## 1 Introduction  
_Arthrospira platensis_ (commonly known as Spirulina) is a photosynthetic cyanobacterium of high nutritional value due to its high content in proteins (up to 70%), polyunsaturated fatty acids, vitamins etc. In fact, _A. platensis_ along with _Chlorella vulgaris_ are the most widely commercially produced microalgal/cyanobacterial species worldwide and are sold mainly as food supplements. _A. platensis_ is exclusively produced under phototrophic mode in open ponds either indoors or outdoors. However, autotrophic mode displays a major drawback related to light limitations due to the poor penetration of light into the higher depths of the cultures, which subsequently result in low biomass concentrations (typically 300-500 mg/L of dry mass) [1]. To overcome this limitation, microalgal/cyanobacterial cultivation could be performed under mixotrophic mode, where organic molecules, such as glucose, acetate, glycerol etc. could be utilized as an energy and/or carbon source. Mixotrophy combines autotrophic photosynthesis and heterotrophic assimilation of organic carbon and it has been demonstrated that is advantageous over phototrophic or heterotrophic modes giving higher biomass concentrations and productivities [2].  
Among various organic compounds that are good candidates for microalgal mixotrophic cultivation, glycerol is an ideal one because it is industrially produced and available at high amounts and low cost, mainly because of the biodiesel production industry, where glycerol is a by-product of the transesterification of fatty acids to methyl esters (b biodiesel). It is estimated that biodiesel market will have an annual growth of around 4.5%, reaching around 40 Mm\({}^{3}\) in 2022 [3]. From each m\({}^{3}\) of biodiesel, approximately 0.1 m\({}^{3}\) of crude glycerol is generated. Moreover, glycerol is widely used as a sweetener in the food industry in pharmaceutical formulations and is therefore safe to be used in microalgal/cyanobacterial biotechnology towards food or feed production.  
_A. platensis_ is a species that has been extensively studied and has been demonstrated that it is capable of growing well heterotrophically or mixotropically on glucose [4; 5; 6]. However, there are only few works examining its cultivation using glycerol as organic source [7; 8]. These studies [7; 8] conclude that _A. platensis_ can utilize glycerol as organic substrate producing higher biomass concentrations than under phototrophic conditions (i.e., using bicarbonate as the sole carbon source). However, one of the main disadvantages of heterotrophic/mixotrophic cultivation is the need for sterilization of the growth medium and the conduction of the cultivation under sterile/aseptic conditions in order to avoid contamination with unwanted heterotrophs (bacteria, fungi etc.) with probably negative impacts on growth and overall cultivation performance. _A. platensis_ is an alkaliophile that thrives at elevated pH values (up to 11) and salt concentrations (\(>\) 25 mS/cm), conditions that may suppress the propagation of many bacteria [9]. However, there is lack of published works on this topic. Moreover, it has been demonstrated that assimilation of organic carbon is affected by light intensity, and more specifically under low light intensities organics are assimilated more efficiently [10; 11]. Therefore, main aim of this study was to cultivate _A. platensis_ mixotropically using glycerol as an organic source under semi-continuous and non-axenic conditions in order to evaluate the effect of glycerol concentration and light intensity on growth and biochemical composition of _A. platensis_. Additionally, the study aimed to examine the interaction between _A. platensis_ and any potential bacteria that might also be grown on glycerol.