## 1 Introduction  
Increasing the ecological, technological, and socio-environmental competitiveness of biofuel technologies based on the production and use of microalgae grown on wastes can be achieved through the use of symbiotic microalgae-bacteria consortia. According to the latest data from the literature, microalgae-bacteria consortia can be used to overcome the barriers and limitations identified in wastewater treatment and bioenergy production [1; 2; 3; 4; 5; 6]. The treatment of municipal and industrial wastewater is usually characterized by a high energy consumption, high production of excess sludge, and high operation costs [7]. Alternatively, using both microalgae and bacteria to treat wastewater is a new approach that ensures a lower energy demand because oxygen can be provided through photosynthesis, and a high carbon-neutral potential through microalgae growth, leading to boosting the wastewater treatment efficiency [1; 5; 8]. Symbiotic microalgae-bacteria consortia could be used to improve the production of microalgae biomass rich in valuable chemical compounds and energy carriers, such as biohydrogen, biodiesel, bioethanol, and biogas [5; 9; 10; 11; 12]. This can be achieved by integrating microalgae processes with bacteria processes in conventional wastewater treatment systems and biofuel technologies [13; 14; 12]. Ecological studies have identified a variety of interactions between microalgae and bacteria. These are mainly food dependencies and environmental relationships that are based primarily on mutualism, commensalism, and, to a lesser extent, on competition and parasitism [15; 16]. The interactions between microalgae and bacteria communities depend on the biodiversity of the emerging biocenoses [15; 17; 18]. These general interactions are related to the excretion of dissolved organic carbon by microalgae, which becomes available to bacteria and, in return, these bacteria remineralize sulphur, nitrogen, and phosphorous to support the further growth of microalgae [3; 5; 19]. Additionally, there are some specific interactions where heterotrophic bacteria provide microalgae B vitamins as organic cofactors or produce siderophores to fix iron, while microalgae provide fixed carbon to bacteria [5]. The identification and determination of the relationship in microalgae-bacteria symbiosis and the development of protocols and growth models allow for a reliable assessment of the technological effects necessary for the further development of these solutions [16; 20; 21; 22]. According to the literature, the conversion of algae biomass into biogas may be highly profitable and the amount of methane produced can reach 140-360 mL CH\({}_{4}\)/g volatile solids (VS) [23]. The biochemical composition of microalgae biomass meets the requirements of anaerobic microorganisms and the amount of biogas produced depends on the species of microalgae used as the feedstock in anaerobic digestion (AD), [24; 25; 26; 27; 28]. However, a factor limiting the effectiveness of methanogenesis is the resistance of algal cell walls to anaerobic degradation, due to their multilayered structure and high content of cellulose [29; 30; 31]. For many years, the appearance of bacterial cultures during the growth of microalgae was an undesirable phenomenon. Nowadays, it is realized that microalgae-bacteria consortia may be utilized to improve algal biomass production and enrich the biomass with valuable chemical and energy compounds, as well as be used as a valuable feedstock in AD [5; 32; 33]. According to [PERSON] et al. [34], bacterial bioaugmentation with _Clostridium thermocellum_ improved the degradation of _Chlorella vulgaris_ biomass, resulting in higher biogas and methane yields. Similarly, the methane yield was enhanced by 37% with _Cyanobacteria_, which was attributed to their proteolytic activity [35]. In turn, the addition of an algalytic bacterial mixture to granular sludge increased the methane yield by 11% and changed the microbial structure in the anaerobic reactor [36].