In developing countries and resource-limited regions, where no power infrastructure is available, photothermal-driven membrane distillation (PMD) has been recognized as an attractive and sustainable technology for freshwater generation.

Engineered cyanobacterium Synechococcus elongatus can use light and CO2 to produce sucrose, making it a promising candidate for use in co-cultures with heterotrophic workhorses. However, this process is challenged by the mutual stresses generated from the multispecies microbial culture. Here we demonstrate an ecosystem where S. elongatus is freely grown in a photo-bioreactor (PBR) containing an engineered heterotrophic workhorse (either β-carotene-producing Yarrowia lipolytica or indigoidine-producing Pseudomonas putida) encapsulated in calcium-alginate hydrogel beads. The encapsulation prevents growth interference, allowing the cyanobacterial culture to produce high sucrose concentrations enabling the production of indigoidine and β-carotene in the heterotroph. Our experimental PBRs yielded an indigoidine titer of 7.5 g/L hydrogel and a β-carotene titer of 1.3 g/L hydrogel, amounts 15–22-fold higher than in a comparable co-culture without encapsulation. Moreover, 13C-metabolite analysis and protein overexpression tests indicated that the hydrogel beads provided a favorable microenvironment where the cell metabolism inside the hydrogel was comparable to that in a free culture. Finally, the heterotroph-containing hydrogels were easily harvested and dissolved by EDTA for product recovery, while the cyanobacterial culture itself could be reused for the next batch of immobilized heterotrophs. This co-cultivation and hydrogel encapsulation system is a successful demonstration of bioprocess optimization under photobioreactor conditions.

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Synechococcus elongatus UTEX 2973 can use light and CO2 to produce sucrose, making them promising candidates to construct cocultures with heterotrophic workhorses. This envisioned process is, however, challenging to implement because of photosynthetic oxidative stress, light shading effect by heterotrophic cells, degradation of light sensitive metabolites, and high cost to separate intracellular products. Here, we demonstrated an effective ecosystem, where the sucrose producing cyanobacterium was freely grown in photo-bioreactors (PBRs), while an engineered heterotrophic workhorse (β-carotene producing Yarrowia lipolytica or indigoidine producing Pseudomonas putida) was encapsulated in calcium-alginate hydrogel beads and then placed inside the PBRs. The compartmentalization by hydrogels prevented growth interference so that the cyanobacterial culture could reach high sucrose concentrations, resulting the production of indigoidine (7.5g/L hydrogel) and β-carotene (1.3g/L hydrogel), respectively (i.e., the titers were 15 ~ 22 folds higher than that in the free cell coculture). Moreover, 13C-metabolic analysis indicated that hydrogels provided a favorable microenvironment so that the flux network of cells inside hydrogel was similar to the free culture. Finally, this novel system allowed the heterotroph- containing hydrogel beads to be easily harvested and dissolved by an EDTA solution for product and cell recovery, while the cyanobacterial culture could be continuously used for growing the next batch of immobilized workhorse heterotrophs.

Effectively recovering phosphate from wastewater streams and reutilizing it as a nutrient will critically support sustainability. Here, to capture aqueous phosphate, we developed novel mineral–hydrogel composites composed of calcium alginate, calcium phosphate (CaP), and calcium silicate hydrate (CSH) (CaP + CSH/Ca-Alg). The CaP + CSH/Ca-Alg composites were synthesized by dripping a sodium alginate (Na-Alg) solution with ionic precursors into a calcium chloride bath. To change the mineral seed’s properties, we varied the calcium bath concentrations and the ionic precursor (sodium dibasic phosphate (NaH2PO4) and/or sodium silicate (Na2SiO3)) amounts and their ratios. The added CSH in the mineral–hydrogel composites resulted in the release of calcium and silicate ions in phosphate-rich solutions, increasing the saturation ratio with respect to calcium phosphate within the mineral–hydrogel composites. The CSH addition to the mineral–hydrogel composites doubled the phosphate removal rate while requiring lesser initial amounts of Ca and P materials for synthesis. By incorporating both CSH and CaP mineral seeds in composites, we achieved a final concentration of 0.25 mg-P/L from an initial 6.20 mg-P/L. Moreover, the mineral–hydrogel composites can remove phosphate even under CaP undersaturated conditions. This suggests their potential to be a widely applicable and environmentally sustainable treatment and recovery method for nutrient-rich wastewater.