Element stoichiometries of individual plankton cells collected during the Southern Ocean Iron Experiment (SOFeX)

Abstract

During the Southern Ocean Iron Experiment (SOFeX), we analyzed Si, P, S, Mn, Ni, and Zn in individual diatoms, autotrophic flagellates, and heterotrophic flagellates with synchrotron-based X-ray fluorescence (SXRF) and calculated cellular C from measurements of cell size. Element stoichiometries for the different types of protists (normalized to either C, S, or P) were generally in good agreement with prior bulk analyses of natural assemblages but also revealed previously undocumented differences in elemental composition among cell types. Flagellated cells contained 39% more P than diatoms, which in turn contained 79% more Mn, 3-fold more Ni, and 2.6-fold more Zn than flagellates. Heterotroph cells contained approximately 40% more P and twice as much Zn as autotrophs. Manganese and Ni stoichiometries were negatively related to cell volume, while larger cells contained more Zn per mole C. Iron fertilization resulted in an approximate doubling of Mn, Ni, and Zn quotas and smaller increases in cellular P, but the timing of the stoichiometric changes varied between the two patches. Silicon contents of diatoms dropped approximately 40% after the first Fe addition but returned to prefertilization levels as cellular P doubled following the second addition, resulting in 40% lower Si : P ratios in the fertilized waters. The mean elemental stoichiometries calculated for all cells analyzed were comparable with previously published extended Redfield ratios for mixed plankton assemblages, but the observed differences between diatoms and flagellates and between autotrophs and heterotrophs indicate that valuable information is lost when all types of co-occurring plankton are grouped together for analysis.Metal biogeochemistry and ocean ecosystem function are closely linked. Several transition metals, including Mn, Fe, Co, Ni, Cu, and Zn, are required as cofactors for common proteins, and both heterotrophic and autotrophic protists accumulate these elements from the surrounding seawater, lowering dissolved concentrations in surface waters. Remineralization of these metals from sinking cells or zooplankton fecal pellets elevates their concentrations below the euphotic zone, resulting in the nutrient-like profiles commonly observed for these bioactive metals. Further, the availability and accumulation of trace metals can exert control over the biological functioning of the organisms. AcknowledgmentsThe authors gratefully acknowledge Michael Landry for providing the opportunity to participate in the SOFeX project and Chief Scientist Ken Johnson and the Captain and crew of the R/V Roger Revelle for enabling the collection of the samples. Jörg Maser, Stefan Vogt, and Dan Legnini provided invaluable technical assistance with the SXRF microprobe, and the manuscript benefited from discussions with Mark Brzezinski and the comments of two anonymous reviewers.

Taxon-specific responses of Southern Ocean diatoms to Fe enrichment revealed by synchrotron radiation FTIR microspectroscopy

“…Additionally, samples from the Fe-enriched stations were observed to have substantially higher carbon productivity, which could have led to increased concentrations of cellular carbon (Jacquet et al, this issue). If the decrease in BSi / POC was caused primarily by a large increase in cellular C concentrations, calculations of cellular C from cell volume may provide underestimates under Fe enrichment, because cell size is constrained by the frustule (Twining et al, 2004a). Such changes in cellular carbon and Si content are consistent with changes in macromolecular composition observed by SR-FTIR microspectroscopy.…”

“…The lack of taxon-specific macromolecular data relates to the large quantities of biomass generally required for biochemical analyses, difficulty sorting algal cells into taxonomic groupings, and to the time and financial costs associated with available measurement techniques. Recently however, elemental analysis of individual Southern Ocean phytoplankton cells using X-ray Fluorescence Microprobe Analysis has revealed species-specific changes in elemental composition with Fe availability (Twining et al, 2004a, b). Here we used synchrotron radiation Fourier Transform Infrared (SR-FTIR) microspectroscopy to analyse taxon-specific changes in macromolecular composition with Fe availability in individual diatom cells.…”

Tracing terrestrial organic matter by δ34S and δ13C signatures in a subarctic estuary

“…After determining the C : S ratio in respective end members (C : S ter from Hessen and Tranvik 1998 and Findlay and Sinsabaugh 2003; C : S mar from Ho et al 2003 andTwining et al 2004), the following equation for converting the fraction of terrestrial DOS to the fraction of terrestrial DOC could be used:…”

“…The ratio between carbon concentration and sulfur concentration (molar ratio) in the marine fraction (derived from phytoplankton production) of the sample, assumed to be 100 (Ho et al 2003;Twining et al 2004). …”

Metal to phosphorus stoichiometries for freshwater phytoplankton in three Scottish lakes

“…In the same 30 context, phytoplankton can exploit iron (Fe), manganese (Mn), zinc (Zn), copper (Cu) and 31 nickel (Ni) for N acquisition, oxygen cycling, chlorophyll synthesis, and sulfate reduction 32 (Moffett et al, 1997;Twining et al, 2004). These nutrient metals can be replaced at their 33 metabolic site by toxic metals such as cadmium (Cd), mercury (Hg), lead (Pb) and chromium 34 (Cr) (Bruland et al, 1978;Sunda and Huntsman, 1998).…”