The relationship between toxicity and mixotrophy in bloom dynamics of
the ichthyotoxic Prymnesium parvum
Abstract
Toxin production in planktonic protists is widespread and comprises an
effective mechanism to eliminate competitors or grazers. Toxins released
into the water can mediate the immobilization of prey for subsequent
consumption or can mediate the lysis and uptake of the released
nutrients. For the worldwide-distributed ichthyotoxic and mixotrophic
haptophyte Prymnesium parvum, the relationship between toxin
production; impact on co-occurring species, and mixotrophy remains
blurred. In the current study, we show that changes in salinities (5 vs
30), phosphorous (P) availabilities (P-replete vs P-deplete) and cell
densities affect growth, toxicity and mixotrophy in P. parvum.
Cell density positively affected cellular toxin content by a factor of
up to 10. Low salinity resulted in a higher mortality of the cryptophyte
prey Teleaulax acuta, with ~80% of cells being
lysed after 2 h of incubation. However, phagotrophic rates were higher
in P-deplete conditions, independent of the salinity. Transcriptomic
analysis of the monocultures revealed the up-regulation of genes
involved in endocytosis under either low salinity and phosphorous,
suggesting that this process is evolutionarily conserved, triggered by
environmental stressors and independent of prey presence. Polyketide
synthase genes, potentially involved in toxin biosynthesis, exhibited
distinct expression patterns, depending on the physiological status,
toxicity and with generally higher expression under the high cell
density conditions. Overall, our study contributes to a better
understanding of the dynamics between the two critical processes of
toxin production and mixotrophy, and has important implications for
bloom formation and its maintenance in this ecologically important
species.