▪ Abstract The evolutionary succession of marine photoautotrophs began with the origin of photosynthesis in the Archean Eon, perhaps as early as 3.8 billion years ago. Since that time, Earth's atmosphere, continents, and oceans have undergone substantial cyclic and secular physical, chemical, and biological changes that selected for different phytoplankton taxa. Early in the history of eukaryotic algae, between 1.6 and 1.2 billion years ago, an evolutionary schism gave rise to “green” (chlorophyll b–containing) and “red” (chlorophyll c–containing) plastid groups. Members of the “green” plastid line were important constituents of Neoproterozoic and Paleozoic oceans, and, ultimately, one green clade colonized land. By the mid-Mesozoic, the green line had become ecologically less important in the oceans. In its place, three groups of chlorophyll c–containing eukaryotes, the dinoflagellates, coccolithophorids, and diatoms, began evolutionary trajectories that have culminated in ecological dominance in the contemporary oceans. Breakup of the supercontinent Pangea, continental shelf flooding, and changes in ocean redox chemistry may all have contributed to this evolutionary transition. At the same time, the evolution of these modern eukaryotic taxa has influenced both the structure of marine food webs and global biogeochemical cycles.

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.

A Mediterranean clone of the red-tide forming dinoflagellate Prorocentrum minimum was studied in vitro for its capacities to adapt to salinity, temperature and light. This clone is euryhaline and shows optimal growth between 15 and 35‰. After adaptation, slow growth was observed at salinities as low as 5‰. An apparatus generating crossed gradients of temperature and light allowed 100 combined experimental conditions to be studied. Variations in lighting between 30 and 500 (μmol photons m⁻² s⁻¹ had little effect on growth, and no photoinhibition occurred. The clone can grow between 8 and 31°C, but is thermophilic with an optimal growth between 18 and 26.5°C. As a result of large variations in temperature from 18°C down to 10°C and maintained at 10°C, small spherical structures (8–10 μjn) were observed; they are described as temporary cysts. These results were compared to those obtained by different authors, in vitro and in situ, notably in the Mediterranean region.

The dinoflagellate Alexandrium minutum strain AM89BM was studied to investigate its capacity to adapt to different salinities and the influence of salinity on cellular content of paralytic shellfish toxins (PST), in batch culture in enriched offshore seawater media. The strain shows optimal growth (average rate ≥0.5 div day⁻¹) at salinities between 20 and 37 p.s.u., peaking at 25 p.s.u. (0.63 ± 0.07 div day⁻¹). Under a high NO₃^–:PO₄^3– mole ratio (76), cell PST content increased during growth phase, peaking toward the end of growth phase or in the early stationary phase. The PST content was low (∼10 fmol PST cell⁻¹) from 30 to 37 p.s.u., but it increased at lower salinities, with the maximum value (∼50 fmol PST cell⁻¹) recorded at 15 p.s.u. We then compared growth rate and toxin content of A. minutum cells grown in 3- and 0.2-μm filtered estuarine water (27 p.s.u.), and a similarly filtered seawater collected from an oyster farming area (36 p.s.u.), with a N- and P-enriched offshore seawater (37 p.s.u.). In both estuarine water treatments, growth was fast (≥0.8 div day⁻¹) and the cell PST content increased only in the early growth phase, peaking in mid-growth phase at 9.56 ± 1.06 and 7.63 ± 0.44 fmol PST cell⁻¹ in the 3- and 0.2-μm filtered waters, respectively. In the 3-μm filtered oyster farm water, although inorganic nutrient concentrations were very low, A. minutum grew well (0.68 ± 0.06 div day⁻¹), suggesting mixotrophic nutrition; as PST production was nearly zero, the cell toxin content decreasing to 1.11 fmol PST cell⁻¹, we hypothesize that toxin biosynthesis was greatly weakened due to the lack of amino acid precursors in prey material. In the offshore seawater, A. minutum grew slowly (0.18 ± 0.04 div day⁻¹) and cells lost toxins down to 1.08 fmol PST cell⁻¹, suggesting that growth was limited and PST production stopped due to the lack of some vitamins, and/or trace metals.