Effect of tertiary sewage effluent additions on Prymnesium parvum cell toxicity and stable isotope ratios

We investigated the ability of the ichthyotoxic haptophyte Prymnesium parvum to use sewage-originated nutrients applying stable carbon (C) and nitrogen (N) isotope techniques. P. parvum was cultured under N and phosphorus (P) sufficient and deficient conditions in either sewage effluent-based medium or in a nitrate- and phosphate-based control. Cell densities and toxicities were monitored and stable carbon N isotopes signatures (δ13C and δ15N) of P. parvum and the sewage effluent analysed. Nitrogen and P sufficient cultures achieved the highest biomass followed by P and N deficient cultures, regardless of sewage effluent additions. The P deficient cultures with sewage effluent had higher toxicity, estimated as haemolytic activity (9.4 ± 0 × 10−5 mg Saponin equiv. cell−1) compared to the P deficient control and to all N deficient and NP sufficient cultures. Nutrient deficient conditions had no effect on the cell δ15N, but a decreasing effect on δ13C in the inorganic N deficient treatment. Growth in sewage-based media was followed by a substantial increase in the cell δ15N (10.4-16.1‰) compared to the control treatments (2.4-4.9‰), showing that P. parvum is capable of direct use of sewage-originated N, inorganic as well as organic. Uptake of terrestrial derived C in the sewage treatments was confirmed by a decrease in cell δ13C, implying that P. parvum is able to utilize organic nutrients in sewage effluent.