Autofermentation of alkaline cyanobacterial biomass to enable biorefinery approach

Abstract

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.

Keywords: Alkaliphiles; Anaerobic digestion; Cyanobacteria; Fermentation; Hydrogen.

Fig. 1

The effect of fermentation temperature on organic acid yield during anoxic dark fermentation. Initial pH in all cases was 10.36 ± 0.05. Values reported corresponding to the average of triplicate measurements ± 95% confidence interval

Fig. 2

Organic acid product distribution at different fermentation temperatures. Initial pH in all cases was 10.36 ± 0.05

Fig. 3

The effect of harvesting and dewatering method on organic acid yields (mmol-organic acids produced per g-initial biomass) during 10 days of anoxic dark fermentation. Initial pH in all cases was 10.48 ± 0.02. Values reported corresponding to the average of triplicate measurements ± 95% confidence interval

Fig. 4

Organic acid product distribution at different concentration of highly alkaline and high pH media during 10 days of anoxic dark fermentation. Initial pH in all cases was 10.48 ± 0.02

Fig. 5

Organic acid yields (mmol-organic acids produced per g-initial biomass) during 10 days of anoxic and hypoxic dark fermentation. Initial pH in all cases was 10.35 ± 0.01. Values reported corresponding to the average of triplicate measurements ± 95% confidence interval

Fig. 6

Organic acid percentage and product distribution under hypoxic and anoxic highly alkaline and high pH media during 10 days of dark fermentation. Initial pH in all cases was 10.35 ± 0.01