic methane source material and adds weight to the suggestion that UV irradiation plays an as yet overlooked role in the aquatic methane budget.

Increased losses of DOC from catchments in the northern hemisphere have been well documented (Clark et al., 2010) and are projected to accelerate with climate change (Worrall et al., 2004). This creates an interesting setting for aquatic methane research, as it follows that in catchments where more DOC is delivered to aquatic systems, methane emissions stimulated via UV irradiation of organic matter will increase concurrently. Whether this hypothesis holds true in either laboratory tests or field based experiments is yet to be determined, however our initial data present plenty of options to explore. The challenge now is to understand what environmental factors affect aerobic methane emissions and to combine both laboratory and field based experiments to determine the importance of this process in catchment level biogeochemical cycles.

Photo: The Black Burn at Auchencorth Moss. Photo courtesy of Fraser Leith


Bloom, A.A., Lee-Taylor, J., Madronich, S., Messenger, D.J., Palmer, P.I., Reay, D.S., McLeod, A.R., 2010. Global methane emission estimates from ultraviolet irradiation of terrestrial plant foliage. New Phytol 187, 417-425

Clark, J.M., Bottrell, S.H., Evans, C.D., Monteith, D.T., Bartlett, R., Rose, R., Newton, R.J., Chapman, P.J., 2010. The importance of the relationship between scale and process in understanding long-term DOC dynamics. Sci Total Environ 408, 2768-2775

Keppler, F., Hamilton, J.T.G., Brass, M., Rockmann, T., 2006. Methane emissions from terrestrial plants under aerobic conditions. Nature 439, 187-191.

McLeod, A.R., Fry, S.C., Loake, G.J., Messenger, D.J., Reay, D.S., Smith, K.A., Yun, B.W., 2008. Ultraviolet radiation drives methane emissions from terrestrial plant pectins. New Phytol 180, 124-132.

Vigano, I., van Weelden, H., Holzinger, R., Keppler, F., McLeod, A., Rockmann, T., 2008. Effect of UV radiation and temperature on the emission of methane from plant biomass and structural components. Biogeosciences 5, 937-947

Worrall, F., Burt, T., Adamson, J., 2004. Can climate change explain increases in DOC flux from upland peat catchments? Sci Total Environ 326, 95-112

Wuebbles, D.J., Hayhoe, K., 2002. Atmospheric methane and global change. Earth-Sci Rev 57, 177-210


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