Growth of harmful marine algae in multispecies cultures

Mixtures of harmful and harmless algae were grown in discontinuously diluted batch cultures under ammonium, nitrate and phosphate limitation, and at different irradiances (20–500 μjnol quanta m−2 s−1). The species used were Chrysochromulina polylepis, Emiliania huxleyi type B, Rhodomonas sp., the dinoflagellales Fibrocapsa japonica, Gymnodinium simplex, Gyrodinium aure-olum, Heterocapsa triquetra, Heterosigma carterae, Prorocentrum micans and Alexandrium tamarense, the diatoms Chaetoceros socialis, Cymatosira belgica, Ditylum brightwellii, Laudcria borealis, Odon-telta aunla, Pseudonitzschia pungens, Streptotheca tamesis, and the cyanobacterium Synechococcus. Their growth response in the mixed algal cultures is discussed in relation to their abundance in different natural habitats. In comparison with the other non-diatoms, the mixotrophic C.polylepis grew fast under all tested nutrient and light limitations.Emiliania huxleyi grew well under nitrogen (N) limitation (with nitrate as N source) and at irradiance levels from 15 up to 500 μmol quanta m−2 s−1. No growth of calcifying cells could be detected under N limitation when ammonium was used as N source. Rhodomonas grew reasonably well under ammonium-N limitation and grew fast at the highest irradi-ance. The dinoflagellates were poor competitors compared to the Prymnesiophyceae. The environmental fitness of the Prymnesiophyceae appears to be closely related to the reproductive capacity of the vegetative stage, whereas the natural distribution of dinoflagellates seems more closely dependent on the generative reproduction-related specific life cycle characteristics of the individual species. The marine diatoms include a mixture of both types of species. Some marine diatom species clearly have the capability to outcompete non-diatoms under different types of nutrient and light limitations when silicate is in excess. Other diatoms seem to be poor competitors.