Magnesium isotope analysis of different chlorophyll forms in marine phytoplankton using multi-collector ICP-MS

A new separation method of magnesium in the different chlorophyll forms, including chlorophyll a, b, c1 + c2 and c3, of phytoplankton, was developed for isotope measurements. Our method is based on a two-step separation using HPLC and a C₁₈ column. The method made it possible to determine the magnesium isotopes (δ²⁵Mg and δ²⁶Mg) of the particular chlorophyll forms in marine living phytoplankton with a high precision using a multi-collector ICP-MS. The differences of δ²⁶Mg values among commercial chlorophylls (four types of chlorophyll a and two types of chlorophyll b) and the different chlorophyll forms of marine diatom-rich phytoplankton collected from the Northwest Pacific show variations of 1.5‰ and 0.65‰, respectively. We found a slight enrichment in the heavier magnesium isotopes in chlorophylls relative to sea-water: 0.58‰, 0.74‰, 0.27‰ and 0.35‰ on δ²⁶Mg for chlorophyll a, b, c1 + c2 and c3, respectively. Our data suggest a mass dependent fractionation of magnesium isotopes among species of photosynthetic organisms and the chlorophyll forms. The magnesium isotope analysis by chlorophyll forms might not only give a new insight into the biosynthetic pathways associated with chlorophylls but also be a potential environmental indicator preserved in marine phytoplankton and other photosynthetic organisms.