The significance of organic nutrients in the nutrition of Pseudo-nitzschia delicatissima (Bacillariophyceae)

Abstract

The influence of organic nutrients on the evolution of Pseudo-nitzschia delicatissima cultures was investigated in an enrichment experiment with high-molecular-weight dissolved organic matter (HMWDOM) and in an uptake assay with 15N-ammonium and 15N-urea. HMWDOM was extracted from seawater collected at a nearby shore station during the decline of a diatom bloom. Four incubations were prepared: L1/5+DOM (P. delicatissima grown in L1 growth medium with 1/5 of the nitrate concentration of standard L1), (L1-N)+DOM (L1 without nitrate, i.e. nitrogen-deficient treatment), L1-DOM (control culture without added DOM) and BV+DOM (bacterial and viral control, free of microalgae). Incubations were carried out for 10 days. Chlorophyll a concentrations differed after day 4 and reached higher levels in the L1-DOM incubation by the end of the experiment; however, similar growth rates were observed in all incubations (1.64 ± 0.05 divisions day-1). The persistently lower cellular chlorophyll content in (L1-N)+DOM during the experiment was consistent with N limitation conditions. The data suggested that the nitrogen needed for the growth of (L1-N)+DOM cells might have originated from the DOM. Based on the results of 15N uptake assays, it was concluded that P. delicatissima more readily acquires ammonium than urea. Nevertheless, under low N conditions, P. delicatissima may use urea as an alternative N source, and comparable photosynthetic rates are attained on either substrate. Taken together, our results suggest a positive effect of organic nutrients on the growth of P. delicatissima. © 2008 The Author 2008. Published by Oxford University Press. All rights reserved.