Growth interactions between marine dinoflagellates in multispecies culture experiments

The growth interactions between the marine dinoflagellates Scrippsiella faeroense (Paulsen) Balech et Soares, Prorocentrum micans Ehrenberg and Gymnodinium splendens Lebour were investigated in batch and continuous-culture combinations. Generation times were 1.3±0.2 days for S. faeroense; 1.6±0.1 days for P. micans and 1.8 ± 0.2 days for G. splendens. In mixed batch cultures, growth depended strongly on the ratio between cell numbers of the two species inoculated at the start of the experiments. In various inocula, the more abundant species prevailed and suppressed the competitor even during exponential growth. Total cell production of the suppressed forms was lowered drastically; initial generation time, however, remained more or less unaffected. In equal inocula, S. faeroense dominated over G. splendens, P. micans over S. faeroense and G. spendens over P. micans. In continuous cultures, the species grew independently of each other during the exponential stage; cell number was regulated only by generation times. Competitive inhibition did not occur until maximum cell densities of the dominating forms had been reached. Filtration experiments indicated inhibiting effects mainly in filtered culture liquids of stagnating cultures: reinoculated cells of all three test populations showed a somewhat reduced total cell production. Short initial lag phases were indicated in filtered culture liquids of P. micans and G. spendens; following this period, generation times of the test population remained unaffected, however. The experiments show that growth in multispecies cultures is regulated during the exponential stage mainly by nutrient competition, while at maximum cell densities an additional effect of inhibiting metabolic products is involved. Whether this effect is caused by toxic algal excretions or by bacterial decomposition products of dead cells could not be determined. Sexual stages occurred in aged cultures of S. faeroense and G. splendens both in monocultures and in multispecies combinations and influenced the equilibrium of the competing species. The results are discussed in the light of the findings of other authors.This paper was presented at a poster presentation of the Second International Conference on Toxic Dinoflagellate Blooms, held in Key Biskayne, Florida, USA, 1978.