Modifications in Filtration Methods for the Measurement of Inorganic 14c Uptake by Photosynthesizing Algae1

SUMMARY In the course, of routine primary productivity experiments with phytoplankton, the filtration step may introduce errors into the determination of inorganic 14C uptake into particulate and filtrate organic fractions. However, when ultrafine glass fiber filters, low filtration pressures, and fuming over HC1 were used, and corrections made for nonbiological adsorption of radioactivity on filters, no significant changes of cpm retained on filters per milliliter of filtration volume were found in tests with various algal populations. Cultures of Lauderia sp. and Gonyaulax polyedra, when photosynthesizing in 1 compartment of a 2‐chambered diffusion apparatus, released similar amounts of radioactive dissolved organic compounds measured after diffusion or upon filtration. Under our conditions, only negligible amounts of dissolved organic compounds were retained by fillers. For many natural waters, therefore, these filtration techniques permit adequate accuracy for primary productivity measurements.