## 4 Conclusions
As demonstrated elsewhere, when cultivated in day-night cycles, the cells accumulated biochemical compounds such as protein and sugars generated by the photosynthetic process [43], with a simultaneous increase in cell size. In periods of darkness (night time), a reduction in biomass concentration and cell size was recorded, as a result of cell division (i.e., increase in cell numbers) and the respiration process (i.e., catabolism of biomass, especially sugar content) in the absence of light [42]. Interestingly, the changing temperature regime was shown to directly influence cell size, which was found to be larger than under continuous conditions. In terms of practical outcomes, although this study confirms the ability of \(N\). sp. QU130 to grow in the harsh summer conditions of the Qatar desert (i.e., the longer season in Qatar), year-round production would clearly imply optimal growth in the winter period [44]. Our findings would also suggest that it is important to control these two factors to maximize the biomass productivity of \(N\). sp. QU130. With constant illumination by artificial light, the application of temperature regimes could lead to an increase in productivity (15% for \(N\). sp. QU130). In actual outdoor cultures, it would be very difficult to control the light intensity (PFD), especially in summer, and impossible considering the open raceways currently used for algae production in Qatar [15]. On the other hand, as detailed by [PERSON] et al., it is unrealistic in practice to maintain a constant temperature in outdoor cultivation systems in harsh desert conditions, due to the excessive amount of energy required for thermal regulation [15]. In this respect, work focusing on reducing the range of temperature variation would be more feasible, so it is essential to characterize the effect of temperature fluctuation on algae growth. The biological response is certainly strain-dependent (as shown here for the desert-adapted strain \(N\). sp. QU130), and metabolic reorganization is certainly supported by specific cellular responses to temperature stress. These responses should be further investigated, with our study being merely an initial step in this regard. An in-depth transcriptomic and metabolomic investigation should provide, for example, valuable information for a better understanding of cell response to temperature or light intensity variations. At the process engineering level, optimization should certainly focus on reducing the temperature fluctuations in the culture system. In harsh conditions, such as the Qatar desert, it would be beneficial to develop a cultivation system adapted to the conditions and the requirements of the strain. For instance, an open system such as a semi-buried raceway providing natural cooling by water evaporation has proved efficient at reducing temperature build-up in the summer. The introduction of a ground heat exchanger in the culture volume, designed to exploit the constant year-round in-ground temperature (28 \({}^{\circ}\)C in Qatar), resulted in further improvement of the technology [15]. By serving either as a heating or cooling source, depending on the time of year and strain temperature requirements, this rather simple solution was found to be efficient at reducing overheating in summer, while providing a heat source (the ground) in winter which can increase the culture temperature, if needed, by around 7-8 \({}^{\circ}\)C.  
Finally, combining a selection of strains adapted to harsh desert conditions (i.e., local strains such as \(N\). sp. QU130), an adequate engineering of culture systems, such as open systems with ground exchangers, and a detailed understanding of the metabolic response of the strain to the various conditions encountered over a year's operation (i.e., including winter period), as carried out at laboratory scale, mimicking outdoor conditions, would help identify a solution for using desert areas for optimized large-scale microalgae production. Conception and design of the work, J.P.; experimental campaign, A.T.; investigation: R.T., I.S., V.G.