Effects of wastewater microalgae harvesting methods on polyhydroxybutyrate production

New insights into Chlorella vulgaris applications

Environmental pollution is a big challenge that has been faced by humans in contemporary life. In this context, fossil fuel, cement production, and plastic waste pose a direct threat to the environment and biodiversity. One of the prominent solutions is the use of renewable sources, and different organisms to valorize wastes into green energy and bioplastics such as polylactic acid. Chlorella vulgaris, a microalgae, is a promising candidate to resolve these issues due to its ease of cultivation, fast growth, carbon dioxide uptake, and oxygen production during its growth on wastewater along with biofuels, and other productions. Thus, in this article, we focused on the potential of Chlorella vulgaris to be used in wastewater treatment, biohydrogen, biocement, biopolymer, food additives, and preservation, biodiesel which is seen to be the most promising for industrial scale, and related biorefineries with the most recent applications with a brief review of Chlorella and polylactic acid market size to realize the technical/nontechnical reasons behind the cost and obstacles that hinder the industrial production for the mentioned applications. We believe that our findings are important for those who are interested in scientific/financial research about microalgae.

The General Composition of Polyhydroxyalkanoates and Factors that Influence their Production and Biosynthesis

Polyhydroxyalkanoates (PHAs) have been a current research topic for many years. PHAs are biopolymers produced by bacteria under unfavorable growth conditions. They are biomaterials that exhibit a variety of properties, including biocompatibility, biodegradability, and high mechanical strength, making them suitable for future applications. This review aimed to provide general information on PHAs, such as their structure, classification, and parameters that affect the production process. In addition, the most commonly used bacterial strains that produce PHAs are highlighted, and details are provided on the type of carbon source used and how to optimize the parameters for bioprocesses. PHAs present a challenge to researchers because a variety of parameters affect biosynthesis, including the variety of carbon sources, bacterial strains, and culture media. Nevertheless, PHAs represent an opportunity to replace plastics, because they can be produced quickly and at a relatively low cost. With growing environmental concerns and declining oil reserves, polyhydroxyalkanoates are a potential replacement for nonbiodegradable polymers. Therefore, the study of PHA production remains a hot topic, as many substrates can be used as carbon sources. Both researchers and industry are interested in facilitating the production, commercialization, and application of PHAs as potential replacements for nonbiodegradable polymers. The fact that they are biocompatible, environmentally biodegradable, and adaptable makes PHAs one of the most important materials available in the market. They are preferred in various industries, such as agriculture (for bioremediation of oil-polluted sites, minimizing the toxicity of pollutants, and environmental impact) or medicine (as medical devices). The various bioprocess technologies mentioned earlier will be further investigated, such as the carbon source (to obtain a biopolymer with the lowest possible cost, such as glucose, various fatty acids, and especially renewable sources), pretreatment of the substrate (to increase the availability of the carbon source), and supplementation of the growth environment with different substances and minerals). Consequently, the study of PHA production remains a current topic because many substrates can be used as carbon sources. Obtaining PHA from renewable substrates (waste oil, coffee grounds, plant husks, etc.) contributes significantly to reducing PHA costs. Therefore, in this review, pure bacterial cultures (Bacillus megaterium, Ralstonia eutropha, Cupriavidus necator, and Pseudomonas putida) have been investigated for their potential to utilize by-products as cheap feedstocks. The advantage of these bioprocesses is that a significant amount of PHA can be obtained using renewable carbon sources. The main disadvantage is that the chemical structure of the obtained biopolymer cannot be determined in advance, as is the case with bioprocesses using a conventional carbon source. Polyhydroxyalkanoates are materials that can be used in many fields, such as the medical field (skin grafts, implantable medical devices, scaffolds, drug-controlled release devices), agriculture (for polluted water cleaning), cosmetics and food (biodegradable packaging, gentle biosurfactants with suitable skin for cosmetics), and industry (production of biodegradable biopolymers that replace conventional plastic). Nonetheless, PHA biopolymers continue to be researched and improved and play an important role in various industrial sectors. The properties of this material allow its use as a biodegradable material in the cosmetics industry (for packaging), in the production of biodegradable plastics, or in biomedical engineering, as various prostheses or implantable scaffolds.

Potential Use of Microbial Enzymes for the Conversion of Plastic Waste Into Value-Added Products: A Viable Solution

The widespread use of commercial polymers composed of a mixture of polylactic acid and polyethene terephthalate (PLA-PET) in bottles and other packaging materials has caused a massive environmental crisis. The valorization of these contaminants via cost-effective technologies is urgently needed to achieve a circular economy. The enzymatic hydrolysis of PLA-PET contaminants plays a vital role in environmentally friendly strategies for plastic waste recycling and degradation. In this review, the potential roles of microbial enzymes for solving this critical problem are highlighted. Various enzymes involved in PLA-PET recycling and bioconversion, such as PETase and MHETase produced by Ideonella sakaiensis; esterases produced by Bacillus and Nocardia; lipases produced by Thermomyces lanuginosus, Candida antarctica, Triticum aestivum, and Burkholderia spp.; and leaf-branch compost cutinases are critically discussed. Strategies for the utilization of PLA-PET's carbon content as C1 building blocks were investigated for the production of new plastic monomers and different value-added products, such as cyclic acetals, 1,3-propanediol, and vanillin. The bioconversion of PET-PLA degradation monomers to polyhydroxyalkanoate biopolymers by Pseudomonas and Halomonas strains was addressed in detail. Different solutions to the production of biodegradable plastics from food waste, agricultural residues, and polyhydroxybutyrate (PHB)-accumulating bacteria were discussed. Fuel oil production via PLA-PET thermal pyrolysis and possible hybrid integration techniques for the incorporation of thermostable plastic degradation enzymes for the conversion into fuel oil is explained in detail.

Challenges and Perspectives of Polyhydroxyalkanoate Production From Microalgae/Cyanobacteria and Bacteria as Microbial Factories: An Assessment of Hybrid Biological System

Polyhydroxyalkanoates (PHAs) are the biopolymer of choice if we look for a substitute of petroleum-based non-biodegradable plastics. Microbial production of PHAs as carbon reserves has been studied for decades and PHAs are gaining attention for a wide range of applications in various fields. Still, their uneconomical production is the major concern largely attributed to high cost of organic substrates for PHA producing heterotrophic bacteria. Therefore, microalgae/cyanobacteria, being photoautotrophic, prove to have an edge over heterotrophic bacteria. They have minimal metabolic requirements, such as inorganic nutrients (CO2, N, P, etc.) and light, and they can survive under adverse environmental conditions. PHA production under photoautotrophic conditions has been reported from cyanobacteria, the only candidate among prokaryotes, and few of the eukaryotic microalgae. However, an efficient cultivation system is still required for photoautotrophic PHA production to overcome the limitations associated with (1) stringent management of closed photobioreactors and (2) optimization of monoculture in open pond culture. Thus, a hybrid system is a necessity, involving the participation of microalgae/cyanobacteria and bacteria, i.e., both photoautotrophic and heterotrophic components having mutual interactive benefits for each other under different cultivation regime, e.g., mixotrophic, successive two modules, consortium based, etc. Along with this, further strategies like optimization of culture conditions (N, P, light exposure, CO2 dynamics, etc.), bioengineering, efficient downstream processes, and the application of mathematical/network modeling of metabolic pathways to improve PHA production are the key areas discussed here. Conclusively, this review aims to critically analyze cyanobacteria as PHA producers and proposes economically sustainable production of PHA from microbial autotrophs and heterotrophs in "hybrid biological system."

Nutrient removal and biomass production: advances in microalgal biotechnology for wastewater treatment

Owing to certain drawbacks, such as energy-intensive operations in conventional modes of wastewater treatment (WWT), there has been an extensive search for alternative strategies in treatment technology. Biological modes for treating wastewaters are one of the finest technologies in terms of economy and efficiency. An integrated biological approach with chemical flocculation is being conventionally practiced in several-sewage and effluent treatment plants around the world. Overwhelming responsiveness to treat wastewaters especially by using microalgae is due to their simplest photosynthetic mechanism and ease of acclimation to various habitats. Microalgal technology, also known as phycoremediation, has been in use for WWT since 1950s. Various strategies for the cultivation of microalgae in WWT systems are evolving faster. However, the availability of innovative approaches for maximizing the treatment efficiency, coupled with biomass productivity, remains the major bottleneck for commercialization of microalgal technology. Investment costs and invasive parameters also delimit the use of microalgae in WWT. This review critically discusses the merits and demerits of microalgal cultivation strategies recently developed for maximum pollutant removal as well as biomass productivity. Also, the potential of algal biofilm technology in pollutant removal, and harvesting the microalgal biomass using different techniques have been highlighted. Finally, an economic assessment of the currently available methods has been made to validate microalgal cultivation in wastewater at the commercial level.

Food waste conversion to microbial polyhydroxyalkanoates

Polyhydroxyalkanoates (PHAs) are biopolymers with desirable material properties similar to petrochemically derived plastics. PHAs are naturally produced by a wide range of microorganisms as a carbon storage mechanism and can accumulate to significantly high levels. PHAs are an environmentally friendly alternative to their petroleum counterparts because they can be easily degraded, potentially reducing the burden on municipal waste systems. Nevertheless, widespread use of PHAs is not currently realistic due to a variety of factors. One of the major constraints of large-scale PHA production is the cost of carbon substrate for PHA-producing microbes. The cost of production could potentially be reduced with the use of waste carbon from food-related processes. Food wastage is a global issue and therefore harbours immense potential to create valuable bioproducts. This article's main focus is to examine the state of the art of converting food-derived waste into carbon substrates for microbial metabolism and subsequent conversion into PHAs.

Microbubble assisted polyhydroxybutyrate production in Escherichia coli

BACKGROUND

One of the potential limitations of large scale aerobic Escherichia coli fermentation is the need for increased dissolved oxygen for culture growth and bioproduct generation. As culture density increases the poor solubility of oxygen in water becomes one of the limiting factors for cell growth and product formation. A potential solution is to use a microbubble dispersion (MBD) generating device to reduce the diameter and increase the surface area of sparged bubbles in the fermentor. In this study, a recombinant E. coli strain was used to produce polyhydroxybutyrate (PHB) under conventional and MBD aerobic fermentation conditions.

RESULTS

In conventional fermentation operating at 350 rpm and 0.8 vvm air flow rate, an OD600 of 6.21 and PHB yield of 23 % (dry cell basis) was achieved. MBD fermentation with similar bioreactor operating parameters produced an OD600 of 8.17 and PHB yield of 43 % PHB, which was nearly double that of the conventional fermentation.

CONCLUSIONS

This study demonstrated that using a MBD generator can increase oxygen mass transfer into the aqueous phase, increasing E. coli growth and bioproduct generation.

Bioprospecting of microalgae for integrated biomass production and phytoremediation of unsterilized wastewater and anaerobic digestion centrate

Eighteen microalgae, including two local isolates, were evaluated for their ability to grow and remove nutrients from unsterilized primary or secondary wastewater effluents as well as wastewater supplemented with nutrient-rich anaerobic digester centrate (ADC). Most of the tested species except several phylogenetically clustered Chlorella sorokiniana including local isolates and Scenedesmus strains were unable to grow efficiently. This may reflect the presence of certain genetic traits important for robust growth in the unsterilized wastewater. The maximum algal-specific growth rates and biomass density obtained in these bacterial-contaminated cultures were in the range of 0.8-1 day(-1) and 250-350 mg L(-1), respectively. ADC supplementation was especially helpful to biologically treated secondary effluent with its lower initial macronutrient and micronutrient content. As a result of algal growth, total nitrogen and orthophosphate levels were reduced by as much as 90 and 70 %, respectively. Biological assimilation was estimated to be the main mechanism of nitrogen removal in primary and secondary effluents with ammonia volatilization and bacterial nitrification-denitrification contributing for cultures supplemented with ADC. Assimilation by algae served as the principal mechanism of orthophosphate remediation in secondary wastewater cultures, while chemical precipitation appeared also to be important for orthophosphate removal in primary wastewater. Overall, cultivation of microalgae in primary and primary + 5 % ADC may be more favorable from an economical and sustainability perspective due to elimination of the costly and energy-intensive biological treatment step. These findings demonstrate that unsterilized wastewater and ADC can serve as critical nutrient sources for biomass generation and that robust microalgae can be potent players in wastewater phytoremediation.