Comparative study of selenium requirements of three phytoplankton species:Gymnodinium catenatum, Alexandrium minutum (Dinophyta) and Chaetoceros cf.tenuissimus (Bacillariophyta)

This study investigated the selenium (Se) requirements of three phytoplankt on species which commonly bloom in southern Australian estuaries. The prese nt study showed that the toxic dinoflagellate Gymnodinium catenatum Graham had an obligate requirement for Se (IV) in culture. After two transfers (si milar to 4 weeks approximate to 7 generations) in Se-deficient seawater med ium, this phytoplankton species exhibited a decline in growth rate (25%) an d biomass yield (90%), while complete cessation of cell division occurred u nder prolonged (8 weeks approximate to 12 generations) Se starvation. Addit ion of 10(-9)-10(-7) M H2SeO3 to nutrient-enriched seawater medium resulted in increased G.catenatum growth and biomass yields in direct proportion to the Se concentrations offered. In contrast to G.catenatum, Se limitation w as observed in the dinoflagellate Alexandrium minutum Halim after four tran sfers (5 weeks approximate to 20 generations) in Se-deficient medium. Expon ential growth rates of A.minutum decreased slightly (5-10%) when Se was not supplied, but biomass yields decreased as much as 80-90%. The diatom Chaet oceros cf. tenuissimus Meunier showed no evidence of Se limitation even aft er eight transfers (8 weeks; >60 generations) in Se-deficient medium. Varia tions in growth rates and biomass yields between transfers provide valuable information about the relative potential for Se limitation in the three sp ecies studied. In addition, differences in Se requirement between these blo om-forming phytoplankton species suggest that this micronutrient may play a role in structuring phytoplankton communities in southern Australian water s.