# Exopolysaccharides from Cyanobacteria: Strategies for Bioprocess Development
## 3 Cyanobacterial Bioprocess (Cyano-EPS) Development  
### Strain Selection  
The classical approach to bioprospecting an exemplar cyanobacterial EPS producer starts by looking at the growth rate and EPS content, to determine product titer and productivity. Cyanobacteria physiology is diverse and translates into a much wider strain-specific response to the stimulus applied during cultivation. Cultivation screenings can be a useful tool to compare the performance of different strains within a predefined environment. A compilation of cultivation screenings in cyanobacteria is presented in Table 2. Despite the diversity of habitats, we can observe that the same does not happen to the orders of strains studied. Most of the reports were done on Nostocales and Oscillatoriales orders, whereas the majority of tested organisms per study were from the _Nostoc_ genus. In addition, the EPS fraction to be recovered varied among screenings to the methodologies applied. A study on forty _Nostoc_ strains from Pasteur Culture Collection isolated from different habitats (soil, soil/water, water, and plant symbiosis), showing that the RPS production was not related to the habitat nor the morphology of the outermost investments. Still, PCC 7413, 8109, and 6720 were identified as the most productive strains 47, 30, and 28 mg/L/day respectively [40]. Moreover, a more comprehensive screening was recently performed in 166 marine algal strains, though only 16 were cyanobacteria. Despite the low number of cyanobacterial strains, the study comprised strains from five different orders some of them evaluated the EPS production potential for the first time. The authors opted to use the culture medium from the culture collection, which is often complex and used for maintenance; however, the screening conducted at 20 \({}^{\circ}\)C and 150 \(\mu\)E revealed _Synechococcus_ sp. as a potential RPS producer (0.12 g/L) [41]. The cultivation screenings were characterized as being too long (30 days). This is explained by the cyanobacteria slow growth rate and also to EPS production associated with the stationary phase [10]. A dilution of N:P in BG11 to a factor of 7 was shown to anticipate the stationary phase in cultures of _Microcytsis aeruginosa_ and stationary phase characteristic EPS production was also observed under these conditions [42]. It is important to understand which is the cyanobacteria's physiological state throughout time and especially to define the period of highest EPS production. Although it is more relevant at the optimization stage, it can be used as a cut-off parameter. Applying a shorter production time (15 days) on four _Cyanothece_ sp. strains from seawater in China's coast grown on modified f/2 with the addition of sea mud extract, aeration, and relatively strong inoculum for a small volume of cultivation was sufficient to highlight _Cyanothece 113_ producing abundant quantities of a CPS (\(\alpha\)-d-1,6-glucan). The cyanobacteria inoculum quality (age and concentration) determines the period of acclimation, thus a lag phase in response to the transition of conditions was applied. The number of cells inoculated should be based on the steady-state of each strain, however, this methodology requires the previous knowledge of each strain's growth curve. The EPS screening studies have rarely provided information of inoculum, though some inoculated according to a determined concentration in terms of the number of cells, the mass of cells, chlorophyll \(a\) or even just a percentage. The culture medium provides the required nutrients for cyanobacterial growth, despite the existent formulations BG11 and BG11\({}_{0}\) (absence of nitrate source) were often applied. BG11\({}_{0}\) is used for nitrogen-fixing strains mostly from Nostocales order. Under diazotrophic conditions, growth is limited and the metabolism is re-directed towards nitrogen fixation with specialized cells and EPS production favored, namely CPS [40].