PHYTOPLANKTON GROWTH AND COMPOSITION IN SHIPBOARD CULTURES SUPPLIED WITH NITRATE, AMMONIUM, OR UREA AS THE NITROGEN SOURCE1

Abstract

Coastal surface seawater off southern California was enriched with phosphate, silicate, vitamins B12, B1, and biotin, a chelated trace metal mixture, and with nitrogen as nitrate, ammonium, or urea, and the cultures were incubated on deck in daylight. Natural phytoplankton present in the seawater served as inoculum. Samples were taken every 6 hr, once the cultures were established. Diel periodicity was noted in diatom cell division, in nitrate and ammonium assimilation rate, in phosphate assimilation rate, and in photosynthetic rate measured at intervals under constant, artificial irradiance, but not in the rate of chlorophyll a synthesis. The chemical composition of the crops was influenced by the nitrogen source used for growth and by diel periodicity in assimilation rates. Most of the vitamin Bi content of the crop was synthesized by the organisms, only a small proportion being supplied initially from the medium. Some species known to require vitamins continued to grow after vitamin depletion from the medium, their requirements apparently satisfied by vitamins released by other species.