Cyanobacteria are promising microorganisms as cell factories due to their ability to perform oxygenic photosynthesis. During this process the cell produces metabolites through CO2 fixation powered by light energy. The main carbon fixation pathway in cyanobacteria is the Calvin-Benson-Bassham (CBB) cycle. Cyanobacteria, for example Synechocystis PCC 6803 (thereafter Synechocystis) have the ability to produce carbon compounds, for example acetate, as side products of their metabolism. Acetate production can be increased through the insertion of a phosphoketolase and overexpression of a phosphotransacetylase. In the present study the production of acetate was further tuned by overexpression of selected enzyme(s) of CBB-cycle. The enzymes selected was aldolase (FBA) and its combination with either fructose-1,6/sedoheptulose-1,7-bisphosphatase (FBP/SBPase) or transketolase (TK). The higher increase was noticed in the strain overexpressing FBA, 1.5 times fold increase, followed by the strain overexpressing both FBA and FBP/SBPase, while the overexpression of both FBA and TK did not influence the acetate production. However, when CP12, a small regulatory protein of the CBB-cycle, was knocked out, the strain overexpressing FBA and TK showed increased acetate titers while the other two combinations showed moderate increase. These results indicate the capability to optimize the acetate production through overexpression of FBA and emphasize the dynamic regulation of the CBB-cycle enzyme(s).

Eukaryote-associated microbes are ubiquitous, but their essential roles in the development and ecology of their host is yet to be fully understood, partly because complex associations cannot be reconstituted and, in many instances, the genetic tools to elucidate those roles are not available. Here, we report the conjugative transfer of DNA into Nostoc azollae within three Azolla fern hosts. N. azollae is a filamentous, N2-fixing, heterocyst-forming cyanobacterium which is the predominant obligate endosymbiont of the complex microbial community associated with the floating ferns of the genus Azolla. The cyanobiont provides fixed nitrogen to its host, supporting maximum growth rates without any N-fertilizer and making Azolla symbioses both ecologically and agriculturally important. Triparental mating protocols and fluorescent reporter detection were optimized for the cyanobiont isolated from the fern, allowing the further demonstration of heterologous gene expression in N. azollae driven by several promoters, including some of a CRISPR-associated transposon system. Azolla was then treated with a cytokinin hormone to render fern shoot apexes amenable to in planta conjugation, permitting DNA transfer to, and gene expression in two distinct developmental stages of N. azollae within Azolla. These included (i) cells of filaments from the Shoot Apical Nostoc colony, the only cyanobacterial stem-cell population vertically transmitted across fern generations, and (ii) cells from differentiated filaments in early formed Azolla leaf cavities. Our approach represents a technically groundbreaking advance for the genetic engineering of cyanobacterial endosymbioses that may be useful for other symbiotic systems, opening a way to investigate these important biological entities.

To overcome the limitations in the production of polyhydroxybutyrate (PHB) in cyanobacteria, we genetically modified Synechocystis sp. PCC 6803 focusing on carbon metabolism at the phosphoketolase pathway. In addition, for the baseline of the control strain (Ct), an inhibition of the pyruvate-to-lactate reaction was created by the disruption of the native ddh gene that encodes lactate dehydrogenase. The effects on cell growth of phosphoketolase (pk) gene expression derived from Pseudomonas aeruginosa ATCC 15442 (Ct_OXpkPa), Bifidobacterium breve strain 203 (Ct_OXpkBb), and Bifidobacterium adolescentis ATCC 15703 (Ct_OXpkBa) were comparable to that observed in the Ct strain. Notably, the PHB production in the Ct_OXpkBb strain increased under the normal growth condition without any stress, reaching 32.5

Photosynthetic microorganisms, such as cyanobacteria, are promising candidates for sustainable production of chemicals. Photosynthesis is a unique process where light energy is used to convert CO2 into carbon metabolites that sustain the cell`s metabolism. One of these products is acetate, a chemical with various applications in industry. Metabolic engineering can be used to increase the titer of extracellular acetate in the unicellular cyanobacterium Synechocystis sp. PCC 6803 (Synechocystis). Simultaneous expression of phosphoketolase (PK) and phosphotransacetylase (Pta) resulted in an enhanced acetate titer in Synechocystis cells (Roussou et al. Metab Eng 88:250-260) [1]. In the present study these two enzymes were expressed in different locus in the genome as well as expressed in the same locus organized as a single operon. The latter design reached higher acetate production. Attempts to further optimize the production through the creation of fused protein did not result in significant higher values than 2.3 g/L previously reported. However, the production was further increased when acetate kinase (AckA) was additionally overexpressed. Cultivation of this strain in high density cultivation (CellDEG system) led to high levels of acetate with a maximum of 7.1 g/L cumulative acetate production after 12 days of experiment when the cultures were sampled every day. Synechocystis sp. PCC 6803 is a candidate for sustainable acetate production driven by sunlight and CO2. The high level of acetate production is result of combining genomic integration of heterogenous genes in the cell and overexpression of native genes through self-replication vector. The production level achieved through the high-density cultivation reveal the strain capabilities when the growth conditions are optimal.

Eukaryote-associated microbes are ubiquitous, but their essential roles in the development and ecology of their host is yet to be fully understood, partly because complex associations cannot be reconstituted and, in many instances, the genetic tools to elucidate those roles are not available. Here, we report the conjugative transfer of DNA into Nostoc azollae within two Azolla fern hosts. N. azollae is a filamentous, N 2 -fixing, heterocyst-forming cyanobacterium which is an obligate endosymbiont of the complex microbial community associated with the floating ferns of the genus Azolla . The cyanobiont provides fixed nitrogen to its host, supporting maximum growth rates without any N-fertilizer and making Azolla symbioses both ecologically and agriculturally important. Triparental mating protocols and fluorescent reporter detection were optimized for the cyanobiont isolated from the fern, allowing further demonstration of heterologous gene expression in N. azollae driven by several promoters, including some of a CRISPR-associated transposon (CAST) system. Azolla was then treated with a cytokinin hormone to render fern shoot apices amenable to in planta conjugation, permitting DNA transfer to, and stable gene expression in two distinct developmental stages of N. azollae within Azolla . These included (i) cells of filaments from the Shoot Apical Nostoc colony, the only cyanobacterial stem-cell population vertically transmitted across fern generations, and (ii) cells from differentiated filaments in early formed Azolla leaf cavities. Our approach represents a technically groundbreaking advance for the genetic engineering of cyanobacterial endosymbioses that may be useful for other symbiotic systems, opening a pathway to investigate these important biological entities.

The photosynthetic cyanobacterium Synechocystis sp. PCC 6803 offers a promising sustainable solution for simultaneous CO2 fixation and compound bioproduction. While various heterologous products have now been synthesised in Synechocystis, limited genetic tools hinder further strain engineering for efficient production. Here, we present a versatile CRISPR activation (CRISPRa) system for Synechocystis, enabling robust multiplexed activation of both heterologous and endogenous targets. Following tool characterisation, we applied CRISPRa to explore targets influencing biofuel production, specifically isobutanol (IB) and 3-methyl-1-butanol (3M1B), demonstrating a proof-of-concept approach to identify key reactions constraining compound biosynthesis. Notably, individual upregulation of target genes, such as pyk1, resulted in up to 4-fold increase in IB/3M1B formation while synergetic effects from multiplexed targeting further enhanced compound production, highlighting the value of this tool for rapid metabolic mapping. Interestingly, activation efficacy did not consistently predict increases in compound formation, suggesting complex regulatory interactions influencing bioproduction. This work establishes a CRISPRa system for targeted upregulation in cyanobacteria, providing an adaptable platform for high-throughput screening, metabolic pathway optimisation and functional genomics. Our CRISPRa system provides a crucial advance in the genetic toolbox available for Synechocystis and will facilitate innovative applications in both fundamental research and metabolic engineering in cyanobacteria.

Cyanobacteria are promising platforms for metabolic engineering to convert carbon dioxide into valuable fuels and chemicals, addressing both energy demands and global climate change. Among various fuels and chemicals, isobutanol (IB) and 3-methyl-1-butanol (3M1B) have gained increasing attention due to their superior physical properties, such as high energy density, low water solubility, and low hygroscopicity. Heterologously expressing α-ketoisovalerate decarboxylase (KivdS286T) in the unicellular cyanobacterium Synechocystis sp. PCC 6803 (Synechocystis) enables microbial production of IB and 3M1B through the 2-keto acid pathway, with KivdS286T identified as a key bottleneck limiting production efficiency. To address this limitation, a high-throughput screening method based on the consumption of the substrate 2-ketoisovalerate was successfully established. This screen was coupled with random mutagenesis, via error-prone PCR, of KivdS286T. Out of the 1600 variants, 1B12, featuring dual substitutions K419E and T186S, exhibited a 55

The water fern Azolla spp. harbors as an endobiont the N2-fixing, filamentous, heterocyst-forming cyanobacterium Nostoc azollae. N. azollae provides the fern with fixed nitrogen permitting its growth in nitrogen-poor environments. In the diazotrophic filaments of heterocyst-forming cyanobacteria, intercellular molecular exchange occurs in which heterocysts provide vegetative cells with fixed nitrogen and vegetative cells provide heterocysts with reduced carbon. Intercellular molecular exchange takes place by diffusion through septal junctions and can be probed by fluorescence recovery after photobleaching (FRAP) analysis with fluorescent markers such as calcein and 5-carboxyfluorescein. The septal junctions traverse the septal peptidoglycan (PG) through nanopores that can be visualized in isolated septal PG disks by electron microscopy. Here, we obtained from Azolla plants material containing the symbiotic cyanobacterium in a viable state and with different morphologies, including heterocyst-containing filaments. FRAP analysis showed effective transfer of the fluorescent markers between vegetative cells, as well as from vegetative cells to heterocysts. Interestingly, communicating and noncommunicating vegetative cells and heterocysts could be distinguished, showing conservation in the endobiont of a mechanism regulating the septal junctions. PG sacculi were also isolated and showed septal disks with arrays of nanopores that conform to those visualized in other heterocyst-forming cyanobacteria. However, a wider range of septal disk size was observed in N. azollae. In spite of its eroded genome, N. azollae maintains the intercellular communication system that is key for its growth as a multicellular organism. Additionally, labeling with the fluorescent sucrose analog esculin suggests sucrose as a source of reduced carbon for the endobiont.IMPORTANCEThe water fern Azolla constitutes a unique symbiotic system in which cyanobacterial endobionts capable of fixing atmospheric nitrogen provide the plant with the nitrogen needed for growth. This symbiosis is an important fertilizer for rice crops worldwide, thereby reducing the reliance on fossil fuel-derived nitrogen fertilizers. The symbiotic cyanobacterium, Nostoc azollae, is a heterocyst-forming strain in which a filament of cells is the organismic unit of growth. Here, we show that the intercellular molecular exchange function necessary for the multicellular behavior of the organism is conserved in the endobiotic N. azollae.

Light-driven water oxidation by photosystem II sustains life on Earth by providing the electrons and protons for the reduction of CO2 to carbohydrates and the molecular oxygen we breathe. The inorganic core of the oxygen evolving complex is made of the earth-abundant elements manganese, calcium and oxygen (Mn4CaO5 cluster), and is situated in a binding pocket that is connected to the aqueous surrounding via water-filled channels that allow water intake and proton egress. Recent serial crystallography and infrared spectroscopy studies performed with PSII isolated from Thermosynechococcus vestitus (T. vestitus) support that one of these channels, the O1 channel, facilitates water access to the Mn4CaO5 cluster during its S2→S3 and S3→S4→S0 state transitions, while a subsequent CryoEM study concluded that this channel is blocked in the cyanobacterium Synechocystis sp. PCC 6803, questioning the role of the O1 channel in water delivery. Employing site-directed mutagenesis we modified the two O1 channel bottleneck residues D1-E329 and CP43-V410 (T. vestitus numbering) and probed water access and substrate exchange via time resolved membrane inlet mass spectrometry. Our data demonstrates that water reaches the Mn4CaO5 cluster via the O1 channel in both wildtype and mutant PSII. In addition, the detailed analysis provides functional insight into the intricate protein-water-cofactor network near the Mn4CaO5 cluster that includes the pentameric, near planar ‘water wheel’ of the O1 channel.

Cyanobacteria of the genera Synechocystis and Synechococcus have emerged as promising platforms for metabolic engineering endeavors aimed at converting carbon dioxide into valuable fuels and chemicals, thus addressing the pressing energy demand and mitigating global climate change. Notably, Synechocystis sp. strain PCC 6803 (Synechocystis) has been engineered to produce isobutanol (IB) and 3-methyl-1-butanol (3M1B) via heterologous expression of α-ketoisovalerate decarboxylase (Kivd). Despite these advances, the achieved IB/3M1B titers remain low. CRISPR interference (CRISPRi), an emerging tool for targeted gene repression, has demonstrated success in various cellular systems to enhance biochemical productivity. In this study, we developed a dCas12a-mediated CRISPRi system (CRISPRi-dCas12a) that effectively blocked the transcriptional initiation/elongation of essential gene(s), resulting in up to 60

This study highlights a new avenue to improve polyhydroxybutyrate (PHB) productivity by optimizing genes related to arginine catabolism, which influences nitrogen metabolism in cyanobacteria based on the carbon/nitrogen metabolism balance. In the Synechocystis sp. PCC 6803 wild type (WT) and its adc1 mutant (Δadc1), the native putA gene, responsible for the oxidation of proline to glutamate, was overexpressed to create the OXPutA and OXPutA/Δadc1 strains, respectively. PHB accumulation was considerably higher in OXPutA and OXPutA/Δadc1 under the nitrogen-deprived condition than in strains that overexpressed the proC gene, involved in proline synthesis. The increased transcript level of glgX, associated with glycogen degradation, confirmed that glycogen served as the primary carbon source for PHB synthesis under nitrogen stress without any carbon source addition. Furthermore, proline and glutamate level changes helped cells deal with nitrogen stress and considerably improve intracellular carbon/nitrogen metabolism. As indicated by elevated levels of proA and argD transcripts as well as chlorophyll a accumulation, this impact was most noticeable in strains that overexpressed putA, which was crucial for the synthesis of glutamate, a precursor for important metabolic pathways that respond to nitrogen stress. Therefore, our metabolic model presents PHB-producing strains as promising candidates for biomaterial biotechnology applications in medical and agricultural fields.

In addition to the external release of free fatty acids (FFAs) brought on by environmental stress, this study has used metabolic engineering approaches to generate strains of Synechocystis sp. PCC 6803 that can both increase the proportion of saturated FAs and secrete an abundance of FFAs. Under both normal and nitrogen-deprived conditions at day 5, it was found that Synechocystis sp. PCC 6803 with desC gene overexpression (OD), which encodes the membrane-bound desaturase enzyme, secreted FFAs outside the cell more than it accumulated intracellular lipids. Additionally, on day 10 in the nitrogen-deficient condition, the OD strain secreted FFAs with a larger proportion of unsaturated fatty acids (C18:1, C18:2, and C18:3) than those in the normal condition. It is crucial to note that when the desC gene was partly disrupted in the KD strain, the cells highly accumulated lipids inside of themselves rather than secreting FFAs outside of them. When the desC gene was co-overexpressed with the tolC gene combined with the disrupted aas gene (KAOTD), it resulted in an increased release of FFAs when stimulated by prolonged nitrogen stress, specifically for 10 days. On the other hand, under the 10-day nitrogendeprived condition, cells with desC overexpression in the OD and KAOTD strains continued to accumulate polyhydroxybutyrate (PHB) at levels around wild-type control, but the KD and KAOT strains collected less PHB. Therefore, this study highlighted the function of the desC gene in Synechocystis sp. PCC 6803, which substantially contributed to the secretion of unsaturated FFAs under prolonged nitrogen shortage.

Acetate is a biological anion with many applications in the chemical and food industries. In addition to being a common microbial fermentative end-product, acetate can be produced by photosynthetic cyanobacteria from CO2 using solar energy. Using wild-type cells of the unicellular model cyanobacterium Synechocystis PCC 6803 only low levels of acetate are observed outside the cells. By inserting a heterologous phosphoketolase (PKPa) in the acs locus, encoding acetyl-CoA synthetase responsible for the irreversible conversion of acetate to acetyl-CoA, an increased level of 40 times was observed. Metabolite analyses indicate an enhanced Calvin-Benson-Bassham cycle, based on increased levels of glyceraldehyde 3-phosphate and fructose-1,6-biphosphate, while the decreased levels of 3-phosphoglycerate and pyruvate suggest a quick consumption of the fixed carbon. Acetyl-P and erythrose-4-phosphate showed significantly increased levels, as products of phosphoketolase, while acetyl-CoA remained stable through the experiment. The results of intra- and extra-cellular acetate levels clearly demonstrate an efficient excretion of produced acetate from the cells in the engineered strain. Knock-out of ach and pta showed a reduction in acetate production however, it was not as low as in cells with a single knock-out of ach. Overexpressing acetyl-CoA hydrolase (Ach) and acetate kinase (AckA) did not significantly increase production. In contrast, overexpressing phosphotransacetylase (Pta) in cells containing an inserted PKPa resulted in 80 times more acetate reaching 2.3 g/L after 14 days of cultivation.

Photobiological hydrogen production offers a sustainable route to clean energy by harnessing solar energy through photosynthetic microorganisms. The pioneering sulfur-deprivation technique developed by Melis and colleagues in the green alga Chlamydomonas reinhardtii successfully enabled sustained hydrogen production by downregulating photosystem II (PSII) activity to reduce oxygen evolution, creating anaerobic conditions necessary for hydrogenase activity. Inspired by this approach, we present the project of the European consortium PhotoSynH2, which builds on these biological insights and employs synthetic biology to replicate and enhance this strategy in cyanobacteria, specifically, Synechocystis sp. PCC 6803. By genetically engineering precise downregulation of PSII, we aim to reduce oxygen evolution without the unintended effects associated with nutrient deprivation, enabling efficient hydrogen production. Additionally, re-engineering endogenous respiration to continuously replenish glycogen consumed during respiration allows matching oxygen production with consumption, maintaining anaerobic conditions conducive to hydrogen production. This review discusses how focusing on molecular-level processes and leveraging advanced genetic tools can lead to a new methodology that potentially offers improved results over traditional approaches. By redirecting electron flow and optimizing redox pathways, we seek to enhance hydrogen production efficiency in cyanobacteria. Our approach demonstrates how harnessing photosynthesis through synthetic biology can contribute to scalable and sustainable hydrogen production, addressing the growing demand for renewable energy and advancing toward a carbon-neutral future.

The physiological connection between glycogen and polyhydroxybutyrate (PHB), two carbon stores, has been found via known control during nutritional deprivation, such as nitrogen. However, limited biomass production is one factor that lowers the productivity of sustainable biosources. The present study highlights the significance of controlled regulation by metabolic engineering to produce beneficial chemicals, which are biodegradable polymers of PHB. This is achieved through the overexpression of RuBisCO and PHB biosynthetic genes, phaA and phaB, in the cyanobacterium Synechocystis sp. PCC 6803. The RuBisCO overexpression contributed to higher biomass accumulation. In contrast to the regular BG11 medium, which typically took 8-10 days for cells to enter log phase, the NaHCO3 addition undoubtedly promoted an increased level of cell growth and hastened the growth phase within 4 days of cultivation. Following substantial biomass growth, these cells acclimated to various nutrient-deficient conditions. The highest level of PHB was significantly induced by nitrogen and phosphorus deficiency, up to 51.6 % of dry cell weight at day 9 of stressed treatment. According to our findings, which were supported by higher photosynthetic efficiency, lower glycogen levels, and higher transcript amounts of the glgX gene associated with glycogen breakdown, PHB accumulation in our engineered PHB-producing strain was a significant result of increased CO2 fixation and glycogen breakdown. Therefore, the metabolic improvement of the PHB-producing Synechocystis strain with increased carbon capture has set the stage for the potential biotechnology implemented for mass production in the future.

Phototrophic microbial communities – groups of tiny organisms whose energy for growth comes from light – play a significant role in global primary production by absorbing carbon dioxide and nitrogen gas. With the growing challenges of energy demands and environmental concerns, researchers are exploring scientifically designed (synthetic) phototrophic communities as a promising alternative to traditional energy generation methods. These consortia can efficiently convert CO₂ and N₂ gases, along with water and solar energy, into bioenergy products, offering a potential solution to today’s energy and sustainability problems. In this context, the development of synthetic phototrophic communities has attracted increased attention due to their ability to divide tasks among different species, allowing them to function more efficiently and remain stable. However, challenges remain, particularly in maintaining balance among strains and ensuring stable performance in environments that do not replicate the complex natural conditions in which these consortia typically thrive. To address these challenges, recent PROMICON studies have focused on how cyanobacteria interact with purple nonsulfur bacteria (PNSB). These bacteria, including Rhodopseudomonas palustris (R. palustris), have shown potential in producing biohydrogen and lipids by capturing nitrogen in oxygen-free environments. Nevertheless, a key limitation is that they need a carbon-based food source (e.g., acetate) to produce energy. A promising approach to overcome this issue involves growing R. palustris with cyanobacteria, which can pull carbon dioxide from the air and turn it into the organic carbon that R. palustris needs to thrive.

Alka(e)nes are produced by many living organisms and exhibit diverse physiological roles, reflecting a high functional versatility. Alka(e)nes serve as waterproof wax in plants, communicating pheromones for insects, and microbial signaling molecules in some bacteria. Although alka(e)nes have been found in cyanobacteria and algal chloroplasts, their importance for photosynthetic membranes has remained elusive. In this study, we investigated the consequences of the absence of alka(e)nes on membrane lipid composition and photosynthesis using the cyanobacterium Synechocystis PCC6803 as a model organism. By following the dynamics of membrane lipids and the photosynthetic performance in strains defected and altered in alka(e)ne biosynthesis, we show that drastic changes in the glycerolipid contents occur in the absence of alka(e)nes, including a decrease in the membrane carotenoid content, a decrease in some digalactosyldiacylglycerol (DGDG) species and a parallel increase in monogalactosyldiacylglycerol (MGDG) species. These changes are associated with a higher susceptibility of photosynthesis and growth to high light in alka(e)ne-deficient strains. All these phenotypes are reversed by expressing an algal photoenzyme producing alka(e)nes from fatty acids. Therefore, alkenes, despite their low abundance, are an essential component of the lipid composition of membranes. The profound remodeling of lipid composition that results from their absence suggests that they play an important role in one or more membrane properties in cyanobacteria. Moreover, the lipid compensatory mechanism observed is not sufficient to restore normal functioning of the photosynthetic membranes, particularly under high-light intensity. We conclude that alka(e)nes play a crucial role in maintaining the lipid homeostasis of thylakoid membranes, thereby contributing to the proper functioning of photosynthesis, particularly under elevated light intensities.

One important aspect of cyanobacterial homoeostasis is reducing the toxicity of excess free fatty acids (FFAs) generated in the cells by means of both secreting these into the medium and recycling them toward membrane lipid synthesis. In this study, the cyanobacterium Synechocystis sp. PCC 6803 served to implement the overexpression of native genes of the transportation system. Specifically, we worked with the Sll0180-Slr2131-Slr1270 homologs of Escherichia coli AcrA-AcrB-TolC, respectively, to create single- and triple-overexpressing strains of OA, OB, OC, and OABC. Remarkably, the OABC strain that triply overexpressed the sll0180_slr2131_slr1270 genes acquired a significant amount of intracellular lipids, up to 23.5% of dry cell weight, under the normal condition. Nitrogen-deficient stress undoubtedly raised extracellular FFAs and intracellular lipids in overexpressing strains, especially in the OABC strain, which exhibited 33.9% and 41.5% of dry cell weight, respectively. During the first 5 days of treatment, salt stress at 256 mM significantly increased the FFA efflux, notably for the OB strain, but had no effect on intracellular lipids. It is noteworthy that the OA and OABC strains outperformed all other strains in terms of growth throughout the 16 days of nitrogen shortage. Furthermore, in comparison to the wild-type control, all the overexpressing strains exhibited a considerable increase in carotenoid accumulation. Thus, our results point to the effective role of the sll0180_slr2131_slr1270 transportation system in facilitating FFA secretion, especially in response to environmental stressors.

Energy is a crucial entity for the development and it has various alternative forms of energy sources. Recently, the synthesis of nanoparticles using benign biocatalyst has attracted increased attention. In this study, silver nanoparticles were synthesized and characterized using Azadirachta indica plant-derived phytochemical as the reducing agent. Biomass of the microalga Chlorella sp. cultivated in BG11 medium increased after exposure to low concentrations of up to 0.48 mg L-1 AgNPs. In addition, algal cells treated with 0.24 mg L-1 AgNPs and cultivated in BG110 medium which contained no nitrogen source showed the highest hydrogen yield of 10.8 mmol L-1, whereas the untreated cells under the same conditions showed very low hydrogen yield of 0.003 mmol L-1. The enhanced hydrogen production observed in the treated cells was consistent with an increase in hydrogenase activity. Treatment of BG110 grown cells with low concentration of green synthesized AgNPs at 0.24 mg L-1 enhanced hydrogenase activity with a 5-fold increase of enzyme activity compared to untreated BG110 grown cells. In addition, to improve photolytic water splitting efficiency for hydrogen production, cells treated with AgNPs at 0.24 mg L-1 showed highest oxygen evolution signifying improvement in photosynthesis. The silver nanoparticles synthesized using phytochemicals derived from plant enhanced both microalgal biomass and hydrogen production with an added advantage of CO2 reduction which could be achieved due to an increase in biomass. Hence, treating microalgae with nanoparticles provided a promising strategy to reduce the atmospheric carbon dioxide as well as increasing production of hydrogen as clean energy.

Mismanaged plastics, upon entering the environment, undergo degradation through physicochemical and/or biological processes. This process often results in the formation of microplastics (MPs), the most prevalent form of plastic debris (<1 mm). MPs pose severe threats to aquatic and terrestrial ecosystems, necessitating innovative strategies for effective remediation. Some photosynthetic microorganisms can degrade MPs but there lacks a comprehensive review. Here we examine the specific role of photoautotrophic microorganisms in water and soil environments for the biodegradation of plastics, focussing on their unique ability to grow persistently on diverse polymers under sunlight. Notably, these cells utilise light and CO2 to produce valuable compounds such as carbohydrates, lipids, and proteins, showcasing their multifaceted environmental benefits. We address key scientific questions surrounding the utilisation of photosynthetic microorganisms for MPs and nanoplastics (NPs) bioremediation, discussing potential engineering strategies for enhanced efficacy. Our review highlights the significance of alternative biomaterials and the exploration of strains expressing enzymes, such as polyethylene terephthalate (PET) hydrolases, in conjunction with microalgal and/or cyanobacterial metabolisms. Furthermore, we delve into the promising potential of photo-biocatalytic approaches, emphasising the coupling of plastic debris degradation with sunlight exposure. The integration of microalgal-bacterial consortia is explored for biotechnological applications against MPs and NPs pollution, showcasing the synergistic effects in wastewater treatment through the absorption of nitrogen, heavy metals, phosphorous, and carbon. In conclusion, this review provides a comprehensive overview of the current state of research on the use of photoautotrophic cells for plastic bioremediation. It underscores the need for continued investigation into the engineering of these microorganisms and the development of innovative approaches to tackle the global issue of plastic pollution in aquatic and terrestrial ecosystems.