tSmax = instant when the starch productivity reach the maximum. Factors influencing starch accumulation in microalga Chlorella sorokiniana Imma Gifuni1, Giuseppe Olivieri1, Antonino Pollio2, Antonio Marzocchella1 1Dipartimento di Ingegneria Chimica, dei Materiali e della Produzione Industriale, Università degli Studi di Napoli Federico II, Piazzale Tecchio80, 80125 Napoli, Italy 2Dipartimento di Biologia Università degli Studi di Napoli Federico II, Via Foria 223, 80139 Napoli,Italy The recent research regarding the microalgal exploitation is focused on the biorefinery of microalgal biomass: all components are valorized in different fields to make the whole process cost-effective. This contribution is focused on the starch fraction of microalgal biomass. Autotrophic batch cultures of Chlorella sorokiniana have been carried out at different concentration of nitrogen sources and of CO2. The effects of the concentration of CO2 and of nitrogen source on growth and starch production have been investigated. Introduction Fig. 2: Starch applications in food and non-food industries Nitrogen depletion is known to enhance energy molecules accumulation (starch and lipids). Microalgal biorefinery is a potential strategy to produce renewable industrial feedstocks for goods and services production[1]. Current biorefinery schemes (e.g. Fig. 1) mainly valorize proteins and value added lipids. The carbohydrate fraction is proposed as feedstock for bioethanol production [2]; [3]. The increase of CO2 concentration can promote carbon rich molecules accumulation. The largest fraction of microalgal carbohydrates is composed by starch which has a lot of industrial applications more profitable than ethanol production. Moreover only a mechanical extraction is required for starch industrial use. Literature studies show conflicting results because of the photobioreactor design, irradiance level and strategy [4], [5]. Fig. 1: Microalgal biorefinery scheme from Cellana. Aim The effects of the concentration of the nitrogen and carbon source concentration on growth and starch accumulation have been investigated for the microalgal strain Chlorella sorokiniana. Results and Discussion Effect of initial Nitrogen concentration The time to reach the maximum starch fraction increases with the nitrogen concentration in the medium because of the nitrogen reserves accumulation inside the cells. Fig. 5: Comparison of the maximum starch fraction (ωStarch), starch productivity (PStarch) and biomass productivity (PX) of tests at different nitrogen concentration Effect of Carbon concentration The CO2 concentration in the gas stream influence the pH of the culture and cause a deflection from the optimal growth conditions (pH=7–7.5). [CO2] =0.04% → pH=8.5 [CO2] =0.5% → pH=7.5 [CO2] =2% → pH=7.5 [CO2] =5% → pH=6.5 [CO2] =10% → pH=6 Fig. 6: Comparison of the maximum starch fraction (ωStarch), starch productivity (PStarch) and biomass productivity (PX) of tests at different CO2 concentration It is interesting that the maximum starch fraction of the biomass remains almost unchanged for the different tests. Therefore CO2 concentration does not influence starch accumulation. Materials and Methods MICROORGANISM: Chlorella sorokiniana, Shihiraet Krauss strain ACUF 318 For the tests at different nitrogen concentration: thickness=8 cm Fig. 3: C. soronianiana Fig. 4: Sketch of the inclined square bubble column photobioreactor Biomass productivity Starch productivity CULTURE SYSTEM: inclined squared bubble column photobioreactors reported in the fig. 4. Operating conditions: Irradiance= 300 μE/m2s 24/24h Temperature=25°C pH= 7 Volume= 1,5 L Gas stream: 20 L/h air + CO2 BBM (NaNO3 as N source) Biomass concentration (X), nitrate concentration, pH of the medium and biomass starch fraction (ωStarch) were analyzed daily. The following parameters were calculated: BBM (Bold Basal Medium) For the test at different carbon concentration: 2% CO2 Air stream + N=32mg L-1 tSmax = instant when the starch productivity reach the maximum. Low increase of Xmax Photoautotrophic batch growth Decrease of PX and PStarch +N Decrease of ωStarch [N] = 32 mg L-1 OPTIMUM [N] =19mg L-1 [N] =32mg L-1 [N] =61mg L-1 [N] =122 mg L-1 [CO2] =0.04% [CO2] =0.5% [CO2] =2% [CO2] =5% [CO2] =10% pH stress + 2% CO2 OPTIMUM - less carbon for supporting the growth Conclusions At continuous illumination of 300µE/m 2s, the increase of nitrogen concentration up to 32 mg L-1 does not led to an increase in biomass and starch production due to a lack of energy substrate (light). The optimal CO2 concentration for C. sorokiniana growth is 2%. The CO2 concentration does not influence starch fraction accumulated by microalgal cells. OPTIMAL RESULTS PX (kg m-3 day-1) Pstarch ωStarch % on DW N= 32 mg L-1 CO2 2%. 443 170 38 References [1]Soh et al. 2014, Bioresource technology. (151) 19-27. [2]Winck et al. 2013, Journal of Proteomics (94) 207-218. [3]Zhu and Hiltunen 2016, Renewable and Sustainable Energy Review. (54) 1285-1290. [4]Adestanya et al. 2014, Bioresource Technology. (157) 293-304. [5]Takeshita et al. 2013, Bioresource Technology. (158) 127-134.