Effects of nutrient limitation on food uptake in the toxic haptophyte Prymnesium parvum

Abstract

The haptophyte Prymnesium parvum Carter is toxic and frequently responsible for harmful algal blooms in coastal waters. It is a mixotrophic species having the capability to feed on various planktonic microorganisms. It is frequently suggested that mixotrophic algae may obtain inorganic nutrients through phagotrophy and that nutrient depletion should then lead to increased food uptake. To study this, we investigated the feeding activity of P. parvum in semi-continuous, nutrient-limited cultures, using the cryptophyte Rhodomonas baltica as prey. P. parvum showed to be an active predator under all conditions investigated. After 2 h of incubation with prey, 40% of P. parvum cells were either feeding or contained recently formed food vacuoles. However, under the conditions used, no difference in feeding activity was found between treatments. On the contrary, the feeding activity was similar in P. parvum cultures that had been grown under N-limiting, P-limiting, N- and P-limiting, as well as under nutrient-replete conditions. It cannot be excluded that P. parvum under limiting nutrient conditions may acquire nutrients to be used in photosynthetic growth through phagotrophy. It is evident, however, that the species also feeds when inorganic nutrients are present in concentrations sufficient to support maximum phototrophic growth.