Magnesium Isotopic Equilibrium in Chlorophylls

摘要

Photosynthesis generates ～100 gigatons of carbon per year and drives many terrestrial geochemical cycles.1 Magnesium is the metal center of all chlorophylls and is thus central to photosynthesis. Plants and microorganisms synthesize a number of structurally distinct chlorophylls. The two main forms are chlorophyll-a (Chl-a) and chlorophyll-b (Chl-b). The ratio of these chlorophylls varies as the plants adapt to specific environmental conditions. The complexity of the enzymatic pathways involved in effecting this variation leads us to speculate that the chlorophyll cycle may produce a difference in the Mg isotope composition of Chl-a and Chl-b. In this study, we attempt to detect such an isotopic difference, which would put constraints on the lifetime of magnesium in these biomolecules. It is well-known that biosynthetic pathways can cause fractionation of light isotopes; ~(13)C is preferentially distributed to certain sites of Chl-a. A few previous studies indicate a potential biogenic fractionation of the three naturally abundant stable isotopes of magnesium (~(24,25,26)Mg) in chlorophyll. Here, using multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS), we demonstrate a mass-dependent isotopic fractionation of Mg between Chl-a and Chl-b and also between the porphyrin species and the inorganic Mg complexes in a leaf's reservoir. Density functional (DFT) electronic structure calculations indicate that the measured Chl-a/Chl-b isotope ratio is consistent with an equilibrium fractionation.