Carbonyl sulfide sulfur isotopes fractionation and leaves' internal conductance

Carbonyl sulfide (OCS) is the major long-lived sulfur-bearing gas in the atmosphere. The main sink for COS occurs when it diffuses through the plant leaves stomata and enters the mesophyll cell, where it reacts with the enzyme carbonic anhydrase. Since CO₂ enters the leaves by a similar pathway, COS has been used to estimate the rates of regional and global photosynthesis. For example, recently ¹, it was suggested that the global GPP is ~30% higher than estimated so far, based on COS observations and new modeling of COS internal conductance, which relates to the diffusion into the active site in the mesophyll. Sulfur isotope analysis (³⁴S/³²S ratio, δ³⁴S) of COS was shown ² to be useful for improving the determination of atmospheric COS sources and sinks. The sulfur isotopic fractionation during COS uptake in plants is needed for using this tool, but so far, has only been established in the lab. In that study, the fractionation was found to be −1.6 ± 0.1‰ for C3 plants,  −5.4 ± 0.5‰  for C4 plants, and the carbonic anhydrase fractionation was estimated indirectly as −15 ± 2‰. Field studies of leaves' COS uptake enable the study of the effects of varying light conditions in the tree canopy. Here, we measured the COS fractionation during uptake in an Austrian alpine forest, using branch chambers at three height levels in a Pinus sylvestris canopy. In addition, we directly measured the fractionation of carbonic anhydrase in vitro in the lab. The isotopic analysis was conducted by pre-concentrating the air samples and subsequent δ³⁴S analysis by gas chromatography (GC) connected to a multi-collector inductively coupled plasma mass spectrometer (MC-ICPMS). The results of this research are important for improving both leaves scale COS transport models and global budgets of COS and CO₂.

 

1. Lai, J., Kooijmans, L. M., Sun, W., Lombardozzi, D., Campbell, J. E., Gu, L., ... & Sun, Y. (2024). Terrestrial photosynthesis is inferred from plant carbonyl sulfide uptake. Nature, 634, 855-861.

2. Davidson, C., Amrani, A., & Angert, A. (2021). Tropospheric carbonyl sulfide mass balance based on direct measurements of sulfur isotopes. Proceedings of the National Academy of Sciences, 118(6), e2020060118.

3. Davidson, C., Amrani, A., & Angert, A. (2022). Carbonyl sulfide sulfur isotope fractionation during uptake by C3 and C4 plants. Journal of Geophysical Research: Biogeosciences, 127(10), e2022JG007035.