Mixotrophic lifestyle of the ichthyotoxic haptophyte, Prymnesium parvum, offered different sources of phosphorus.

Many harmful algae are mixoplanktonic, i.e. they combine phototrophy and phagotrophy, and this ability may explain their ecological success, especially when environmental conditions are not optimal for autotrophic growth. In this study, laboratory experiments were conducted with the mixotrophic and ichthyotoxic haptophyte Prymnesium parvum (strain CCAP 946/6) to test the effects of phosphorus (P) sufficiency and deficiency on its growth rate, phagotrophic and lytic activities. P-deficient P. parvum cultures were grown without or with addition of P in the form of inorganic phosphorus (nutrients) and/or living algal prey (i.e. the cryptophyte Teleaulax amphioxeia). The ingestion rate of P. parvum and prey mortality rate were calculated using flow cytometry measurements based on pigment-derived-fluorescence to distinguish between prey, predators and digesting predators. The first aim of the study was to develop a method taking into account the rate of digestion, allowing the calculation of ingestion rates over a two-week period. Growth rates of P. parvum were higher in the treatments with live prey, irrespective of the concentration of inorganic P, and maximum growth rates were found when both inorganic and organic P in form of prey were added (0.79 ± 0.07 d

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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.