Harmful algal blooms of Prymnesium parvum have recurrently been associated with the killing of fish. The causative ichthyotoxic agents of this haptophyte are believed to be prymnesins, a group of supersized ladder-frame polyether compounds currently divided into three types. Here, the development of a quantitative method to assess the molar sum of prymnesins in water samples and in algal biomass is reported. The method is based on the derivatization of the primary amine group and subsequent fluorescence detection using external calibrants. The presence of prymnesins in the underivatized sample should be confirmed by liquid chromatography mass spectrometry. The method is currently only partly applicable to water samples due to the low amounts that are present. The growth and cellular toxin content of two B-type producing strains were monitored in batch cultures eventually limited by an elevated pH. The cellular toxin contents varied by a factor of ~2.5 throughout the growth cycle, with the highest amounts found in the exponential growth phase and the lowest in the stationary growth/death phases. The strain K-0081 contained ~5 times more toxin than K-0374. Further investigations showed that the majority of prymnesins were associated with the biomass (89% ± 7%). This study provides the basis for further investigations into the toxicity and production of prymnesins.

Harmful algal blooms kill fish populations worldwide, as exemplified by the haptophyte microalga Prymnesium parvum. The suspected causative agents are prymnesins, categorized as A-, B-, and C-types based on backbone carbon atoms. Impacts of P. parvum extracts and purified prymnesins were tested on the epithelial rainbow trout fish gill cell line RTgill-W1 and on the human colon epithelial cells HCEC-1CT. Cytotoxic potencies ranked A > C > B-type with concentrations spanning from low (A- and C-type) to middle (B-type) nM ranges. Although RTgill-W1 cells were about twofold more sensitive than HCEC-1CT, the cytotoxicity of prymnesins is not limited to fish gills. Both cell lines responded rapidly to prymnesins; with EC₅₀ values for B-types in RTgill-W1 cells of 110 ± 11 nM and 41.5 ± 0.6 nM after incubations times of 3 and 24 h. Results of fluorescence imaging and measured lytic effects suggest plasma membrane interactions. Postulating an osmotic imbalance as mechanisms of toxicity, incubations with prymnesins in media lacking either Cl⁻, Na⁺, or Ca²⁺ were performed. Cl⁻ removal reduced morphometric rearrangements observed in RTgill-W1 and cytotoxicity in HCEC-1CT cells. Ca²⁺-free medium in RTgill-W1 cells exacerbated effects on the cell nuclei. Prymnesin composition of different P. parvum strains showed that analog composition within one type scarcely influenced the cytotoxic potential, while analog type potentially dictate potency. Overall, A-type prymnesins were the most potent ones in both cell lines followed by the C-types, and lastly B-types. Disturbance of Ca²⁺ and Cl⁻ ionoregulation may be integral to prymnesin toxicity.

Chaetoceros peruvianus is a marine diatom species with circumglobal distribution. While frequently observed, it appears never to dominate the marine phytoplankton community hence it can be characterized as a rather opportunistic, generalistic species. Here we present ecological interpretations from a long-term data set on marine microphytoplankton in the northern Adriatic Sea, where the abundancies and relative contributions of C. peruvianus were observed along a set of steep ecological gradients. Limited supply of dissolved inorganic phosphate was identified as the driving ecological factor for this ecosystem. In parallel C. peruvianus was cultivated in monoclonal cultures and its morphological and physiological reaction to replete and phosphorus depleted medium was analysed. C. peruvianus reacted to phosphorus depletion by an increase in cell height and length as well as thickness and length of setae. This morphological reaction included an increase in cellular volume and calculated carbon content. Additionally, it represents the transition between two described morphological varieties, C. peruvianus and C. peruvianus var. robusta. C. peruvianus showed a significant induction of extracellular alkaline phosphatase activity if grown in phosphate depleted medium. Microscopical analysis demonstrated this activity to be located exclusively on the setae of the cells.

Marine life is threatened by global warming and its indirect consequences, which include, among others, increased stratification leading to phosphorus (P) and nitrogen depletion in the upper water column. Phosphorus plays a key role in all biochemical systems; storage of metabolic energy, formation of genetic material, and subcellular compartmentalization. Our multi‐year study of lipid biogeochemistry in the northern Adriatic (Mediterranean), which is becoming warmer and nutrient‐poorer, particularly regarding P, has shown that under conditions of P scarcity, phospholipids are relatively more abundant and smaller plankton cells dominate. Consistent with the field data, experiments with seven phytoplankton monocultures, comprising microplankton, nanoplankton, and picoplankton, confirmed a relatively higher phospholipid content in the smaller phytoplankton species and, in particular, an increase in those grown under stress conditions in general, including, unexpectedly, P‐limitation. We suggest two reasons for the observed “P paradox” of P‐limited phytoplankton: (1) cell geometry: volume of the plasma membrane relative to the volume of the entire cell is greater in smaller cells and, therefore, the proportion of plasma membrane phospholipids to intracellular lipids is greater in smaller cells, (2) higher proportion of densely packed saturated fatty acids found in stressful conditions, including P oligotrophy, additionally increase the proportion of membrane phospholipids relative to intracellular lipids. Our findings contribute to the understanding of P cycling in the sea. In addition, our data suggest that higher phospholipid export to deep waters is possible by smaller plankton.