Environmental risks of large scale cultivation of microalgae: Mitigation of spills

Nutraceutical prospects of genetically engineered cyanobacteria- technological updates and significance

Genetically engineered cyanobacterial strains that have improved growth rate, biomass productivity, and metabolite productivity could be a better option for sustainable bio-metabolite production. The global demand for biobased metabolites with nutraceuticals and health benefits has increased due to their safety and plausible therapeutic and nutritional utility. Cyanobacteria are solar-powered green cellular factories that can be genetically tuned to produce metabolites with nutraceutical and pharmaceutical benefits. The present review discusses biotechnological endeavors for producing bioprospective compounds from genetically engineered cyanobacteria and discusses the challenges and troubleshooting faced during metabolite production. This review explores the cyanobacterial versatility, the use of engineered strains, and the techno-economic challenges associated with scaling up metabolite production from cyanobacteria. Challenges to produce cyanobacterial bioactive compounds with remarkable nutraceutical values have been discussed. Additionally, this review also summarises the challenges and future prospects of metabolite production from genetically engineered cyanobacteria as a sustainable approach.

Microalgae-bacteria nexus for environmental remediation and renewable energy resources: Advances, mechanisms and biotechnological applications

Microalgae and bacteria, known for their resilience, rapid growth, and proximate ecological partnerships, play fundamental roles in environmental and biotechnological advancements. This comprehensive review explores the synergistic interactions between microalgae and bacteria as an innovative approach to address some of the most pressing environmental issues and the demands of clean and renewable freshwater and energy sources. Studies indicated that microalgae-bacteria consortia can considerably enhance the output of biotechnological applications; for instance, various reports showed during wastewater treatment the COD removal efficiency increased by 40%-90.5 % due to microalgae-bacteria consortia, suggesting its great potential amenability in biotechnology. This review critically synthesizes research works on the microalgae and bacteria nexus applied in the advancements of renewable energy generation, with a special focus on biohydrogen, reclamation of wastewater and desalination processes. The mechanisms of underlying interactions, the environmental factors influencing consortia performance, and the challenges and benefits of employing these bio-complexes over traditional methods are also discussed in detail. This paper also evaluates the biotechnological applications of these microorganism consortia for the augmentation of biomass production and the synthesis of valuable biochemicals. Furthermore, the review sheds light on the integration of microalgae-bacteria systems in microbial fuel cells for concurrent energy production, waste treatment, and resource recovery. This review postulates microalgae-bacteria consortia as a sustainable and efficient solution for clean water and energy, providing insights into future research directions and the potential for industrial-scale applications.

Polyphosphate kinase deletion increases laboratory productivity in cyanobacteria

Identification and manipulation of cellular energy regulation mechanisms may be a strategy to increase productivity in photosynthetic organisms. This work tests the hypothesis that polyphosphate synthesis and degradation play a role in energy management by storing or dissipating energy in the form of ATP. A polyphosphate kinase (ppk) knock-out strain unable to synthesize polyphosphate was generated in the cyanobacterium Synechocystis sp. PCC 6803. This mutant strain demonstrated higher ATP levels and faster growth than the wildtype strain in high-carbon conditions and had a growth defect under multiple stress conditions. In a strain that combined ppk deletion with heterologous expression of ethylene-forming enzyme, higher ethylene productivity was observed than in the wildtype background. These results support the role of polyphosphate synthesis and degradation as an energy regulation mechanism and suggest that such mechanisms may be effective targets in biocontainment design.

Overview and Challenges of Large-Scale Cultivation of Photosynthetic Microalgae and Cyanobacteria

Microalgae and cyanobacteria are diverse groups of organisms with great potential to benefit societies across the world. These organisms are currently used in food, feed, pharmaceutical and cosmetic industries. In addition, a variety of novel compounds are being isolated. Commercial production of photosynthetic microalgae and cyanobacteria requires cultivation on a large scale with high throughput. However, scaling up production from lab-based systems to large-scale systems is a complex and potentially costly endeavor. In this review, we summarise all aspects of large-scale cultivation, including aims of cultivation, species selection, types of cultivation (ponds, photobioreactors, and biofilms), water and nutrient sources, temperature, light and mixing, monitoring, contamination, harvesting strategies, and potential environmental risks. Importantly, we also present practical recommendations and discuss challenges of profitable large-scale systems associated with economical design, effective operation and maintenance, automation, and shortage of experienced phycologists.

Potential use of saline resources for biofuel production using halophytes and marine algae: prospects and pitfalls

There exists a global challenge of feeding the growing human population of the world and supplying its energy needs without exhausting global resources. This challenge includes the competition for biomass between food and fuel production. The aim of this paper is to review to what extent the biomass of plants growing under hostile conditions and on marginal lands could ease that competition. Biomass from salt-tolerant algae and halophytes has shown potential for bioenergy production on salt-affected soils. Halophytes and algae could provide a bio-based source for lignoceelusic biomass and fatty acids or an alternative for edible biomass currently produced using fresh water and agricultural lands. The present paper provides an overview of the opportunities and challenges in the development of alternative fuels from halophytes and algae. Halophytes grown on marginal and degraded lands using saline water offer an additional material for commercial-scale biofuel production, especially bioethanol. At the same time, suitable strains of microalgae cultured under saline conditions can be a particularly good source of biodiesel, although the efficiency of their mass-scale biomass production is still a concern in relation to environmental protection. This review summaries the pitfalls and precautions for producing biomass in a way that limits environmental hazards and harms for coastal ecosystems. Some new algal and halophytic species with great potential as sources of bioenergy are highlighted.

Harnessing the Algal Chloroplast for Heterologous Protein Production

Photosynthetic microbes are gaining increasing attention as heterologous hosts for the light-driven, low-cost production of high-value recombinant proteins. Recent advances in the manipulation of unicellular algal genomes offer the opportunity to establish engineered strains as safe and viable alternatives to conventional heterotrophic expression systems, including for their use in the feed, food, and biopharmaceutical industries. Due to the relatively small size of their genomes, algal chloroplasts are excellent targets for synthetic biology approaches, and are convenient subcellular sites for the compartmentalized accumulation and storage of products. Different classes of recombinant proteins, including enzymes and peptides with therapeutical applications, have been successfully expressed in the plastid of the model organism Chlamydomonas reinhardtii, and of a few other species, highlighting the emerging potential of transplastomic algal biotechnology. In this review, we provide a unified view on the state-of-the-art tools that are available to introduce protein-encoding transgenes in microalgal plastids, and discuss the main (bio)technological bottlenecks that still need to be addressed to develop robust and sustainable green cell biofactories.

Fourth generation biofuel from genetically modified algal biomass: Challenges and future directions

Genetic engineering applications in the field of biofuel are rapidly expanding due to their potential to boost biomass productivity while lowering its cost and enhancing its quality. Recently, fourth-generation biofuel (FGB), which is biofuel obtained from genetically modified (GM) algae biomass, has gained considerable attention from academic and industrial communities. However, replacing fossil resources with FGB is still beset with many challenges. Most notably, technical aspects of genetic modification operations need to be more fully articulated and elaborated. However, relatively little attention has been paid to GM algal biomass. There is a limited number of reviews on the progress and challenges faced in the algal genetics of FGB. Therefore, the present review aims to fill this gap in the literature by recapitulating the findings of recent studies and achievements on safe and efficient genetic manipulation in the production of FGB. Then, the essential issues and parameters related to genome editing in algal strains are highlighted. Finally, the main challenges to FGB pertaining to the diffusion risk and regulatory frameworks are addressed. This review concluded that the technical and biosafety aspects of FGB, as well as the complexity and diversity of the related regulations, legitimacy concerns, and health and environmental risks, are among the most important challenges that require a strong commitment at the national/international levels to reach a global consensus.

Role of Biofuels in Energy Transition, Green Economy and Carbon Neutrality

Modern civilization is heavily reliant on petroleum-based fuels to meet the energy demand of the transportation sector. However, burning fossil fuels in engines emits greenhouse gas emissions that harm the environment. Biofuels are commonly regarded as an alternative for sustainable transportation and economic development. Algal-based fuels, solar fuels, e-fuels, and CO2-to-fuels are marketed as next-generation sources that address the shortcomings of first-generation and second-generation biofuels. This article investigates the benefits, limitations, and trends in different generations of biofuels through a review of the literature. The study also addresses the newer generation of biofuels highlighting the social, economic, and environmental aspects, providing the reader with information on long-term sustainability. The use of nanoparticles in the commercialization of biofuel is also highlighted. Finally, the paper discusses the recent advancements that potentially enable a sustainable energy transition, green economy, and carbon neutrality in the biofuel sector.

Enhanced lipid accumulation in microalgae through nanoparticle-mediated approach, for biodiesel production: A mini-review

Nanoparticle application in microalgae for enhanced lipid production is an ongoing work that leads towards the contribution in biodiesel production. During this decade, metal nanoparticles are constantly being reported to have numerous applications in diverse fields, because of their unique optical, electrical, and magnetic properties. They can interact with the biomolecules of cells and thereby alters cellular metabolisms, which in turn reflects their ability to regulate some primary or secondary metabolic pathways. Nanoparticles derived from metals like Fe, Cu, and Se are taking part in redox processes and their presence in many enzymes may modulate algal metabolisms. Besides by upregulating or downregulating the expression of several genes, nanoparticle exposure can alter gene expressions in many organisms. In microalgae such as Chlorella vulgaris, C. pyrenoidosa, Scenedesmus obliquus, S. rubescens, Trachydiscus minut u s, Parachlorella kessleri, and Tetraselmis suecica; metal nanoparticle exposure in different environmental conditions have impacts on various physiological or molecular changes, thereby increasing the growth rate, biomass and lipid production. The present mini-review gives an insight into the various advantages and a future outlook on the application of nanoparticles in microalgae for biofuel production. Also, it can be proposed that nanoparticles could be useful in blocking or deactivating the AGPase enzyme (involved in the glucose to starch conversion pathway), binding to its active site, thereby increasing lipid production in microalgae that could be utilized for enhanced biodiesel production.

Effects of fluctuating temperature in open raceway ponds on the biomass accumulation and harvest efficiency of Spirulina in large-scale cultivation

It is of great significance to select strains with wide adaptability to temperature range for large-scale commercial cultivation of Spirulina. The aim of this study was to comprehend how the strain H-208 grew and whether this strain had any advantages in temperature adaptation compared with local production strain during the large-scale cultivation in Inner Mongolia. The results showed that the strain H-208 could adapt to the new environmental condition quickly, and the daily average biomass dry weight of strain H-208 was 49% and 52% more than that of production strain M-1 in first cycle (20.24 g/m2/day) and second cycle (16.90 g/m2/day) of acclimation experiment, respectively. The growth rate of strain H-208 was 0.055 and 0.066 g/L/day from July 22 to July 25 and from July 26 to July 29, respectively, while the growth rate of strain M-1 was only 0.036 and 0.032 g/L/day, respectively, during the same cultured days in 605-m2 raceway ponds before high temperature. The harvesting efficiency of H-208 and M-1 was 95.1% and 72.1% before high temperature, and that was 95.3% and 52.5% after being stressed by high temperature, respectively. Meanwhile, it was also observed that the filaments of the two strains contracted and their pitches were smaller than that before high temperature stress, especially the strain M-1. In 20-m2 raceway ponds of recovery experiment after high temperature, the percentage of daily average biomass dry weight of strain H-208 was 68% more than that of strain M-1, which demonstrated that strain H-208 could recover and grow rapidly, and its self-regulation ability was superior to that of strain M-1.

Applications of Microalgal Biotechnology for Disease Control in Aquaculture

Aquaculture industries, and in particular the farming of fish and crustaceans, are major contributors to the economy of many countries and an increasingly important component in global food supply. However, the severe impact of aquatic microbial diseases on production performance remains a challenge to these industries. This article considers the potential applications of microalgal technology in the control of such diseases. At the simplest level, microalgae offer health-promoting benefits as a nutritional supplement in feed meal because of their digestibility and high content of proteins, lipids and essential nutrients. Furthermore, some microalgal species possess natural anti-microbial compounds or contain biomolecules that can serve as immunostimulants. In addition, emerging genetic engineering technologies in microalgae offer the possibility of producing ‘functional feed additives’ in which novel and specific bioactives, such as fish growth hormones, anti-bacterials, subunit vaccines, and virus-targeted interfering RNAs, are components of the algal supplement. The evaluation of such technologies for farm applications is an important step in the future development of sustainable aquaculture.

Improvement of the Uranium Sequestration Ability of a Chlamydomonas sp. (ChlSP Strain) Isolated From Extreme Uranium Mine Tailings Through Selection for Potential Bioremediation Application

The extraction and processing of uranium (U) have polluted large areas worldwide, rendering anthropogenic extreme environments inhospitable to most species. Noticeably, these sites are of great interest for taxonomical and applied bioprospection of extremotolerant species successfully adapted to U tailings contamination. As an example, in this work we have studied a microalgae species that inhabits extreme U tailings ponds at the Saelices mining site (Salamanca, Spain), characterized as acidic (pH between 3 and 4), radioactive (around 4 μSv h⁻¹) and contaminated with metals, mainly U (from 25 to 48 mg L⁻¹) and zinc (from 17 to 87 mg L⁻¹). After isolation of the extremotolerant ChlSP strain, morphological characterization and internal transcribed spacer (ITS)-5.8S gene sequences placed it in the Chlamydomonadaceae, but BLAST analyses identity values, against the nucleotide datasets at the NCBI database, were very low (<92%). We subjected the ChlSP strain to an artificial selection protocol to increase the U uptake and investigated its response to selection. The ancestral strain ChlSP showed a U-uptake capacity of ≈4.30 mg U g⁻¹ of dry biomass (DB). However, the artificially selected strain ChlSG was able to take up a total of ≈6.34 mg U g⁻¹ DB, close to the theoretical maximum response (≈7.9 mg U g⁻¹ DB). The selected ChlSG strain showed two possible U-uptake mechanisms: the greatest proportion by biosorption onto cell walls (ca. 90%), and only a very small quantity, ~0.46 mg g⁻¹ DB, irreversibly bound by bioaccumulation. Additionally, the kinetics of the U-uptake process were characterized during a microalgae growth curve; ChlSG cells removed close to 4 mg L⁻¹ of U in 24 days. These findings open up promising prospects for sustainable management of U tailings waters based on newly evolved extremotolerants and outline the potential of artificial selection in the improvement of desired features in microalgae by experimental adaptation and selection.

Competitive growth experiments with a high-lipid Chlamydomonas reinhardtii mutant strain and its wild-type to predict industrial and ecological risks

Key microalgal species are currently being exploited as biomanufacturing platforms using mass cultivation systems. The opportunities to enhance productivity levels or produce non-native compounds are increasing as genetic manipulation and metabolic engineering tools are rapidly advancing. Regardless of the end product, there are both environmental and industrial risks associated to open pond cultivation of mutant microalgal strains. A mutant escape could be detrimental to local biodiversity and increase the risk of algal blooms. Similarly, if the cultivation pond is invaded by a wild-type (WT) microalgae or the mutant reverts to WT phenotypes, productivity could be impacted. To investigate these potential risks, a response surface methodology was applied to determine the competitive outcome of two Chlamydomonas reinhardtii strains, a WT (CC-124) and a high-lipid accumulating mutant (CC-4333), grown in mixotrophic conditions, with differing levels of nitrogen and initial WT to mutant ratios. Results of the growth experiments show that mutant cells have double the exponential growth rate of the WT in monoculture. However, due to a slower transition from lag phase to exponential phase, mutant cells are outcompeted by the WT in every co-culture treatment. This suggests that, under the conditions tested, outdoor cultivation of the C. reinhardtii cell wall-deficient mutant strains does not carry a significant environmental risk to its WT in an escape scenario. Furthermore, lipid results show the mutant strain accumulates over 200% more TAGs per cell, at 50 mg L^-1 NH₄Cl, compared to the WT, therefore, the fragility of the mutant strain could impact on overall industrial productivity.

Engineering cyanobacteria as photosynthetic feedstock factories

Carbohydrate feedstocks are at the root of bioindustrial production and are needed in greater quantities than ever due to increased prioritization of renewable fuels with reduced carbon footprints. Cyanobacteria possess a number of features that make them well suited as an alternative feedstock crop in comparison to traditional terrestrial plant species. Recent advances in genetic engineering, as well as promising preliminary investigations of cyanobacteria in a number of distinct production regimes have illustrated the potential of these aquatic phototrophs as biosynthetic chassis. Further improvements in strain productivities and design, along with enhanced understanding of photosynthetic metabolism in cyanobacteria may pave the way to translate cyanobacterial theoretical potential into realized application.

Fungal-assisted algal flocculation: application in wastewater treatment and biofuel production

BACKGROUND

The microalgal-based industries are facing a number of important challenges that in turn affect their economic viability. Arguably the most important of these are associated with the high costs of harvesting and dewatering of the microalgal cells, the costs and sustainability of nutrient supplies and costly methods for large scale oil extraction. Existing harvesting technologies, which can account for up to 50% of the total cost, are not economically feasible because of either requiring too much energy or the addition of chemicals. Fungal-assisted flocculation is currently receiving increased attention because of its high harvesting efficiency. Moreover, some of fungal and microalgal strains are well known for their ability to treat wastewater, generating biomass which represents a renewable and sustainable feedstock for bioenergy production.

RESULTS

We screened 33 fungal strains, isolated from compost, straws and soil for their lipid content and flocculation efficiencies against representatives of microalgae commercially used for biodiesel production, namely the heterotrophic freshwater microalgae Chlorella protothecoides and the marine microalgae Tetraselmis suecica. Lipid levels and composition were analyzed in fungal-algal pellets grown on media containing alternative carbon, nitrogen and phosphorus sources from wheat straw and swine wastewater, respectively. The biomass of fungal-algal pellets grown on swine wastewater was used as feedstock for the production of value-added chemicals, biogas, bio-solids and liquid petrochemicals through pyrolysis. Co-cultivation of microalgae and filamentous fungus increased total biomass production, lipid yield and wastewater bioremediation efficiency.

CONCLUSION

Fungal-assisted microalgal flocculation shows significant potential for solving the major challenges facing the commercialization of microalgal biotechnology, namely (i) the efficient and cost-effective harvesting of freshwater and seawater algal strains; (ii) enhancement of total oil production and optimization of its composition; (iii) nutrient supply through recovering of the primary nutrients, nitrogen and phosphates and microelements from wastewater. The biomass generated was thermochemically converted into biogas, bio-solids and a range of liquid petrochemicals including straight-chain C12 to C21 alkanes which can be directly used as a glycerine-free component of biodiesel. Pyrolysis represents an efficient alternative strategy for biofuel production from species with tough cell walls such as fungi and fungal-algal pellets.

Lipid production in association of filamentous fungi with genetically modified cyanobacterial cells

BACKGROUND

Numerous strategies have evolved recently for the generation of genetically modified or synthetic microalgae and cyanobacteria designed for production of ethanol, biodiesel and other fuels. In spite of their obvious attractiveness there are still a number of challenges that can affect their economic viability: the high costs associated with (1) harvesting, which can account for up to 50 % of the total biofuel's cost, (2) nutrients supply and (3) oil extraction. Fungal-assisted bio-flocculation of microalgae is gaining increasing attention due to its high efficiency, no need for added chemicals and low energy inputs. The implementation of renewable alternative carbon, nitrogen and phosphorus sources from agricultural wastes and wastewaters for growing algae and fungi makes this strategy economically attractive.

RESULTS

This work demonstrates that the filamentous fungi, Aspergillus fumigatus can efficiently flocculate the unicellular cyanobacteria Synechocystis PCC 6803 and its genetically modified derivatives that have been altered to enable secretion of free fatty acids into growth media. Secreted free fatty acids are potentially used by fungal cells as a carbon source for growth and ex-novo production of lipids. For most of genetically modified strains the total lipid yields extracted from the fungal-cyanobacterial pellets were found to be higher than additive yields of lipids and total free fatty acids produced by fungal and Synechocystis components when grown in mono-cultures. The synergistic effect observed in fungal-Synechocystis associations was also found in bioremediation rates when animal husbandry wastewater was used an alternative source of nitrogen and phosphorus.

CONCLUSION

Fungal assisted flocculation can complement and assist in large scale biofuel production from wild-type and genetically modified Synechocystis PCC 6803 strains by (1) efficient harvesting of cyanobacterial cells and (2) producing of high yields of lipids accumulated in fungal-cyanobacterial pellets.

Cultivated microalgae spills: hard to predict/easier to mitigate risks

Cultivating algae on a large scale will inevitably lead to spills into natural ecosystems. Most risk analyses have dealt only with transgenic algae, without considering the risks of cultivating the corresponding non-transgenic wild type species. This is despite the long-studied 'paradox of the plankton', which describes the unsuitability of laboratory experimentation or modeling to predict the outcome of introducing non-native algae into a new ecosystem. Risk analyses of transgenic strains of native algae can be based on whether they are more fit or less fit than their wild type, but these are not possible with non-native species. Risks from spills can be minimized by mutagenically or transgenically deleting genes that are unnecessary in culture but obligatory in nature.