25th International Meeting on Organic Geochemistry IMOG 2011

O-65
Fluxes and isotope composition of selected halocarbons from
sea grass meadows
Enno Bahlmann, Ingo Weinberg, Richard Seifert, Walter Michaelis
Institute for Biogeochemistry and Marine Chemistry, University of Hamburg, Hamburg, Germany
(corresponding author: enno.bahlmann@zmaw.de)
Coastal areas are considered to be a significant
source of reactive trace gases to the atmosphere. In
the past, emissions of halocarbons and the underlying
mechanisms have been studied for macroalgae [1, 2],
plankton [3], and saltmarsh plants [4, 5]. In contrast,
data about trace gas emissions from sea grass
meadows are hardly available. Sea grass beds
covering about 10% of the coastal oceans are
amongst the most productive marine ecosystems [6]
and might represent an additional significant source of
halocarbons to the atmosphere.
We conducted flux chamber measurements on a sea
grass meadow in List/Sylt, Northern Germany to study
halocarbon fluxes. The isotopic compositions of the
emitted halocarbons were determined to elucidate the
underlying biogeochemical processes.
The stable carbon isotope ratios of chloromethane
and iodomethane ranged from δ 13 C of -46‰ to -58‰
and from -52‰ to –68‰, respectively. On average,
both compounds were depleted in 13 C by 40– 60‰ as
compared to the bulk biomass of sea grass. This
large isotopic shift relative to the plant material is in
the same range as previously reported for
chloromethane formed by higher plants and ascribed
to a methyltransferase catalysed reaction [7]. In
contrast, the carbon isotope compositions of
bromoform, iodoethane, iodopropane, and
vinylchloride were not more than 10‰ depleted in 13 C,
relative to the bulk sea grass biomass. This small
isotopic shift points towards a different building
mechanism, presumably the haloperoxidase pathway
(Fig. 1). The emitted bromomethane showed
intermediate carbon isotope ratios (δ 13 C -28‰ to -
41‰) as compared to the other compounds.
Measurements over adjacent bare sediments
revealed up to 45% of the newly formed
bromomethane to become readily degraded on the
sediment surface with this degradation being
associated with a fractionation factor of 45‰.
Although we cannot rule out that the production of
bromomethane is at least partly catalyzed by a
haloperoxidase reaction, we rather presume the shift
in 13 C of bromomethane to result from its immediate
degradation in the surface sediments.
δ 13 δ C [‰] 13C [‰]
-70
-60
-50
-40
-30
-20
-10
Methyltransferase
Methyltransferase
Degradation Degradation Degradation ? ? ?
Haloperoxidase
Haloperoxidase
seagrass CH CH3Cl 3Cl CH CH3Br 3Br CH CH3I 3I CHBr 3 C C2H 2H5I 5I C C3H 3H7I 7I C C2H 2H3Cl 3Cl
Fig. 1. δ 13 C values of emitted halocarbons and presumed
building mechanism from sea grass biomass
References
[1] Carpenter, L.; Malin, G.; Liss, P.; Küpper, F. 2000. Global
Biogeochemical, 14, 1191-1204.
[2] Laturnus, F.; Giese, B.; Wiencke, C.; Adams, F. 2000.
Fresenius Journal of Analytical Chemistry, 368, 297-302.
[3] Class, T.; Ballschmiter, K. 1998. Journal of Atmospheric
Chemistry, 6, 35-46.
[4] Bill, M.; Rhew, R.; Weiss, R.; Goldstein, A. 2002.
Geophysical Research Letter, 29, 2-5.
[5] Manley, S.; Wang, N.; Walser, M.; RJ. 2006. Global
Biogeochemical Cycles, 20, 1-13.
[6] Yamamuro, M.et al. 2002. Recent Advances in Marine
Science and Technology, 1-6.
[7] Harper et al. 2003. Chemosphere, 52, 433-436
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