Evidence for a major missing source in the global chloromethane budget from stable carbon isotopes

Chloromethane (CH3Cl) is the most important natural input of reactive chlorine to the stratosphere, contributing about 16 % to stratospheric ozone depletion. Due to the phase-out of anthropogenic emissions of chlorofluorocarbons, CH3Cl will largely control future levels of stratospheric chlorine. The tropical rainforest is commonly assumed to be the strongest single CH3Cl source, contributing over half of the global annual emissions of about 4000 to 5000 Gg (1 Gg = 109 g). This source shows a characteristic carbon isotope fingerprint, making isotopic investigations a promising tool for improving its atmospheric budget. Applying carbon isotopes to better constrain the atmospheric budget of CH3Cl requires sound information on the kinetic isotope effects for the main sink processes: the reaction with OH and Cl in the troposphere. We conducted photochemical CH3Cl degradation experiments in a 3500 dm3 smog chamber to determine the carbon isotope effect (ε=k13C/k12C-1) for the reaction of CH3Cl with OH and Cl. For the reaction of CH3Cl with OH, we determined an ε value of (-11.2±0.8) ‰ (n=3) and for the reaction with Cl we found an ε value of (-10.2±0.5) ‰ (n=1), which is 5 to 6 times smaller than previously reported. Our smaller isotope effects are strongly supported by the lack of any significant seasonal covariation in previously reported tropospheric δ13C(CH3Cl) values with the OH-driven seasonal cycle in tropospheric mixing ratios. Applying these new values for the carbon isotope effect to the global CH3Cl budget using a simple two hemispheric box model, we derive a tropical rainforest CH3Cl source of (670±200) Gg a−1, which is considerably smaller than previous estimates. A revision of previous bottom-up estimates, using above-ground biomass instead of rainforest area, strongly supports this lower estimate. Finally, our results suggest a large unknown CH3Cl source of (1530±200) Gg a−1.

Differences in Carbon Isotope Discrimination of Three Variants of D-Ribulose-1,5-bisphosphate Carboxylase/Oxygenase Reflect Differences in Their Catalytic Mechanisms

The carboxylation kinetic (stable carbon) isotope effect was measured for purified d-ribulose-1,5-bisphosphate carboxylases/oxygenases (Rubiscos) with aqueous CO2 as substrate by monitoring Rayleigh fractionation using membrane inlet mass spectrometry. This resulted in discriminations (Δ) of 27.4 ± 0.9‰ for wild-type tobacco Rubisco, 22.2 ± 2.1‰ for Rhodospirillum rubrum Rubisco, and 11.2 ± 1.6‰ for a large subunit mutant of tobacco Rubisco in which Leu335 is mutated to valine (L335V). These Δ values are consistent with the photosynthetic discrimination determined for wild-type tobacco and transplastomic tobacco lines that exclusively produce R. rubrum or L335V Rubisco. The Δ values are indicative of the potential evolutionary variability of Δ values for a range of Rubiscos from different species: Form I Rubisco from higher plants; prokaryotic Rubiscos, including Form II; and the L335V mutant. We explore the implications of these Δ values for the Rubisco catalytic mechanism and suggest that Rubiscos that are associated with a lower Δ value have a less product-like carboxylation transition state and/or allow a decarboxylation step that evolution has excluded in higher plants.

Viewpoint: Carbon isotope effect predictions for enzymes involved in the primary carbon metabolism of plant leaves

Carbon isotope effects of enzymes involved in primary carbon metabolism are key parameters in our understanding of plant metabolism. Nevertheless, some of them are poorly known because of the lack of in vitro experimental data on purified enzymes. Some studies have focused on theoretical predictions of isotope effects. Here we show how quantum chemical calculations can be adapted for calculation of isotope effects for the Rubisco-catalysed carboxylation and oxygenation reactions and the citrate synthase reaction. The intrinsic isotope effect of the carboxylation by Rubisco appears to be much smaller than previously thought, being close to the overall isotope effect of the reaction that is, between 25 and 30 per mil. The same applies to the enzyme citrate synthase, that catalyses the first step of the Krebs cycle, with an isotope effect of around 23 per mil. Combined with the isotope effects of equilibrium reactions calculated with β-factors, the Krebs cycle then has an overall isotope effect that depletes organic acids in 13C.