## 3 Results and Discussion  
### Schizochytrium Imacinum Growth on Synthetic VFAs and Resulting Fatty Acid Profile  
#### 3.1.1 Biomass Production and VFA Consumption in Batch Fermentations  
The potential of _Schizochytrium limacinum_ to produce DHA and OCFA utilizing volatile fatty acids was evaluated in a first step. Therefore, acetic, propionic, butyric, valeric and caproic acid, the main short-chain fatty acid present in the dark fermentation effluents, were used in separated cultures, and their effect on biomass production and related parameters was evaluated for each culture. At the same time, the rate of VFA consumption was also evaluated. When the microalgae _Schizochytrium limacinum_ was grown in batch culture on 10 g/L of each VFA for 72 h, acetic acid showed the highest dry cell weight reaching 3.2 g/L, while propionic acid showed the lowest production with 0.7 g/L. Butyric and valeric acid achieved 3.0 g/L and 2 g/L of dry cell weight, respectively. For caproic acid, no biomass growth could be observed after the time of growth. The evaluation of biomass productivity (g/L/h) and biomass yield (g.g\({}^{-1}\)substrate) resulted in a similar behaviour for acetic and butyric acid, 0.04 g/L/h and 0.3 g\(\cdot\)g\({}^{-1}\)substrate in both experiments, as it can be observed in Table 1. On the other hand, propionic acid showed low productivity (0.01 g/L/h) and the yields obtained were below 0.1 g\(\cdot\)g\({}^{-1}\) substrate. Every 24 h, an aliquot of each culture was analysed for the evaluation of VFA consumption. After 72 h of growth, 96.7% of the initial acetic acid concentration was consumed, leaving a residual concentration of 0.4 g/L. On the contrary, no consumption was observed after 72 h of growth on caproic acid. Propionic and valeric acid showed a similar rate of consumption, after 72 h of growth, 79.0% and 74.0%, respectively, of initial carbon source was still detected in the media (Figure 1). The evolution of valeric acid showed an interesting behaviour. As it can be observed in Figure 2, after 24 h, the VFA analysis revealed that at the same time that the valeric acid was disappearing in the culture media, propionic acid was beginning to be detected. The experiment was then allowed to continue for another 96 h and, after seven days of the experiment, the valeric acid was almost consumed (0.70 \(\pm\) 0.03 g/L) and propionic acid reached a concentration of 5.69 \(\pm\) 0.09 g/L in the media. These results are in line with those obtained for the _Thraustochytriacea and Crypthe codiniacea_ family, that are the two main families most studied for their capacity to grow under heterotrophic conditions. In particular, _schizochytrium_ and _ulkenia_ species are able to produce high Omega-3 PUFA content, especially DHA [47]. The evaluation of the effect of every short chain fatty acid that comprises VFA on the growth of _Schizochytrium limacinum_, resulted in acetic acid and butyric acid appearing to be the most suitable fatty acid to be used when a concentration of 10 g/L of VFA was studied. Meanwhile, propionic acid resulted in a very low capacity to produce biomass by itself (0.78 g/L). _Avantiochytrium_ sp. from _Thraustochytrid_ exhibited similar behaviour when [PERSON] et al. studied its growth on VFA [26]. Acetic and butyric acid resulted in a higher cell dry weight (2.10 g/L and 3.23 g/L, respectively) at 10 g/L carbon source [26]. \begin{table}
\begin{tabular}{c c c c c c} \hline
**Parameters** & **Acetic Acid** & **Propionic Acid** & **Butyric Acid** & **Valeric Acid** & **Caproic Acid** \\ \hline Cell dry weight (g/L) & 3.2 \(\pm\) 0.05 & 0.70 \(\pm\) 0.03 & 3.0 \(\pm\) 0.05 & 1.2 \(\pm\) 0.03 & - \\ \hline Biomass productivity (g/L/h) & 0.04 \(\pm\) 0.01 & 0.07 \(\pm\) 0.01 & 0.04 \(\pm\) 0.01 & 0.02 \(\pm\) 0.01 & - \\ \hline Biomass yield & & & & & \\ (g\(\cdot\)g\({}^{-1}\)substrate) & & & & & \\ \hline \end{tabular}
\end{table} 
Table 1: Parameters relating to cultivation in flask of _Schizochytrium limacinum_ on each VFA.