## 3 Results
### Assessment of Light and Temperature Fluctuations on the Number and Morphology of Algal Cells  
Cell size and number were also monitored during \(N\). sp. QU130 growth under the three cultivation regimes. In general, a decrease in cell numbers was observed when light intensity or temperature was increased during the separate cycles (Figure 6A,B). However, the number of cells did not change significantly in response to combined light and temperature fluctuations (Figure 6C). However, an increase in cell size was also observed when shifting from continuous conditions to each of the cycles (Table 1). This might be explained by the cell size increasing under cycle stresses in response to the accumulation of energy-rich compounds, such as carbohydrates and lipids, or as a way of acclimating to temperature-increase stress [33]. In each case, the physiology of the cells was affected by time-fluctuation conditions.  
In the experiment under light cycle (Experiment C), note that cell numbers decreased by 10% at midday due to the greater amount of light received, but they increased again, by almost the same percentage, with the decrease in PFD at the end of the day, and remained at the same level overnight (Figure 6A). By contrast, cell size started to decrease from 2.70 \(\pm\) 0.18 \(\upmu\)m at midday to about 2 \(\pm\) 0.04 \(\upmu\)m at the end of the night, with a total 25% reduction in cell volume (Table 1). This can be explained by the cell division process  
\begin{table}
\begin{tabular}{c c c c c} \hline  & **Dawn** & **Midday** & **Dusk** & **End of the Day** \\ \hline Regime B & 2.5 \(\pm\) 0.06 & 3.13 \(\pm\) 0.06 & 2.7 \(\pm\) 0.1 & 2.4 \(\pm\) 0.1 \\ Regime C & 2.31 \(\pm\) 0.03 & 2.7 \(\pm\) 0.18 & 2.68 \(\pm\) 0.09 & 2 \(\pm\) 0.04 \\ Regime D & 2.57 \(\pm\) 0.06 & 2.8 \(\pm\) 0.06 & 2.99 \(\pm\) 0.06 & \\ \hline \end{tabular}
\end{table}
Table 1: Variation of \(N\). sp. QU130 cell diameter (in \(\upmu\)m) over the day and under different cultivation regimes: Regime B: Temperature fluctuation; Regime C: Light fluctuation; Regime D: Light and temperature fluctuation.  
Figure 6: Evolution of cells number under different cultivation regimes: (**A**): Light fluctuation (Regime C); (**B**): Temperature fluctuation (Regime B); (**C**): Light and temperature fluctuations (Regime D). Only Trial 1 is shown in Figure 6c—similar results for other trials.  
following the accumulation of necessary metabolic compositions during the (light) photosynthesis process [34]. A similar situation was observed for the temperature cycle with constant light (Experiment B); the number of cells mainly dropped when the temperature was highest, then stabilized when the temperature fell again. This tends to emphasize the stressful conditions of both light and temperature at midday, which both resulted in a drop in cell numbers. However, the cycle periods with low light (i.e., night) or low temperature (below 30-35 \({}^{\circ}\)C) produced different physiological effects. For light cycles, darkness resulted in a marked increase in cell numbers, as a result of night division. For the temperature cycle (Regime B), a low temperature resulted in a negligible variation in cell numbers. This concurs with several other works which reported a decrease in cell division when the temperature dropped [35]. Note, however, that the temperature was sufficiently high to allow metabolic activity. During the period of low temperature, a decrease in cell size from 3.13 \(\pm\) 0.06 \(\upmu\)m at 41 \({}^{\circ}\)C to 2.4 \(\pm\) 0.03 \(\upmu\)m at 32 \({}^{\circ}\)C was recorded.