Variations in Synechococcus Cell Quotas of Phosphorus, Sulfur, Manganese, Iron, Nickel, and Zinc within Mesoscale Eddies in the Sargasso Sea

The quotas of P, S, Mn, Fe, Ni, and Zn in individual Synechococcus cells collected from the surface and deep chlorophyll maximum (DCM) layer of three mesoscale eddies in the Sargasso Sea were measured using synchrotron X-ray fluorescence microscopy. Cells in a mode-water eddy had significantly higher P (57 +/- 10 amol) and Mn (28 +/- 7 zmol) cell quotas than cells collected from a cyclone (22 +/- 2 amol and 10 +/- 1 zmol, respectively) or anticyclone (25 +/- 3 amol and 18 +/- 3 zmol, respectively). Conversely, Ni and Zn quotas were significantly higher in the cells from the anticyclone (92 +/- 19 and 561 +/- 150 zmol, respectively) than in cells from the cyclonic (25 +/- 4 and 35 +/- 7 zmol, respectively) or mode-water (30 +/- 9 and 21 +/- 8 zmol, respectively) eddies. These changes may reflect biochemical responses (e. g., production of urease and alkaline phosphatase) to gradients in inorganic N and P supplies. Cellular quotas of Fe (111 +/- 17 zmol in the cyclone) and S (52 +/- 6 amol in the cyclone) did not vary significantly among eddies despite two-to threefold higher dissolved and particulate Fe concentrations in the anticylone. Cells collected from 10-m depth contained approximately 80% more Ni and S than cells collected from the DCM, potentially reflecting cell responses to heightened oxidative stress. Depth-related trends varied by eddy for the other elements. Cellular P and Zn varied significantly during repeated samplings of the cyclone, with quotas of both elements dropping as bulk chlorophyll biomass in the DCM increased. These data demonstrate the dynamic responses of phytoplankton elemental composition to physical and chemical environmental gradients

Single-Cell Measurements of Element Concentration In Phytoplankton Using Synchrotron X-Ray Fluorescence

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 (\u3c 1 µm) and high elemental sensitivity ( 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 (\u3c2 \u3eµm), 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