In a laboratory study, we determined the influence of temperature, salinity and irradiance on the growth of the paralytic shellfish poisoning (PSP) toxin producer Alexandrium catenella, which can form toxic blooms in the Thau lagoon (western Mediterranean Sea). The strain studied, ACT03, was grown in an artificial seawater medium. The influence of temperature and that of salinity were analysed using 48 different combinations of 6 salinities (10–40 psu) and 8 temperatures (9–30°C) under saturating irradiance (100 µmol photons m⁻² s⁻¹). ACT03 appeared to be an euryhaline strain that can survive at salinities as low as 10 psu and can grow at salinities up to 40 psu. This strain can grow between 15 and 30°C. The highest growth rates (>0.4 day⁻¹) were observed between 35 and 40 psu and 15 and 27°C. The influence of irradiance on growth and cell pigment content was tested between 10 and 260 µmol photons m⁻² s⁻¹ at 20°C and 38 psu. The results revealed both a low compensation irradiance and that light saturation was reached at 90 µmol photons m⁻² s⁻¹. Temperature had the greatest influence on growth. The ecophysiological characteristics reported here are consistent with the environmental conditions encountered in the Thau lagoon. A. catenella exhibited important adaptive capacities over the large range of tested physical factors. This flexibility helps us to explain its ability to bloom seasonally on the Mediterranean coast, where the physico-chemical environmental conditions are characterized by high seasonal variations.

The multifactorial etiology of massive Crassostrea gigas summer mortalities results from complex interactions between oysters, opportunistic pathogens and environmental factors. In a field survey conducted in 2014 in the Mediterranean Thau Lagoon (France), we evidenced that the development of the toxic dinoflagellate Alexandrium catenella, which produces paralytic shellfish toxins (PSTs), was concomitant with the accumulation of PSTs in oyster flesh and the occurrence of C. gigas mortalities. In order to investigate the possible role of toxic algae in this complex disease, we experimentally infected C. gigas oyster juveniles with Vibrio tasmaniensis strain LGP32, a strain associated with oyster summer mortalities, after oysters were exposed to Alexandrium catenella. Exposure of oysters to A. catenella significantly increased the susceptibility of oysters to V. tasmaniensis LGP32. On the contrary, exposure to the non-toxic dinoflagellate Alexandrium tamarense or to the haptophyte Tisochrysis lutea used as a foraging alga did not increase susceptibility to V. tasmaniensis LGP32. This study shows for the first time that A. catenella increases the susceptibility of Crassostrea gigas to pathogenic vibrios. Therefore, in addition to complex environmental factors explaining the mass mortalities of bivalve mollusks, feeding on neurotoxic dinoflagellates should now be considered as an environmental factor that potentially increases the severity of oyster mortality events.