Alkaliphilic cyanobacteria have gained significant interest due to their robustness, high productivity, and ability to convert CO2 into bioenergy and other high value products. Effective nutrient management, such as re-use of spent medium, will be essential to realize sustainable applications with minimal environmental impacts. In this study, we determined the solubility and uptake of nutrients by an alkaliphilic cyanobacterial consortium grown at high pH and alkalinity. Except for Mg, Ca, Co, and Fe, all nutrients are in fully soluble form. The cyanobacterial consortium grew well without any inhibition and an overall productivity of 0.15 g L−1 d−1 (AFDW) was achieved. Quantification of nutrient uptake during growth resulted in the empirical formula CH1.81N0.17O0.20P0.013S0.009 for the consortium biomass. We showed that spent medium can be reused for at least five growth/harvest cycles. After an adaptation period, the cyanobacterial consortium fully acclimatized to the spent medium, resulting in complete restoration of biomass productivity.

Phycocyanin is a natural blue pigment produced by cyanobacteria and is a valuable compound for food and cosmetic industries. At present, phycocyanin is manufactured with expensive and resource-heavy biotechnology, impeding its widespread use as a blue dye substitute. Here we show that cells of an alkaliphilic cyanobacterium lyse spontaneously in dark incubations mimicking natural soda lake environments, releasing concentrated phycocyanin. Proteogenomics showed that lysis likely resulted from a programmed response triggered by a failure to maintain osmotic pressure in the wake of severe energy limitation. This response explains the high turnover rates of cyanobacterial cells observed in soda lakes. Cells of Arthrospira platensis (Spirulina), currently used worldwide for phycocyanin production, lyse and release their pigments in the same manner. We propose this natural form of programmed cell death could reduce the costs and resources needed to produce phycocyanin, enabling displacement of current artificial blue colourants associated with adverse health effects.

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Background Carbon capture using alkaliphilic cyanobacteria can be an energy-efficient and environmentally friendly process for producing bioenergy and bioproducts. The inefficiency of current harvesting and downstream processes, however, hinders large-scale feasibility. The high alkalinity of the biomass also introduces extra challenges, such as potential corrosion, inhibitory effects, or contamination of the final products. Thus, it is critical to identify low cost and energy-efficient downstream processes. Results Autofermentation was investigated as an energy-efficient and low-cost biomass pre-treatment method to reduce pH to levels suitable for downstream processes, enabling the conversion of cyanobacterial biomass into hydrogen and organic acids using cyanobacteria’s own fermentative pathways. Temperature, initial biomass concentration, and oxygen presence were found to affect yield and distribution of organic acids. Autofermentation of alkaline cyanobacterial biomass was found to be a viable approach to produce hydrogen and organic acids simultaneously, while enabling the successful conversion of biomass to biogas. Between 5.8 and 60% of the initial carbon was converted into organic acids, 8.7–25% was obtained as soluble protein, and 16–72% stayed in the biomass. Interestingly, we found that extensive dewatering is not needed to effectively process the alkaline cyanobacterial biomass. Using natural settling as the only harvesting and dewatering method resulted in a slurry with relatively low biomass concentration. Nevertheless, autofermentation of this slurry led to the maximum total organic acid yield (60% C mol/C mol biomass) and hydrogen yield (326.1 µmol/g AFDM). Conclusion Autofermentation is a simple, but highly effective pretreatment that can play a significant role within a cyanobacterial-based biorefinery platform by enabling the conversion of alkaline cyanobacterial biomass into organic acids, hydrogen, and methane via anaerobic digestion without the addition of energy or chemicals.