INTERNAL NITRATE CONCENTRATIONS IN SINGLE CELLS OF LARGE PHYTOPLANKTON FROM THE SARGASSO-SEA

Nitrate concentrations within individual cells of Ethmodiscus, Pyrocys tis, and Halosphaera and chains of Rhizosolenia were determined from s amples collected in the Sargasso Sea. In all cases, field populations exhibited a wide range of internal nitrate concentrations (INCs) withi n a single sampling date. Halosphaera INCs reached 100 mM, in contrast to diatom and dinoflagellate INCs, which did not exceed 22 mM. Sinkin g Rhizosolenia, Ethmodiscus and Pyrocystis had significantly lower int ernal NO3- pools than did floating cells (P < 0.05). Ethmodiscus incub ations in surface seawater resulted in a dramatic reduction in the pro portion of high INC cells concurrent with decreases in average INCs an d an increased proportion of sinking cells. Population buoyancy was in versely related to INC, and negatively buoyant cells rarely exceeded 1 mM INC, suggesting that a critical INC threshold may exist. The photo synthetic parameters P-max and alpha decreased with time as internal N O3- pools were depleted. Internal nitrate depletion rates were consist ent with oxygen production rates during this time. Based on the known characteristics of Pyrocystis and Ethmodiscus, we conclude that virtua lly all of the > 100 mu m-sized phytoplankton present in the Sargasso Sea can vertically migrate. However, the appropriate time scale for mi grators such as Halosphaera that reproduce by swarmer formation is unc lear and may be significantly different than the other taxa studied. C hanges in the frequency distributions, buoyancy-internal pool relation ships, and general P-1 phatosynthesis-irradiance time series data in E thmodiscus suggest that nutrient limitation is related to these migrat ions. High INC appears to be a fundamental property of the largest mic roalgal cells present in oligotrophic seas and suggests that nitrate t ransport by these nonmotile cells is widespread.