Tunisian microalgae are diverse and rarely been studied. This study reports a first investigation of thermophile Chlorophyta isolated from mats community colonizing the geothermal springs in the north of Tunisia at water temperature 60 °C. In the study, the combined effect of temperature and light intensity was investigated on the cell growth, the mother and daughter cells abundance and the extracellular polymeric substances synthesis in batch culture of the isolated species. Three levels were tested for each factor, 20, 30, 40 °C for temperature; and 20, 70, 120 μmol photons m(-2) s(-1) for light intensity, using full factorial design and response surface methodology. The thermophile strain was identified as a genus Graesiella and showed 99.8% similarity with two Graesiella species: Graesiella emersonii and Graesiella vacuolata based on the 18S rDNA molecular identification. The optimal growth condition was found at 30 °C and 120 µmol photons m(-2) s(-1) (7 MC mL(-1) day(-1)), with the abundance of vegetative cells (daughter cells). In contrast, the number of mother cells increased significantly as the growth decreased; consequently, the highest ratio of auto spore mother cells versus daughter cells (19.4) was obtained at 20 °C and 20 µmol photons m(-2) s(-1). The highest yield of EPS production (11.7 mg L(-1) day(-1)) was recorded at the highest temperature (40 °C) and lowest light intensity (20 µmol photons m(-2)s(-1)). These results revealed how the species respond to high and low temperatures and suggest that the species should be considered as facultative thermophile.

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A thermo-tolerant diatom species has been isolated from Tunisian hot spring water (40°C). The isolated diatom has been molecularly identified and classified into the genus Halamphora. The growth kinetics, lipid content and distribution of fatty acids were assessed at 20 and 30°C temperature levels and constant irradiance in controlled batch cultures (11 days). Halamphora sp. showed better growth (μ = 0.53 day−1) and a higher lipid yield (25% of the dry weight) at a higher temperature (30°C). Under the two temperatures tested, the highest lipid and fatty acid contents were mainly reached during the stationary growth phase. The fatty acid profile showed a significant content of two essential fatty acids, eicosapentaenoic acid (EPA, 20:5n-3) and arachidonic acid (AA, 20:4n-6), reaching ~15% and ~21% of the total fatty acids, respectively, at 20°C and 30°C. The distribution of the different components of the fatty acids showed that EPA and AA were mainly located in the neutral lipid fraction in the stationary phase.