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In batch cultures of flagellates and non-flagellate cells of Phaeocystis globosa, the biomass yield was significantly enhanced by the addition of a mixture of the vitamins thiamine (B1), cyanocobalamin (B12) and biotin (H). A bioassay with B1 and B12 using the non-flagellate cells of P.globosa showed that this prymnesiophyte is a B1 auxotroph. The bioassay also indicated a significant difference in growth rate between culture medium with 10 nmol l–1 B1 (μ = 0.80 day–1) and culture medium with 10 nmol l–1 B12 (μ = 0.52 day–1). These findings are discussed in relation to the hypothesis that centric diatoms, through vitamin B1 excretion or B12 depletion, initiate Phaeocystis blooms. It is concluded, however, that an alternative hypothesis, that diatoms provide a solid substrate for colony initiation, has more experimental support. [ABSTRACT FROM PUBLISHER]

Two cell types of the same clone of Phaeocystis globosa, solitary non-flagellate cells and flagellates, were grown in batch cultures under identical conditions. The non-flagellate cells had a shorter lag phase (1.4 versus 2.8 days) and a higher growth rate (0.72 versus 0.65 day–1) than flagellate cells. The flagellates had a longer stationary phase (15.6 versus 9.5 days) and a lower death rate (0.07 versus 0.52 day–1) than non-flagellate cells. All differences were statistically significant. Biomass yield did not differ between the two cell types. The short lag phase and high growth rate of nonflagellate cells corresponds to field observations of rapidly developing non-flagellate Phaeocystis blooms that are typically observed in nutrient-rich environments such as temperate seas in spring. The flagellate cell type, with its longer stationary phase and lower death rate than non-flagellate cells, is better equipped for survival in oligotrophic environments. This explains why the flagellates of Phaeocystis are abundant after the spring phytoplankton bloom in temperate seas and in other nutrient-poor environments such as the open ocean. [ABSTRACT FROM PUBLISHER]