Exploring ocean biogeochemistry by single-cell microprobe analysis of protist elemental composition

The biogeochemical cycles of many elements in the ocean are linked by their simultaneous incorporation into protists. In order to understand these elemental interactions and their implications for global biogeochemical cycles, accurate measures of cellular element stoichiometries are needed. Bulk analysis of size-fractionated particulate material obscures the unique biogeochemical roles of different functional groups such as diatoms, calcifying protists, and diazotrophs. Elemental analysis of individual protist cells can be performed using electron, proton, and synchrotron X-ray microprobes. Here we review the capabilities and limitations of each approach and the application of these advanced techniques to cells collected from natural communities. Particular attention is paid to recent studies of plankton biogeochemistry in low-iron waters of the Southern Ocean. Single-cell analyses have revealed significant inter-taxa differences in phosphorus, iron, and nickel quotas. Differences in the response of autotrophs and heterotrophs to iron fertilization were also observed. Two-dimensional sub-cellular mapping indicates the importance of iron to photosynthetic machinery and of zinc to nuclear organelles. Observed changes in diatom silicification and cytoplasm content following iron fertilization modify our understanding of the relationship between iron availability and silicification. These examples demonstrate the advantages of studying ocean biogeochemistry at the level of individual cells.