Submit Manuscript | http://medcraveonline.com Introduction Many studies have been performed to explore the techniques for production large quantities of microalgae biomass. Two type of bioreactors and cultivation techniques are usually used to growth microalgae: open raceway pond system and closed Photobioreactor system. 1,2 Disadvantages of the open pond system is: limitation in controlling contaminations and less control of process parameters on the other hand photo bioreactors provide an easy system of controlling nutrients for growth, cultivation parameters such as temperature, dissolved CO 2 and pH, and to prevent contaminations. However, photo bioreactors have a high capital and operational costs. 3 Therefore, selection of microalgae production techniques is an important factor to be considered for the effcient production of a microalgal species. Microalgae are an important source of vitamins, minerals, proteins, lipids, antioxidants and can be use as valuable nutrients in the human and cattle feeding products. 4 Some microalgae species can accumulate high levels of lipids which can be used as valuable food supplements. Additionally, lipids can be transesterifed into biodiesel. 5 Microalgae are also interesting microorganisms for the ecological and environmental issues, such as the greenhouse effect and industrial water pollution. Carbon dioxide released from power plants can be used as the carbon source for microalgae growth and production of valuable nutrients effciency at a minimal cost. 6 In this investigation Euglena gracilis photo-mixotrophic cultivation was done in modifed stirred tank bioreactor. Optimization of light source and carbon sources was applied in order to increase algae biomass and lipid production. The second focus was on potential of Euglena gracilis to growth phototrophic and fxates CO 2 as main carbon source. Materials and methods Algae strain, media and cultivation conditions Euglena gracilis 1224-5/25 from Sammlung von Algenkulturen Götting was used in all experiments. E. gracilis inoculum cultures were grown for 72h at 28°C in 500mL Erlenmeyer fasks flled with 200mL of a liquid medium prepared according Hutner medium. Bioprocess operations Photomixotrophic batch-cultivation were performed in a 2L bioreactor (Biostat MD, B. Braun, Germany) with a working volume of 1L. Bioreactor was equipped with four lamp located vertically around bioreactor vessel, at the 5cm distance from the surface of bioreactor to provided continuous light to the system. Two different light source was used: Sun-glo lamp Hagen-Deutschland, Japan, 15W, lux80, 4500K and Aqua-glo lamp Hagen-Deutschland, Japan, 15W, lux 80, 4500K. 900mL of a Hutner medium was added to the reactor and autoclaved for 20min at 121°C. After cooling, cultivation was started by the addition of the inoculums culture and the temperature was maintained at 28°C. The stirrer speed was in the range from 1001/ min to 3001/min. The aeration rate was changed from 127 to 800mL/ min. After 120h of heterotrophic cultivation glucose and organic acids was depilated and autotrophic growth condition was started. During autotrophic growth, the effect of increase CO 2 concentration on biomass and lipids production was investigated. By taking into account the CO 2 concentration in the air is about 0.03%, during autotrophic growth CO 2 concentration was increase to 3%. The initial E. gracilis cell number in the bioreactor was 5·10 5 cell/mL. The pH was monitored but was allowed to foat freely during the fermentation. Analysis Total lipid analysis: Cells were harvested by centrifugation at 4500rpm for 5min at 4°C. After centrifugation samples were extracted with chloroform–methanol–water (1:2:0.8, v/v/v) containing 0.5% pyrogallol (w/v) for preventing oxidation. The extracts were evaporated to dryness under vacuum at 40°C using a rotary evaporator. Extracts were used for gravimetric determination of total lipid. Sugar analysis: Concentrations of glucose was quantifed by HPLC on a Supelcogel C-610H column using a refractive index detector (RID, Schimadzu 10A VP, Kyoto, Japan). Analytes were separated MOJ Food Process Technol. 2017;4(5):125‒127. 125 ©2017 Šantek et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and build upon your work non-commercially. Cultivation of microalgae euglena gracilis: mixotrophic growth in photobioreactor Volume 4 Issue 5 - 2017 Božidar Šantek, Tonči Rezić* Department of Biochemical Engineering, University of Zagreb, Croatia Correspondence: Tonči Rezić, Department of Biochemical Engineering, Faculty of Food Technology and Biotechnology, University of Zagreb, Pierottijeva 6/IV, HR-10000 Zagreb, Croatia, Tel +385 1 4605 056, Fax +385 1 4836 424, Email trezic@pbf.hr Received: April 18, 2017 | Published: July 17, 2017 Abstract Microalgae Euglena gracilis was used for lipid production. Photo-mixotrophic cultivation was done in self-constructed photobioreactor. During cultivation carbon source, stirrer speed, aeration rate and light source were changed to provide suitable cultivation condition for algae biomass and lipid production. It was find out that the changing from heterotrophic to autotrophic condition increase lipid production. Stirrer speed and aeration rate has a more pronounced effect on the biomass production. Due to the optimization of cultivation conditions, lipid production was increased from 0.4% to 30% of biomass dry weight in a single bioreactor. During autotrophic cultivation CO 2 increase lipid production in the E. gracilis cells but it has negative impact on the biomass production. Keywords: photo-mixotrophic cultivation, euglena gracilis, lipid production, cultivation conditions, photobioreactor MOJ Food Processing & Technology Short Communication Open Access