The stable isotopic composition of photosynthetic pigments and related biochemicals

Abstract

The stable carbon and nitrogen isotopic composition of bulk organic matter, lipids, chlorophylla, chlorophyllb and β,β-carotene were determined for selected photosynthetic plants. Lipids are depleted in¹³C (∼ − 6‰) compared to bulk organic matter. C₄- and C₃-type biosynthesis can be differentiated at the compound class and individual compound level by their stable carbon isotope ratios. The stable carbon isotopic ratios of chlorophylls isolated from C₃ and C₄ plants differ by ∼ 10ℵ.. The stable nitrogen isotope ratios of lipids vary over an ∼ 15‰ range, excluding one very positive sample. Various sources of carbon and nitrogen, differing assimilation mechanisms, and nutrient limitation contribute to the observed isotopic compositions. The stable isotopic composition of chlorophyllf and bulk organic matter are linearly correlated. Based on this relationship it is possible to resolve the assimilation of carbon and nitrogen by algae within complex mixture of detritus, bacteria and phytoplankton. The isotopic record of photosynthetic fixation of carbon and nitogren in chlorophylls suggests that geoporphyrins do retain information on paleo-productivity. The stable isotopic composition of chlorophylla, chlorophyllb and β,β-carotene, and their breakdown products provide unique insight into a wide range of biogeochemical processes.