## 4 Discussion
### Application of Spent Tomato Byproduct as an Alternative Medium  
The STB used as a substrate for _E. gracilis_ cultivation was acidic (pH 4.2) and contained a large amount of various nutrients. The carbon and nitrogen source concentrations in the STB were higher than the optimal concentrations set in the previous experiment (Section 3.2). In particular, the STB mainly comprised fructose and glucose, which are reported as efficient organic carbon sources for _E. gracilis_ cultivation [8]. Accordingly, STB was assumed to be an effective carbon source for biomass and paramylon production. [PERSON] et al. (2010) reported that the growth rate of _Chlorella_ sp. was affected when the turbidity of the media was beyond 100 NTU [25]. Although the turbidity of the STB reactor was presumed to inhibit growth for photosynthesis (Figure S1), no such affection occurred due to results from the enhanced productivities. Since there would be differences between microalgal species, further study on the impact of turbidity on _E. gracilis_ growth is necessary. This is the first study to investigate upcycling tomato processing byproducts as substrates for _E. gracilis._ Enhanced cell growth, biomass production, and paramylon production demonstrated the strong potential of STB. Although various food processing byproducts have been applied as substrates for other microalgal species, no study has reported the use of _E. gracilis._ The alternative medium with kinnow peels demonstrated the biomass and lipid productivity of _Tetraselmis indica_, which was comparable to that of synthetic medium [4]. _Chlorella_ sp. would be the most studied microalgae to date in the valorization of food waste hydrolysate. Out of 67 kinds of fruit waste hydrolysates, 59 kinds were proven to be effective for biomass and chlorophyll production from _Chlorella pyrenoidosis_[26]. Using a pH-adjusted waste papaya fruit as a sole substrate, _Chlorella protothecoides_ showed robust cell growth and oil production [27]. Biomass and lipid yield were enhanced by using cassava bagasse hydrolysates for the cultivation of _C. pyrenoidosis_ when mixed with yeast _Rhodotorula glutinis_[28]. Additionally, food waste hydrolysate and vegetable waste hydrolysate have been reported to enhance the biomass production of _Chlorella vulgaris_[29]. [PERSON] et al. (2017) reported that the dilution ratio of waste hydrolysates affects the cell growth rate [29]. The byproducts from _Citrus limetta_ used as an alternative medium for _C. culgaris_ enhanced biomass and lipid productivity by more than 2-fold [3]. [PERSON] et al. (2020) reported enhanced biomass, lipid, and lutein production from _Chlorella_ sp. using food waste hydrolysate, probably because glucose was a major organic component in the source [30]. In this study, a maximum of 5.2 g L\({}^{-1}\) of _E. gracilis_ biomass was shown using STB, which is more than 1.6-fold compared with the results using the synthetic medium. Therefore, STB can be utilized as an effective alternative medium for _E. gracilis_ because it has high contents of glucose. Higher biomass and paramylon production was observed using STB as an alternative medium over synthetic medium (1.6-fold and 1.5-fold, respectively). Statistical analysis showed that biomass production between STB and CM was significantly different (_t_-test, \(p\) < 0.05). Using the STB, paramylon production was significantly different between cultures with initial pH 3 and 8 (_t_-test, \(p\) < 0.05). The valuable materials to be applied for biofuel can be produced from _E. gracilis_ even with nonedible industrial byproducts, such as wastewater, wood materials, and food waste. [PERSON] and [PERSON] (2018) reported that ferulic acid extracted from rice bran promoted 1.4-fold higher growth of _E. gracilis_ under phototrophic conditions than the control group [31]. However, high-value materials that can be applied to functional foods or pharmaceuticals can be produced with byproducts generated from highly sterilized processes treating foods. Accordingly, STB as an alternative medium for _E.