Sustainable production of algae-bacteria granular consortia based biological hydrogen: New insights

Microalgae growth-promoting bacteria for cultivation strategies: Recent updates and progress

Microalgae growth-promoting bacteria (MGPB), both actinobacteria and non-actinobacteria, have received considerable attention recently because of their potential to develop microalgae-bacteria co-culture strategies for improved efficiency and sustainability of the water-energy-environment nexus. Owing to their diverse metabolic pathways and ability to adapt to diverse conditions, microalgal-MGPB co-cultures could be promising biological systems under uncertain environmental and nutrient conditions. This review proposes the recent updates and progress on MGPB for microalgae cultivation through co-culture strategies. Firstly, potential MGPB strains for microalgae cultivation are introduced. Following, microalgal-MGPB interaction mechanisms and applications of their co-cultures for biomass production and wastewater treatment are reviewed. Moreover, state-of-the-art studies on synthetic biology and metabolic network analysis, along with the challenges and prospects of opting these approaches for microalgal-MGPB co-cultures are presented. It is anticipated that these strategies may significantly improve the sustainability of microalgal-MGPB co-cultures for wastewater treatment, biomass valorization, and bioproducts synthesis in a circular bioeconomy paradigm.

Two cyanobacterial species exhibit stress responses when grown together in visible light or far-red light

Although most cyanobacteria grow in visible light (VL; λ = 400-700 nm), some cyanobacteria can also use far-red light (FRL; λ = 700-800 nm) for oxygenic photosynthesis by performing far-red light photoacclimation. These two types of cyanobacteria can be found in the same environment. However, how they respond to each other remains unknown. Here, we reveal that coculture stresses FRL-using Chlorogloeopsis fritschii PCC 9212 and VL-using Synechocystis sp. PCC 6803. No significant growth difference was found in Synechocystis sp. PCC 6803 between the coculture and the monoculture. Conversely, the growth of Chlorogloeopsis fritschii PCC 9212 was suppressed in VL under coculture. According to transcriptomic analysis, Chlorogloeopsis fritschii PCC 9212 in coculture shows low transcript levels of metabolic activities and high transcript levels of ion transporters, with the differences being more noticeable in VL than in FRL. The transcript levels of stress responses in coculture were likewise higher than in monoculture in Synechocystis sp. PCC 6803 under FRL. The low transcript level of metabolic activities in coculture or the inhibition of cyanobacterial growth indicates a possible negative interaction between these two cyanobacterial strains.IMPORTANCEThe interaction between two cyanobacterial species is the primary focus of this study. One species harvests visible light, while the other can harvest far-red and visible light. Prior research on cyanobacteria interaction concentrated on its interactions with algal, coral, and fungal species. Interactions between cyanobacterial species were, nevertheless, rarely discussed. Thus, we characterized the interaction between two cyanobacterial species, one capable of photosynthesis using far-red light and the other not. Through experimental and bioinformatic approaches, we demonstrate that when one cyanobacterium thrives under optimal light conditions, it stresses the remaining cyanobacterial species. We contribute to an ecological understanding of these two kinds of cyanobacteria distribution patterns. Cyanobacteria that utilize far-red light probably disperse in environments with limited visible light to avoid competition with other cyanobacteria. From a biotechnological standpoint, this study suggests that the simultaneous cultivation of two cyanobacterial species in large-scale cultivation facilities may reduce the overall biomass yield.

Enhancing strategies of photosynthetic hydrogen production from microalgae: Differences in hydrogen production between prokaryotic and eukaryotic algae

Hydrogen production through the metabolic bypass of microalgae photosynthesis is an environmentally friendly method. This review examines the genetic differences in hydrogen production between prokaryotic and eukaryotic microalgae. Additionally, the pathways for enhancing microalgae-based photosynthetic hydrogen generation are summarized. The main strategies for enhancing microalgal hydrogen production involve inhibiting the oxygen-generating process of photosynthesis and promoting the oxygen tolerance of hydrogenase. Future research is needed to explore the regulation of physiological metabolism through quorum sensing in microalgae to enhance photosynthetic hydrogen production. Moreover, effective evaluation of carbon emissions and sequestration across the entire photosynthetic hydrogen production process is crucial for determining the sustainability of microalgae-based production approaches through comprehensive lifecycle assessment. This review elucidates the prospects and challenges associated with photosynthetic hydrogen production by microalgae.

Fermentation of micro- and macroalgae as a way to produce value-added products

Fermentation of both microalgae and macroalgae is one of the most efficient methods of obtaining valuable value-added products due to the minimal environmental pollution and the availability of economic benefits, as algae do not require arable land and drift algae and algal bloom biomass are considered waste and must be recycled and their fermentation waste utilized. The compounds found in algae can be effectively used in the fuel, food, cosmetic, and pharmaceutical industries, depending on the type of fermentation used. Products such as methane and hydrogen can be produced by anaerobic digestion and dark fermentation of algae, and lactic acid and its polymers can be produced by lactic acid fermentation of algae. Article aims to provide an overview of the different types potential of micro- and macroalgae fermentation, the advantages and disadvantages of each type considered, and the economic feasibility of algal fermentation for the production of various value-added products.

A potential paradigm in CRISPR/Cas systems delivery: at the crossroad of microalgal gene editing and algal-mediated nanoparticles

Microalgae as the photosynthetic organisms offer enormous promise in a variety of industries, such as the generation of high-value byproducts, biofuels, pharmaceuticals, environmental remediation, and others. With the rapid advancement of gene editing technology, CRISPR/Cas system has evolved into an effective tool that revolutionised the genetic engineering of microalgae due to its robustness, high target specificity, and programmability. However, due to the lack of robust delivery system, the efficacy of gene editing is significantly impaired, limiting its application in microalgae. Nanomaterials have become a potential delivery platform for CRISPR/Cas systems due to their advantages of precise targeting, high stability, safety, and improved immune system. Notably, algal-mediated nanoparticles (AMNPs), especially the microalgae-derived nanoparticles, are appealing as a sustainable delivery platform because of their biocompatibility and low toxicity in a homologous relationship. In addition, living microalgae demonstrated effective and regulated distribution into specified areas as the biohybrid microrobots. This review extensively summarised the uses of CRISPR/Cas systems in microalgae and the recent developments of nanoparticle-based CRISPR/Cas delivery systems. A systematic description of the properties and uses of AMNPs, microalgae-derived nanoparticles, and microalgae microrobots has also been discussed. Finally, this review highlights the challenges and future research directions for the development of gene-edited microalgae.

Genetic engineering for biohydrogen production from microalgae

The development of biohydrogen as an alternative energy source has had great economic and environmental benefits. Hydrogen production from microalgae is considered a clean and sustainable energy production method that can both alleviate fuel shortages and recycle waste. Although algal hydrogen production has low energy consumption and requires only simple pretreatment, it has not been commercialized because of low product yields. To increase microalgal biohydrogen production several technologies have been developed, although they struggle with the oxygen sensitivity of the hydrogenases responsible for hydrogen production and the complexity of the metabolic network. In this review, several genetic and metabolic engineering studies on enhancing microalgal biohydrogen production are discussed, and the economic feasibility and future direction of microalgal biohydrogen commercialization are also proposed.

Nutrient removal and biogas upgrade using co-cultivation of Chlorella vulgaris and three different bacteria under various GR24 concentrations by induction with 5-deoxystrigol

Algae symbiosis technology shows great potential in the synchronous treatment of biogas slurry and biogas, which has promising applications. For improving nutrients and CO₂ removal rates, the present work constructed four microalgal systems: Chlorella vulgaris (C. vulgaris) monoculture, C. vulgaris-Bacillus licheniformis (B. licheniformis), C. vulgaris-activated sludge, and C. vulgaris-endophytic bacteria (S395-2) to simultaneously treat biogas as well as biogas slurry under GR24 and 5DS induction. Our results showed that the C. vulgaris-endophytic bacteria (S395-2) showed optimal growth performance along with photosynthetic activity under the introduction of GR24 (10^-9 M). Under optimal conditions, CO₂ removal efficiency form biogas, together with chemical oxygen demand, total phosphorus and total nitrogen removal efficiencies from biogas slurry reached 67.25 ± 6.71%, 81.75 ± 7.93%, 83.19 ± 8.32%, and 85.17 ± 8.26%, respectively. The addition of symbiotic bacteria isolated from microalgae can promote the growth of C. vulgaris, and the exogenous addition of GR24 and 5DS can strengthen the purification performance of the algae symbiosis to achieve the maximum removal of conventional pollutants and CO₂.

Advances in the catalyzed photo-fermentative biohydrogen production through photo nanocatalysts with the potential of selectivity, and customization

Photo nanocatalyst have shownpromise in a variety of fields, including biohydrogen production where their catalytic efficiency is related to size, surface-to-volume ratio, and increasing the number of atoms on the surface. They can harvest solar light to create electron-hole pairs which is the key mechanism to define its catalytic efficiency, thus requiring suitable excitation wavelength, band energy, and crystal imperfections. In this review, a discussion on the role of photo nanocatalysts to catalyze biohydrogen production has been carried out. Photo nanocatalysts feature a large bandgap, andhigh defect concentration, thus having the ability to be tuned for their characteristics. Customization of the photo nanocatalyst has been addressed. Mechanism of the photo nanocatalysts in catalyzing biohydrogen has been discussed. Limiting factors of photo nanocatalysts were highlighted and several recommendations have been made to enhance the effective utilization of these photo nanocatalysts to enhance photo-fermentative biohydrogen production from biomass wastes.

Light-dependent biohydrogen production: Progress and perspectives

The fourth industrial revolution anticipates energy to be sustainable, renewable and green. Hydrogen (H₂) is one of the green forms of energy and is deemed a possible solution to climate change. Light-dependent H₂ production is a promising method derived from nature's most copious resources: solar energy, water and biomass. Reduced environmental impacts, absorption of carbon dioxide, relative efficiency, and cost economics made it an eye-catching approach. However, low light conversion efficiency, limited ability to utilize complex carbohydrates, and the O₂ sensitivity of enzymes result in low yield. Isolation of efficient H₂ producers, development of microbial consortia having a synergistic impact, genetically improved strains, regulating bidirectional hydrogenase activity, physiological parameters, immobilization, novel photobioreactors, and additive strategies are summarized for their possibilities to augment the processes of bio-photolysis and photo-fermentation. The challenges and future perspectives have been addressed to explore a sustainable way forward in a bio-refinery approach.

Advanced insights on removal of antibiotics by microalgae-bacteria consortia: A state-of-the-art review and emerging prospects

Antibiotics abuse has triggered a growing environmental problem, posing a major threat to both ecosystem and human health. Unfortunately, there are still several shortcomings to current antibiotics removal technologies. Microalgae-bacteria consortia have been shown to be a promising antibiotics treatment technology owing to advantages of high antibiotics removal efficiency, low operational cost, and carbon emission reduction. This review aims to introduce the removal mechanisms, influencing factors, and future research perspectives for using microalgae-bacteria consortia to remove antibiotics. The interaction mechanisms between microalgae and bacteria are comprehensively revealed, and their exclusive advantages have been summarized in a "Trilogy" strategy, including "reinforced physical contact", "upgraded substance utilization along with antibiotics degradation", and "robust biological regulation". What's more, the relationship between different interaction mechanisms is emphatically analyzed. The important influencing factors, including concentration and classes of antibiotics, environmental conditions, and operational parameters, of antibiotics removal were also assessed. Three innovative treatment systems (microalgae-bacteria fuel cells (MBFCs), microalgae-bacteria membrane photobioreactors (MB-MPBRs), and microalgae-bacteria granular sludge (MBGS)) along with three advanced techniques (metabolic engineering, machine learning, and molecular docking and dynamics) are then introduced. In addition, concrete implementing schemes of the above advanced techniques are also provided. Finally, the current challenges and future research directions in using microalgae-bacteria consortia to remove antibiotics have been summarized. Overall, this review addresses the current state of microalgae-bacteria consortia for antibiotics treatment and provides corresponding recommendations for enhancing antibiotics removal efficiency.

Advances in the utilisation of carbon-neutral technologies for a sustainable tomorrow: A critical review and the path forward

Global industrialisation and overexploitation of fossil fuels significantly impact greenhouse gas emissions, resulting in global warming and other environmental problems. Hence, investigations on capturing, storing, and utilising atmospheric CO2 create novel technologies. Few microorganisms, microalgae, and macroalgae utilise atmospheric CO2 for their growth and reduce atmospheric CO2 levels. Activated carbon and biochar from biomasses also capture CO2. Nanomaterials such as metallic oxides, metal-organic frameworks, and MXenes illustrate outstanding adsorption characteristics, and convert CO2 to carbon-neutral fuels, creating a balance between CO2 production and elimination, thus zeroing the carbon footprint. The need for a paradigm shift from fossil fuels and promising technologies on renewable energies, carbon capture mechanisms, and carbon sequestration techniques that help reduce CO2 emissions for a better tomorrow are reviewed to achieve the world's sustainable development goals. The challenges and possible solutions with future perspectives are also discussed.

Microalgae-derived hydrogen production towards low carbon emissions via large-scale outdoor systems

Hydrogen as a clean fuel is receiving attention because it generates only water and a small amount of nitrogen oxide upon combustion. Biohydrogen production using microalgae is considered to be a highly promising carbon-neutral technology because it can secure renewable energy while efficiently reducing CO2 emissions. However, previous studies have mainly focused on improving the biological performance of microalgae; these approaches have struggled to achieve breakthroughs in commercialization because they do not heavily consider the complexity of the entire production process with microalgae, including large-scale cultivation, biomass harvest, and biomass storage. This work presents an in-depth analysis of the state-of-the-art technologies focused on large-scale cultivation systems with efficient downstream processes. Considering the individual processes of biohydrogen production, strategies are discussed to minimize carbon emissions and improve productivity simultaneously. A comprehensive understanding of microalgae-derived biohydrogen production suggests future directions for realizing environmental and economic sustainability.