Silicic acid uptake and incorporation by natural marine phytoplankton populations1

68Ge‐germanic acid was used as a tracer to study the uptake and incorporation of silicic acid by natural phytoplankton populations in the Gulf of California. The technique was useful in measuring silicic acid incorporation rates as low as 0.5 nM h‒1. The incorporation followed saturation kinetics. Ks values at two stations were 1.59 and 2.53 µM. The incorporation was light dependent, maximum rates being achieved at 0.28 × 1021 photons cm‒2 d‒1 (0.044 ly min‒1; 9% of surface irradiance) and half‐maximum rate at 0.02 × 1021 photons cm‒2 d‒1 (0.003 ly min‒1; 0.6% of surface irradiance); rate of incorporation in the dark was 44% of that at light saturation. Enrichment with silicic acid caused stimulation of photosynthetic carbon fixation at two stations.