Physiological responses of Sargasso Sea picoplankton to nanomolar nitrate perturbations

Abstract

A study was conducted in July 1989 at three stations in the northern Sargasso Sea, where picoplankton (<1 μm) provided approximately half of the standing crop of chlorophyll. Temporal changes in the position of the nitracline at a single location indicated that the vertical supply of nitrate was not at 'steady-state' and phytoplankton distributions tracked the nitracline. Our main experimental objective was to examine the short-term effects of ecologically significant nitrate perturbations (+20 and +100 nM) on the physiology of <1 μm communities growing at low (nanomolar) ambient nitrate concentrations. A chemiluminescent nitrate method was used to measure the time course (up to 4 h) of nitrate disappearance at in situ irradiance, in parallel with measurements of photosynthetic 14CO2 assimilation. Picoplankton growing at <60 nM nitrate rapidly responded to nanomolar nitrate supplements with luxury consumption and enhanced photosynthesis in proportion to their ambient nitrate environment. Light-saturated Synechococcus populations from the most nitrate-depleted waters (13 nM) had doubled their cellular rate of photosynthesis after 4 h, in response to a 20 nM nitrate pulse.