Utilization efficiency of nitrogen associated with riverine dissolved organic carbon (>1 kDa) by two toxin-producing phytoplankton species

Riverine high molecular weight dissolved organic material (HMWDOM) >1 kDa contains varying amounts of nitrogen. This nitrogen is partly available for growth of phytoplankton and can therefore contribute to algal bloom formation. However, there is a lack of knowledge on species-specific utilization efficiency of this HMWDOM-bound nitrogen. A specific question is whether this nitrogen can be used by toxic or otherwise harmful species and, thus, contribute to unwanted eutrophication effects in coastal waters. In this context, 2 phytoplankton species and known toxic bloom-formers in marine coastal waters were cultured in nitrogen-limited semi-continuous cultures with river extracted HMWDOM and nitrate as nitrogen source in the following proportions (%): 0:100, 25:75, 50:50, 75:25 and 100:0 (HMWDOM-bound nitrogen). Prymnesium parvum f. patelliferum (Green, Hibberd et Pienaar) A. Larsen, an ichthyotoxic prymnesiophyte, was not able to utilize the nitrogen bound to HMWDOM. The toxicity of this species, measured as hemolytic activity, did not show any significant correlation with HMWDOM concentration. In contrast, Alexandrium tamarense, a toxin-producing dinoflagellate responsible for paralytic shellfish poisoning (PSP), was able to use nitrogen from riverine HMWDOM as efficiently as nitrate, when the HMWDOM fraction was up to 75 % of the total dissolved nitrogen. When HMWDOM was supplied as the sole nitrogen source, the growth efficiency was reduced by approximately half. At steady state, the total cellular toxin content and toxicity of A. tamarense was negatively correlated with the fraction of HMWDOM. The use of organic nitrogen sources by marine dinoflagellates is well known. Here, we show that nitrogen from a terrestrial origin can be very efficiently used by toxic A. tamarense and could therefore contribute to unwanted eutrophication effects.