Engineering Biocatalytic Solar Fuel Production: The PHOTOFUEL Consortium

Outdoor cultivation and metabolomics exploration of Chlamydomonas engineered for bisabolene production

Demonstrating outdoor cultivation of engineered microalgae at considerable scales is essential for their prospective large-scale deployment. Hence, this study focuses on the outdoor cultivation of an engineered Chlamydomonas reinhardtii strain, 3XAgBs-SQs, for bisabolene production under natural dynamic conditions of light and temperature. Our preliminary outdoor experiments showed improved growth, but frequent culture collapses in conventional Tris-acetate-phosphate medium. In contrast, modified high-salt medium (HSM) supported prolonged cell survival, outdoor. However, their subsequent outdoor scale-up from 250 mL to 5 L in HSM was effective with 10 g/L bicarbonate supplementation. Pulse amplitude modulation fluorometry and metabolomic analysis further validated their improved photosynthesis and uncompromised metabolic fluxes towards the biomass and the products (natural carotenoids and engineered bisabolene). These strains could produce 906 mg/L bisabolene and 54 mg/L carotenoids, demonstrating the first successful outdoor photoautotrophic cultivation of engineeredC. reinhardtii,establishing it as a one-cell two-wells biorefinery.

Biomass generation and heterologous isoprenoid milking from engineered microalgae grown in anaerobic membrane bioreactor effluent

Wastewater (WW) treatment in anaerobic membrane bioreactors (AnMBR) is considered more sustainable than in aerobic reactors. However, outputs from AnMBR are a mixed methane and carbon dioxide gas stream as well as ammonium- (N) and phosphate- (P) containing waters. Using AnMBR outputs as inputs for photoautotrophic algal cultivation can strip the CO₂ while removing N and P from effluent which feed algal biomass generation. Recent advances in algal engineering have generated strains that produce high-value side products concomitant with biomass, although only shown in heavily domesticated, lab-adapted strains. Here, it was investigated whether engineered Chlamydomonas reinhardtii could be grown directly in AnMBR effluent with CO₂ concentrations found in AnMBR off-gas. The strain was found to proliferate over bacteria in the non-sterile effluent, consume N and P to levels that meet general discharge or reuse limits, and tolerate cultivation in modelled (extreme) outdoor environmental conditions prevalent along the central Red Sea coast. In addition to ∼2.4 g CDW L^-1 biomass production in 96 h, a high-value heterologous sesquiterpene co-product could be obtained from 'milking' up to 837 µg L^-1 culture in 96 h. This is the first demonstration of a combined bio-process that employs a heavily engineered algal strain to enhance the product generation potentials from AnMBR effluent treatment. This study shows it is possible to convert waste into value through use of engineered algae while also improving wastewater treatment economics through co-product generation.

Base editing for reprogramming cyanobacterium Synechococcus elongatus

Cyanobacteria can directly convert carbon dioxide (CO₂) at the atmospheric level to biofuels, value-added chemicals and food products, making them ideal candidates to alleviate global climate change. Despite decades-long pioneering successes, the development of genome-editing tools, especially the CRISPR-Cas-based approaches, seems to lag behind other microbial chassis, slowing down the innovations of cyanobacteria. Here, we adapted and tailored base editing for cyanobacteria based on the CRISPR-Cas system and deamination. We achieved precise and efficient genome editing at a single-nucleotide resolution and demonstrated multiplex base editing in the model cyanobacterium Synechococcus elongatus. By using the base-editing tool, we successfully manipulated the glycogen metabolic pathway via the introduction of premature STOP codons in the relevant genes, building engineered strains with elevated potentials to produce chemicals and food from CO₂. We present here the first report of base editing in the phylum of cyanobacteria, and a paradigm for applying CRISPR-Cas systems in bacteria. We believe that our work will accelerate the metabolic engineering and synthetic biology of cyanobacteria and drive more innovations to alleviate global climate change.

Cyanobacteria as a Promising Alternative for Sustainable Environment: Synthesis of Biofuel and Biodegradable Plastics

With the aim to alleviate the increasing plastic burden and carbon footprint on Earth, the role of certain microbes that are capable of capturing and sequestering excess carbon dioxide (CO2) generated by various anthropogenic means was studied. Cyanobacteria, which are photosynthetic prokaryotes, are promising alternative for carbon sequestration as well as biofuel and bioplastic production because of their minimal growth requirements, higher efficiency of photosynthesis and growth rates, presence of considerable amounts of lipids in thylakoid membranes, and cosmopolitan nature. These microbes could prove beneficial to future generations in achieving sustainable environmental goals. Their role in the production of polyhydroxyalkanoates (PHAs) as a source of intracellular energy and carbon sink is being utilized for bioplastic production. PHAs have emerged as well-suited alternatives for conventional plastics and are a parallel competitor to petrochemical-based plastics. Although a lot of studies have been conducted where plants and crops are used as sources of energy and bioplastics, cyanobacteria have been reported to have a more efficient photosynthetic process strongly responsible for increased production with limited land input along with an acceptable cost. The biodiesel production from cyanobacteria is an unconventional choice for a sustainable future as it curtails toxic sulfur release and checks the addition of aromatic hydrocarbons having efficient oxygen content, with promising combustion potential, thus making them a better choice. Here, we aim at reporting the application of cyanobacteria for biofuel production and their competent biotechnological potential, along with achievements and constraints in its pathway toward commercial benefits. This review article also highlights the role of various cyanobacterial species that are a source of green and clean energy along with their high potential in the production of biodegradable plastics.

Current advances in engineering cyanobacteria and their applications for photosynthetic butanol production

Cyanobacteria are natural photosynthetic microbes which can be engineered for sustainable conversion of solar energy and carbon dioxide into chemical products. Attempts to improve target production often require an improved understanding of the native cyanobacterial host system. Valuable insights into cyanobacterial metabolism, biochemistry and physiology have been steadily increasing in recent years, stimulating key advancements of cyanobacteria as cell factories for biochemical, including biofuel, production. In the present review, we summarize the current progress in engineering cyanobacteria and discuss the achieved and potential utilization of these advances in cyanobacteria for the production of the bulk chemical butanol, specifically isobutanol and 1-butanol.

Improved Bioproduction of 1-Octanol Using Engineered Synechocystis sp. PCC 6803

1-Octanol has gained interest as a chemical precursor for both high and low value commodities including fuel, solvents, surfactants, and fragrances. By harnessing the power from sunlight and CO₂ as carbon source, cyanobacteria has recently been engineered for renewable production of 1-octanol. The productivity, however, remained low. In the present work, we report efforts to further improve the 1-octanol productivity. Different N-terminal truncations were evaluated on three thioesterases from different plant species, resulting in several candidate thioesterases with improved activity and selectivity toward octanoyl-ACP. The structure/function trials suggest that current knowledge and/or state-of-the art computational tools are insufficient to determine the most appropriate cleavage site for thioesterases in Synechocystis. Additionally, by tuning the inducer concentration and light intensity, we further improved the 1-octanol productivity, reaching up to 35% (w/w) carbon partitioning and a titer of 526 ± 5 mg/L 1-octanol in 12 days. Long-term cultivation experiments demonstrated that the improved strain can be stably maintained for at least 30 days and/or over ten times serial dilution. Surprisingly, the improved strain was genetically stable in contrast to earlier strains having lower productivity (and hence a reduced chance of reaching toxic product concentrations). Altogether, improved enzymes and environmental conditions (e.g., inducer concentration and light intensity) substantially increased the 1-octanol productivity. When cultured under continuous conditions, the bioproduction system reached an accumulative titer of >3.5 g/L 1-octanol over close to 180 days.

A novel, robust and mating-competent Chlamydomonas reinhardtii strain with an enhanced transgene expression capacity for algal biotechnology

In the future, algae biotechnology could play an important role in sustainable development, especially with regard to the production of valuable chemicals. Among the established laboratory strains with efficient transgene expression, there are none that have demonstrated the required robustness for industrial applications, which generally require growth at larger scale. Here, we created a robust and mating-competent cell line of the green microalga Chlamydomonas reinhardtii, which also possesses a high transgene expression capacity. This strain shows a comparably high resistance to shear stress by accumulating increased amounts of biomass under these conditions. As a proof-of-concept, a high phototrophic productivity of cadaverine from CO₂ and nitrate was demonstrated by efficiently expressing a bacterial l-lysine decarboxylase. In contrast to other established strains, this novel chassis strain for phototrophic production schemes is equipped with the traits required for industrial applications, by combining mating-competence, cell wall-mediated robustness and high level transgene expression.