# Exopolysaccharides from Cyanobacteria: Strategies for Bioprocess Development
## 1 Introduction  
Polysaccharides are complex polymeric carbohydrates that have a huge impact on the industry as hydrocolloids, bio-sourced materials, biological agents, among others [1]. These biopolymers are often used as raw materials in many industries and have a chemical diversity that can go from linear homopolymers to 3D structures of heteropolymers. The majority of commercialized polysaccharides are extracted from plants, seaweeds, and microorganisms [2]. Polysaccharides excreted from microbial sources hold a wide range of functionalities and are highlighted for easier downstream when compared to other natural sources [2; 3]; however, the costs of production, mainly for carbon source, constant oxygenation, and intense energy demand, limit the generalized application of these technology [4]. Eco-friendlier cellular factories such as phototrophic organisms have been highlighted to raise the sustainability of the whole bioprocess [5]. Cyanobacteria can be light-powered, consume inorganic carbon to produce oxygen, and even fix nitrogen under diazotrophic conditions [6]. They are widely known for being rich in protein and a prolific source of secondary metabolites [7]. Nonetheless, their relative low carbohydrate content can be over-exceeded in the forms of sugars and polysaccharides when another relevant nutrient is missing for the production of proteins, nucleic acids, and otherbiomolecules. This is commonly defined as photosynthesis \"carbon overflow\" and is similar to lipid accumulation in microalgae [8]. When produced in considerable amounts, the extracellular polysaccharides or exopolysaccharideacharides (EPS) can be combined with proteins (proteoglycans) and lipids (glycolipids) [9]. There is no current consensus in regards to EPS classification in cyanobacteria [10]. Though many authors agree they should be classified into two main groups according to their location: (1) cell-bound polysaccharides (CPS) comprising slime, sheaths, and \"capsular\"; (2) released polysaccharides (RPS) [4]. A rule of thumb EPS from cyanobacteria are heteropolysaccharides of anionic nature due to uronic acids and are soaked in neutral sugars. In a nutshell, cyanobacterial EPS holds higher complexity among microbes [6]. Physico-chemical properties of cyanobacterial EPS have shown promising performances suitable for the industry as gums, bioflocculants, soil conditioners, biolubrificants, and biosorbents [11; 12; 13]. The presence of non-carbohydrate substituents is proven to act as biological weapons, showing anti-viral, anti-microbial, anti-tumoral activity [14; 15; 16; 17]. Their potential has been already explored as drug delivery technology [18; 19].  
Cyanobacterial EPS, from the bioprocess point of view, is seen as a by-product of biomass and/or metabolite production such as proteins and to a lesser extent lipid [10]. The slimy texture of culture medium after EPS production hinders handling while structure elucidation and productivities gave a tough reputation to these classes of polysaccharides within the industrial sector; however, the uniqueness of the few resolved structures has opened markets to these polysaccharides.  
The overlooked research of cyanobacterial EPS is dominated by their generic production and extraction which is then used to provide insights on EPS partial characterization, physico-chemical, and/or bioactive properties [10]. There is a lot of anticipation regarding their application as metal bioremediation [20] and soil restoration [21]; however, this review is focused on the bioprocess development, for wild type strains in particular. Within this scope, we approached the EPS cyanobacterial in terms of their chemical-diversity and what techniques were used for strain selection and EPS production. Main developments and strategies are outlined and proposed, respectively.