## 3 Results and Discussion
### Short-Chain Fatty Acids  
The concentration of short-chain fatty acids (SCFA) was determined as they were previously reported to contribute to the bad odor of algae stored under inappropriate conditions [22]. SCFA levels, expressed on an organic matter base, were assessed to evaluate the effect of storage approach, time, and temperature (Figure 3). Acetic acid was always the main SCFA and its concentration was affected by storage temperature, time, and storage approach where the effect of storage approach interacted with that of storage temperature and time (\(p<0.001\)). A higher concentration of acetic acid was present after concentrate storage than after dilute culture storage (\(p<0.001\)). Propionic, butyric, and isovaleric acid levels were significantly influenced by all studied factors (storage approach, storage time, and storage temperature) and their interactions (\(p<0.01\)). Higher levels were observed with increasing storage time, increasing storage temperature, and after concentrate storage (Figure 3). Propionic, butyric, and isovaleric acid have an unpleasant odor and they are known to be present in foot sweet [26] and human stool [27], contributing to pungent off-odors. To evaluate their effect on _P. purpureum_ odor more directly, their concentrations expressed per liter algae suspension were plotted (Supplementary Materials Figure S1) and compared to their odor threshold values (OTV) in pure water [28]. Propionic acid only surpassed its OTV (20 mg/L) when the algae were stored at 20 \({}^{\circ}\)C. Although the initial dry matter concentration of the algae pellet obtained after dilute culture storage was about two times higher than that after concentrate storage (Supplementary Materials Table S1), lower propionic acid concentrations (in mg/L) were observed for dilute culture storage. Butyrate levels were usually of the same order of magnitude as the OTV in pure water (0.24 mg/L), except for concentrate storage at 20 \({}^{\circ}\)C. In fact, after 14 days of concentrate storage at 20 \({}^{\circ}\)C, the butyrate concentration was about 100 times the OTV. The concentration of isovaleric acid was only clearly higher than its OTV (0.12-0.7 mg/L) after 20 \({}^{\circ}\)C concentrate storage (Supplementary Materials Figure S1) and this compound was never detected in samples that were stored as a dilute culture (detection limit = 0.046 mg/L). Figure 2: Average number of colony-forming units (CFU) (**a**) at \(t_{0}\) and (**b**) after 7 and 14 days of storage. The \(p\)-values of the factorial ANOVA analysis (without \(t_{0}\) data) are shown above the right graph. Error bars represent standard deviations. For the bar marked with an asterisk, the lower error bar is not displayed as it must go below the minimal value on the vertical axis. As observed before [22], the temperature had a major influence on SCFA levels but also the storage approach had a clear impact. The latter is possibly a result of a partial release and dissolution of SCFA in the medium during storage and/or during subsequent centrifugation in the case of dilute storage. Indeed, after storing the dilute culture, the majority of the medium including the dissolved  
Figure 3: Short-chain fatty acid (SCFA) levels-storage test 1. SCFA levels during concentrate storage (**left**) and dilute culture storage (**right**). Error bars represent standard deviations. Propionic acid, butyric acid, and isovaleric acid levels that are not labeled by a common letter are significantly different. For acetic acid, there was no significant interaction between storage approach, time, and temperature (\(p=0.138\)), so no post hoc analysis was performed and no letter labels were added in the acetic acid figures. No isovaleric acid was detected in the samples obtained after dilute culture storage (isovaleric acid detection limit = 0.046 mg/L). Supplementary Materials Figure S1 shows SCFA levels expressed in mg/L. metabolites is removed by centrifugation.