## 4 Conclusions  
Qatar is an arid country with almost the highest irradiation conditions in the world. Qatar country mainly presents two seasons: long and extreme summer from March to the end of October, and moderate winter. Being able to cultivate microalgae in summer conditions is therefore important to exploit microalgae as a feedstock of interest in such a location. Figure 9: Total sugar and protein contents of \(N\). sp QU130 evolutions over 24 h under three different cultivation regimes: (**A**): Cultivation under day/night cycles; (**B**): Cultivation under temperature fluctuation and (**C**): Cultivation under combined light and temperature cultivation. Results are given for trial 1 (similar results for other trials). Conditions representing a culture in the summer period in the Qatar desert resulted in stressful conditions for the growth of \(N\). sp. QU130, as indicated by a decrease in growth rate combined with a simultaneous increase in lipids content. However, the desert strain \(N\). sp. QU130 revealed to acclimate to all the harsh light and temperature variation conditions tested in this study. Even when acclimated to constant light and temperature conditions, the strain survived all the sudden changes imposed. The set-up for the experiment enabled dissociation of the respective effects of temperature and light regimes:  
* Light cycles with constant temperature demonstrated a sharp 50% decrease in biomass concentration when compared to steady-state culture at constant continuous light and temperature (1.9 kg.m\({}^{-3}\) to 0.96 kg.m\({}^{-3}\)), following an acclimation time of around 5 days. This resulted in a biomass productivity of 0.67 kg.m\({}^{-3}\).day\({}^{-1}\) (i.e., 20.1 g.m\({}^{-2}\).day\({}^{-1}\)), compared to constant light and optimal temperature, which resulted in a maximal volumetric biomass productivity of 1.1 kg.m\({}^{-3}\)d\({}^{-1}\) (i.e., 32 g.m\({}^{-2}\).d\({}^{-1}\)). * When temperature cycle stress was applied to the steady-state culture under constant light and temperature, a 15% decrease in biomass concentration was first observed (1.75 kg.m\({}^{-3}\) to 1.5 kg.m\({}^{-3}\)), followed by a quick recovery of algae growth indicating rapid acclimation to temperature, leading to a biomass concentration 50% higher than under optimal constant conditions (2.3 kg.m\({}^{-3}\)). This surprising result can be explained by the \(N\). sp. QU130 strain adapting to the harsh environmental conditions of the Qatar desert from where the strain was isolated [17; 18]. In terms of benefits, it also highlights the potentially transitory effect of high temperature on growth. Exposure for a few hours revealed that it benefited the growth rate, with biomass productivity higher than under constant conditions. Consequently, the temperature cycle with constant light recorded the highest volumetric biomass productivity, with a rate of 1.10 kg.m\({}^{-3}\) day\({}^{-1}\) (i.e., 33 g.m\({}^{-2}\).day\({}^{-1}\)). * Under conditions representing summer in the Qatar desert, with combined light and temperature cycles, the clear ability of \(N\). sp. QU130 to acclimate to these extreme conditions has been demonstrated. However, the effects of such stressful conditions are also emphasized. The sudden shift from constant conditions to temperature/light cycles requires an acclimation period of around 25 days, with a significant decrease in productivity for the first 2 weeks preceding a slight increase, achieving a stable regime once the culture is acclimated. This resulted in the lowest rate of volumetric biomass productivity of 0.62 kg.m\({}^{-3}\).day\({}^{-1}\) (i.e., 18.6 g.m\({}^{-2}\).day\({}^{-1}\)). Biomass composition and cell morphology were found to be strongly influenced by light-temperature regimes investigated in our study. A strong reorganization of \(N\). sp. QU130 was observed in relation to constant conditions, with a marked increase in cell volume and lipid content and a simultaneous reduction in protein.