Single-cell measurements of element concentration in phytoplankton using synchrotron x-ray fluorescence.

摘要

The elemental composition of phytoplankton is of interest because of their role in the biogeochemical cycles of carbon and other bioactive elements. Phytoplankton also can serve as the point of introduction for metal contaminants into aquatic food webs. Single-cell analysis of phytoplankton elemental composition enables determinations free of interferences from abiotic material and other (non-target) cells present in natural particle assemblages. Synchrotron x-ray fluorescence (SXRF) is a microanalytical technique used for the localization and quantification of elemental distributions. This technique enables quantitative and qualitative analysis with high spatial resolution (1 mum) and high elemental sensitivity (1 mug/g) on a single cell basis.;Standards are required to convert characteristic x-ray fluorescence into element concentrations, but few biologically appropriate standards are available, particularly for the biomass element phosphorus. In the first aspect of this work, P quantification by SXRF was tested using the marine diatom Thalassiosira pseudonana as a reference material. The cellular P quota measured with SXRF was statistically comparable with the P quota measured by a bulk spectrophotometic technique. Additionally, several grid substrates were tested (gold, nickel and nylon) for suitability for single-cell phytoplankton samples. While the P content of cells on Au, Ni and nylon grids was similar, the nylon grids were found to have the lowest background concentrations and limits of detection for P.;In the second aspect of this work, cyanobacteria from three eddies on the Sargasso Sea were analyzed using SXRF. Despite the small size of the target cells (2 mum), iron and other trace metals were successfully quantified. Essential elements and metal quotas were found to be 10-17--10 -21 mol/cell. The biogeochemical implications of these measurements are discussed.