ARREST OF CELL-DIVISION BUT NOT PROTEIN-SYNTHESIS IN SODIUM-DEFICIENTCELLS OF THE MARINE DIATOM PHAEODACTYLUM-TRICORNUTUM BOHLIN

Cell division in the marine diatom Phaeodactylum tricornutum was preve nted when cultures were maintained in the absence of sodium, regardles s of the nitrogen status of the cells or medium. Addition of 10 mM amm onium and 50 mM sodium to cultures preconditioned in nitrogen and sodi um-deficient medium for 5 d led to a recovery in cell division and chl orophyll a, and net protein synthesis. Sodium added in the absence of ammonium led to a recovery in cell division, but not net protein synth esis. Ammonium added in the absence of sodium was partially assimilate d (as NH3) and resulted in a small amount of protein synthesis, but wi thout cell division. This effect was enhanced if the cells had lower p rotein quotas prior to ammonium addition, with total consumption of th e added 1 mM ammonium and appreciable net protein synthesis. Respirati on was enhanced by 1 or 10 mM ammonium or 10 mM methylammonium additio n to nitrogen-deficient cultures maintained in the presence or absence of sodium. In contrast to respiration, photosynthesis was inhibited b y these additions in sodium-replete cultures, but was enhanced in sodi um-deficient cultures.