## 3 Results and Discussion
### Evaluation of C. curvatus Growth on Different Concentrations of VFAs
curatus_ on VFAs and glucose (GC) with (**A**) 10 g/L concentration and C/N 50, (**B**) 20 g/L concentration and C/N 50, (**C**) 40 g/L concentration and C/N 50, and (**D**) 60 g/L concentration with C/N 50 and C/N 100.  
observed that acetic and propionic acid have a similar trend toward lipid accumulation (2.9 g/L for C2 and 2.6 g/L for C3) and content. When glucose was used as a substrate, lipid accumulation and content had the lowest values (2.6 g/L and 22.9% \(w\)/\(w\)). At 40 g/L initial concentration (Figure 5C), there was a significant increase in the DCW with butyric acid as a substrate (22.7 g/L) and the lipid accumulation was also the highest (9.2 g/L). The lipid content, however, did not increase (40.5%) when compared to that of 20 g/L butyric acid (39.5%). After butyric acid, the highest DCW was observed with glucose (14.4 g/L), and a higher DCW was obtained with propionic than with acetic acid (13.4 g/L and 10.5 g/L, respectively). Propionic acid resulted in the highest lipid concentration (6.1 g/L) after butyric acid, followed by glucose (4.7 g/L) and acetic acid (4.3 g/L). The lipid content, however, had a different trend, with propionic acid having the highest value (45.6% \(w\)/\(w\)), followed by acetic acid (41.1% \(w\)/\(w\)), butyric acid (40.5% \(w\)/\(w\)), and lastly, glucose (32.3% \(w\)/\(w\)). At 60 g/L (Figure 5D) and C/N 50, the DCW and lipids for glucose (18.2 g/L and 8.1 g/L, respectively) were double than acetic acid (9.2 g/L and 3.9 g/L, respectively). However, acetic acid had higher lipid content than glucose (52.2% \(w\)/\(w\) for C2 and 44.4% \(w\)/\(w\) for GC). When the C/N ratio increased to 100 for 60 g/L, the substrate consumption of _C. curvatus_ with glucose and acetic acid had the same response as _R. toruloides_. Specifically, a slower consumption rate was observed for both substrates compared to C/N 50. Additionally, the biomass accumulation was similar to that with C/N 50 (19.5 g/L for glucose and 9.8 g/L for C2), but the lipid accumulation and lipid content increased for both substrates. For glucose, the lipid accumulation increased to 9.0 g/L, and the lipid content to 45.8% \(w\)/\(w\). For acetic acid, the increase was bigger, and lipid concentration reached 5.3 g/L, while the lipid content increased to 57.9% \(w\)/\(w\), which is also the maximum lipid content for acetic acid among all the tested conditions. The aforementioned observations are supported by the biomass and lipid yields that are depicted in Figure 6. By increasing the carbon source concentration for _C. curvatus_, the biomass yield on glucose and acetic acid decreases (Figure 6A). The lipid yield (Figure 6B) on glucose remained relatively stable through 10-40 g/L and it increased at 60 g/L, where it reached a maximum with a C/N 100 (0.15 g/g/substrate). For acetic acid, the lipid yield had the highest value at 10 g/L (0.15 g/gsubstrate) and thereafter it decreased when the concentration increased. For propionic and butyric acid, a decrease in the biomass and lipid yield is observed when increasing from 10 g/L to 20 g/L, but a further increase in the yields was observed when the concentration increased to 40 g/L. It is worth noting that for _C. curvatus_, the biomass and lipid yields have the highest values for butyric acid (0.23 g/gsubstrate; lipid yield) compared to the other VFAs, but also compared to glucose, which signifies that there is a higher conversion to cell biomass and lipids when butyric acid is used as a substrate. This observation has been previously reported by [PERSON] et al. and [PERSON] et al. (2019); [PERSON] et al. (2020). The findings of this study indicate that butyric acid in concentrations up to 40 g/L is more beneficial for biomass and lipid accumulation for _C. curvatus_ among the tested VFAs.