Prey Lysate Enhances Growth and Toxin Production in an Isolate of Dinophysis acuminata

Effect of a short-term salinity stress on the growth, biovolume, toxins, osmolytes and metabolite profiles on three strains of the Dinophysis acuminata-complex (Dinophysis cf. sacculus)

Dinophysis is the main dinoflagellate genus responsible for diarrheic shellfish poisoning (DSP) in human consumers of filter feeding bivalves contaminated with lipophilic diarrheic toxins. Species of this genus have a worldwide distribution driven by environmental conditions (temperature, irradiance, salinity, nutrients etc.), and these factors are sensitive to climate change. The D. acuminata-complex may contain several species, including D. sacculus. The latter has been found in estuaries and semi-enclosed areas, water bodies subjected to quick salinity variations and its natural repartition suggests some tolerance to salinity changes. However, the response of strains of D. acuminata-complex (D. cf. sacculus) subjected to salinity stress and the underlying mechanisms have never been studied in the laboratory. Here, a 24 h hypoosmotic (25) and hyperosmotic (42) stress was performed in vitro in a metabolomic study carried out with three cultivated strains of D. cf. sacculus isolated from the French Atlantic and Mediterranean coasts. Growth rate, biovolume and osmolyte (proline, glycine betaine and dimethylsulfoniopropionate (DMSP)) and toxin contents were measured. Osmolyte contents were higher at the highest salinity, but only a significant increase in glycine betaine was observed between the control (35) and the hyperosmotic treatment. Metabolomics revealed significant and strain-dependent differences in metabolite profiles for different salinities. These results, as well as the absence of effects on growth rate, biovolume, okadaic acid (OA) and pectenotoxin (PTXs) cellular contents, suggest that the D. cf. sacculus strains studied are highly tolerant to salinity variations.

Cell cycle regulation of the mixotrophic dinoflagellate Dinophysis acuminata: Growth, photosynthetic efficiency and toxin production

The mixotrophic dinoflagellate Dinophysis acuminata is a widely distributed diarrhetic shellfish poisoning (DSP) producer. Toxin variability of Dinophysis spp. has been well studied, but little is known of the manner in which toxin production is regulated throughout the cell cycle in these species, in part due to their mixotrophic characteristics. Therefore, an experiment was conducted to investigate cell cycle regulation of growth, photosynthetic efficiency, and toxin production in D. acuminata. First, a three-step synchronization approach, termed "starvation-feeding-dark", was used to achieve a high degree of synchrony of Dinophysis cells by starving the cells for 2 weeks, feeding them once, and then placing them in darkness for 58 h. The synchronized cells started DNA synthesis (S phase) 10 h after being released into the light, initiated G2 growth stage eight hours later, and completed mitosis (M phase) 2 h before lights were turned on. The toxin content of three dominant toxins, okadaic acid (OA), dinophysistoxin-1 (DTX1) and pectenotoxin-2 (PTX2), followed a common pattern of increasing in G1 phase, decreasing on entry into the S phase, then increasing again in S phase and decreasing in M phase during the diel cell cycle. Specific toxin production rates were positive throughout the G1 and S phases, but negative during the transition from G1 to S phase and late in M phase, the latter reflecting cell division. All toxins were initially induced by the light and positively correlated with the percentage of cells in S phase, indicating that biosynthesis of Dinophysis toxins might be under circadian regulation and be most active during DNA synthesis.