# Exploring an Integrated Manure-Sewater System for Sustainable Cyanobacterial Biomass Production
## 4 Discussion  
### Effect of NaCl Salt Concentration on Growth of Cyanobacterial Strain HSaC  
The salinity tolerance capacity for the obtained cyanobacterial strain HSaC was tested. The biomass of the fresh weight increased from 0 mM NaCl salt concentration to 120 mM. Although the maximum biomass yield was obtained at 120 mM, a further increase in salt concentration resulted in a higher biomass yield than that at 0 mM NaCl concentration (Figure 1). This finding shows that the current tested cyanobacterial strain HSaC is a salt-loving microorganism. [PERSON] and [PERSON] [25] conducted an experiment on marine _spirulina_ to study the biochemical and physiological indicators of salinity tolerance. The authors reported that an increased cell growth rate in hypersaline medium suggested that the organism is salt-loving rather than salt-resistant.  
The fact that the highest biomass yield, chlorophyll a and carotenoid contents were obtained at 120 mM NaCl salt concentration indicated that 120 mM is a threshold NaCl salt concentration for improved growth and development of cyanobacterial strain HSaC. Although further increasing the salt concentration from 120 mM to 300 mM resulted in a reduction in the chlorophyll a and carotenoid contents, increasing the salt concentration from 0 mM to 120 mM significantly increased the photosynthetic pigment content (Figure 3 and Figure S1).  
The highest exo-polysaccharide (EPS) content was obtained at 180 mM NaCl concentration, which is unlike the results of biomass yield and photosynthetic pigments, whose contents peaked at 120 mM (Figure 2). This behavior could probably result from the cells having invested more energy in EPS production than in cell division at 180 mM salt concentration, and less energy in EPS production than  
Figure 8: Effect of natural seawater and cow manure media on salt sensitive cyanobacteria (LC) biomass production. DW = cow manure with distilled water, 0.5 dS = cow manure with natural seawater at an EC of 0.5 dS/m, 1.5 dS = cow manure with natural seawater at an EC of 1.5 dS/m. The error bars represent standard deviation (SD). The data are the mean of three replicates. Differing letters indicate significant differences of means of LC cyanobacterial biomass production in pairwise comparisons (Tukey’s test \(p<0.05\)) for each treatment by LSD.  
in cell division at 120 mM salt concentration. As the salt concentration increased, the EPS production also increased until the threshold level, but further increases in salinity reduced the EPS content. Similarly, [PERSON] et al. [34] explained that, an increase in salt concentration increased the microbial EPS production, and a high salinity decreased EPS production in micro-organisms. In another study [35], it was reported that the EPS content increased with an increase in NaCl from 52.3 mg-L\({}^{-1}\) to 62 mg-L\({}^{-1}\) in the culture. A similar study [36] stated that the production of cyanobacterial exopolysaccharideacharides (EPSs) was found to be stimulated by salts. This increase in EPS might be because EPSs on the cell membrane help cyanobacteria to maintain the osmotic balance or the excess flow of salt ions into the cell. [PERSON] et al. [36] concluded that the increased EPSs had a strong ability to remove Na+ from the cyanobacterial culture solution. It has been suggested that the accumulation of EPSs on the bacterial cell membrane is a protective response against salinity stress [34].