Phosphorus Content and Rate of Growth in the Diatoms Cyclotella Nana and Thalassiosira Fluviatilis

Cyclotella nana and Thalassiosira fluviatilis were grown in chemostat and turbidostat units in 1/3 strength artificial seawater supplemented with nutrients and vitamin B12 in excess except for phosphorus (P) which in concentrations of 50 or 100 μg/liter was limiting. Different chemostat and turbidostat settings produced different cell numbers per unit volume which, since external P was practically nil, corresponded to varying amounts of bound P per cell. Growth rate plotted vs. P per cell follows a saturation curve. While no growth occurs at a certain minimum content of bound P, half the maxi-mum growth rate is observed at twice the minimum level of P,3/4 of the maximum at 3 times the minimum level, etc., the maximum growth rate being defined as that observed during growth unrestricted by P supply at a given set of conditions (light, temperature, salinity, etc.). Temperature and light were varied from 13.5 to 24 C and from 2000 to 6000 lux (continuous and intermittent). An abnormal growth curve was found with C. nana at 24 C and 6000 lux continuous light. Any change in P content of the cells significantly alters cell composition as reflected in P distribution in acid-soluble, lipid, and acid-insoluble fractions and in organic carbon content. External P does not immediately enter our equation for growth rate but is one of the factors deter-mining rate of P uptake and therefore bound P per cell. Conditions governing uptake, however, cannot be adequately controlled in a chemostat and there fore should be determined in a different experimental approach.