Competition of two marine diatom algae for urea and nitrate nitrogen under three levels of irradiance

Biomass dynamics of the plankton diatoms Thalassiosira weissflogii and Pseudo-nitzschia delicatissima were analyzed in batch mono- and mixed cultures grown on media with urea or nitrate as the sources of nitrogen, under irradiance 13, 38, and 115 mu E/(m(2 .) s). At the initial enrichment, nitrogen concentration was 0.18 mmol, and the nitrogen : phosphorus ratio was 5 : 1. The mechanisms of competition for the limiting resource satisfactorily described the interactions between the algae grown on urea. Competitive ability of algae was characterised according to the value of competitive eddect (CE), which was calculated as the ratio of growth rate and accumulated biomass decrease in mixed culture to that in monoculture CE of algae grown on urea increased with the increasing of irradiance and was lower than that of algae grown on nitrate. CE of P. delicatissima was hi-her than that T. weissflogii, independently of the source of nigrogen and the level of irradiance. At 38 and 115 mu E/(m(2). s) the growth of T. weissflogii ceased earlier than that of P. delicatissima, independently of the source o nitrogen. At 13 mu E/(m(2 .) s) the growth of P. delicatissima ceased earlier than of T. weisflogii in on cultures grown urea, but the growth of T. weissflogii was the first to cease on nitrate. The competition revealed in experimental communities for the nitrogen of urea between plankton algae gives reasons to suggest that in natural communities plankton algae also compere under inorganic nitrogen deficiency and organic nitrogen abundance.