## 1 Introduction  
Microalgae have attracted attention as a promising source for carbon dioxide (CO\({}_{2}\)) utilization and, recently, as raw materials for functional foods. _E. gracilis_, a fast-growing microalgal species, can produce value-added materials, including immune-functional paramylon (\(\beta\)-1,3-glucan). Higher amounts of paramylon can accumulate under heterotrophic and mixotrophic conditions in the presence of organic carbon than under photoautotrophic conditions [1]. However, the use of the carbon source for cultivation is expensive and may account for 50% of the total medium cost [2].  
Upcycling of food processing byproducts can be a compelling option to address the cost problem of organic carbon sources. Several nutrients in food processing byproducts can be potentially useful for microbial cultivation [3]. The use of food processing byproducts as low-cost substrates to produce value-added materials from microalgae may simultaneously enhance productivities and reduce costs [4, 5, 6]. To date, potato liquor [7], corn steep solid [8], corn steep liquor, and brewer's spent grain [9] have been used to produce paramylon efficiently and economically from _E. gracilis_.  
Spent tomato byproducts (STBs), such as pomace and peels, are generated from the tomato processing industry, and their amounts are expected to be 20-50 g kg\({}^{-1}\) of tomato. STB contains high moisture and thus requires an additional drying process in order to be utilized in a powdered form [10]. In general, STB is mixed for use in animal feed [10] or used as a resource for extracting value-added chemicals, such as carotenoids and lycopene [11; 12; 13]. Peels or seeds of tomatoes contain 27% to 31% carbohydrates [14], which are preferred substrates for _E. gracilis_ growth.  
_E. gracilis_ can utilize various carbon sources, such as polysaccharides, fatty acids, and alcohols [15]. However, knowledge on how _E. gracilis_ consumes such carbon sources from food processing byproducts is still limited. The growth of _E. gracilis_ is more competent under mixotrophic cultivation conditions than under other conditions. Cultivation factors related to organic carbon (e.g., carbon source type and concentration) have an impact on paramylon accumulation. Therefore, it is imperative to investigate the optimal carbon sources for paramylon production.  
Cultivation conditions related to pH are also important factors affecting the growth and biomass production of _E. gracilis_. While the optimal pH for the cultivation of _E. gracilis_ has been reported to be 3.5, a pH range of 7.4 to 8 has been shown to enhance the activity of paramylon synthase [16]. However, the impact of pH on the biomass and paramylon productivity of _E. gracilis_ using food processing byproducts has not yet been reported.  
The objectives of this study were (1) to investigate the optimal conditions of the carbon source type and concentration based on the biomass and paramylon production, and (2) to develop a strategy for the application of STB depending on the initial pH condition. Glucose, ethanol, lactate, and acetate were tested to determine the optimal carbon source. The concentration range of the optimal carbon source was 5-60 g L\({}^{-1}\). The organic carbon concentration of the STB was adjusted to the optimal conditions before its application for the cultivation experiments. The impact of initial pH conditions on biomass and paramylon production from _E. gracilis_ was also investigated.