Studies on the Growth of Marine Phytoplankton I.Asterionella Japonica Gran

Abstract

A planktonic marine diatom, Asterionella japonica, has been grown in unialgal, but not bacteria-free, culture under controlled conditions and its growth has been measured by means of optical density determinations and cell counts in combination.The relative growth constant has been found to be usually between 0·7 and 1·2 log_e units per day, and to be little affected by variation of the concentration of nitrate, phosphate and silicate within wide limits. Rapid growth occurred in waters having salinities between 20 and 40‰ S. The optimum temperature for growth was 20–25° C and the optimum light intensity from 4000 to 10,000 lux. The relative growth rate was affected most markedly by variation in hydrogen-ion concentration. This was manifest in stagnant cultures in which the rise of pH above 8·3, caused by absorption of carbon dioxide, resulted in the inhibition of growth. This could be countered by increased aeration or by the addition of the buffer tris(hydroxymethyl)aminomethane.Evidence has been obtained which suggests that A. japonica requires a thermostable substance present in natural sea water and that a thermolabile substance stimulating its growth is produced in culture.Final yields of the order of 4000 cells per mm³ were obtained and, within limits, were dependent on the amounts of nitrate and silicate supplied. The nitrogen requirements per cell was determined as 0·255 μμg atom. Indications were obtained of the presence of a growth-inhibiting substance in the filtrates from old cultures.The optical density of the cells was increased at high nitrate concentrations and when exhaustion of silicate prevented cell division.