Bioplastic: An accost towards sustainable development

In the current era, plastic has become a global environmental menace affecting terrestrial and aquatic ecosystems. Regular plastic resilient nature towards decomposition, and it pollutes the environment. Conventional plastic is widely used in various industrial setups with no alternate substitute available. The quest to find an alternate solution to the emerging problem development of bioplastic that is eco-friendlier and adds no pollution to the environment has been much focussed. Bioplastic is plastic synthesized from renewable biomass sources rather than petroleum origin. The development of bioplastic of microbial origin will be a promising innovation to keep our world plastic-free and promote sustainability. It can be degraded easily and gets broken down into carbon dioxide, biomass, and water rapidly. The present reviews highlight the sources of microbial-derived bioplastic, polyhydroxybutyrate (PHB), polyhydroxyalkanoates (PHAs), extraction methodologies, optimization strategies to improve yield, degradation, application areas, present challenges, and prospects in production. We have also provided a brief insight into gene and gene clusters responsible for bioplastic production. Overall, the article will provide a comprehensive update on bioplastic to help mitigate our current problem associated with conventional plastic usage. -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Introduction One of the 17 UN sustainable goals aims for a clean environment where bioplastic use could achieve sustainability to a great extent. Synthetic polymers are incredibly stable in the environment and do not degrade by biogeochemical cycles of the biosphere, which has to lead them to be a global pollutant and poses a threat to sustainable development in the world.1 A primary environmental concern is a gradual accumulation of synthetic plastics, which are non-biodegradable, directly impacting global warming, eco-toxicity, ozone depletion, and soilwater pollution. Thus, it seeks to develop means to reduce plastic use by practicing bioplastics in dayto-day use. One aspect of incorporating PHA bioplastic production within plants with the aid of genetic manipulation could significantly link agriculture with the material industry's demands. Sources of bioplastics range from a wide variety of renewable sources such as cellulose, starch, chitin, proteins, lipids, and majorly polymers from plant/microbial origin, i.e., PHAs and PHBs.2 Polylactic acid has also shown great potential in bioplastic production apart from its other polymer like polycaprolactone, polyglycolic, PHB is used mainly in pharmaceutical applications.3 Compared 163 NEUROPHARMAC JOURNAL 6 (2021) 162-168 to petroleum-based plastics, degradation of bioplastic supports its sustainable development approach, which quickly gets degraded into carbon dioxide, water, and biomass under aerobic conditions. Compositing under microbial action is useful in degrading polylactic acid, PHA, or other plant-derived bioplastics like bio-polyethylene terephthalate and 1,3-propanediol4, but PLA showed a slow degradation processing taking 11month time for decomposition. Thus, to further enhance biodegradability, either the polymer's blending could be an effective solution or improve the composting methodology. Classification of Plastics Based on biodegradability, plastic can be classified into biodegradable and non-biodegradable. Chemical structure and degradation patterns have been explored mainly for various plastics. Fossil fuel-based plastics are conventional plastic, nonbiodegradable due to their extensive repetition of monomer units and higher molecular weight.5 The domain of non-biodegradable plastic includes PVC, PP, PS, PET, and PUR. Due to their harsh nature and poor waste management practices, it has posed a severe threat of pollution to the earth. Polyolefinsbased plastics are widely used in various domains of our day-to-day life; because of their highly stable nature in the environment, it is vital to look for proper waste management.6 Although to enhance its degradability, starch and prooxidants are preferred for better fragmentation of plastics. The diverse range of bioplastic is summarized in Figure 1. While on the other hand, biodegradable bioplastics have a high biodegradability rate and get easily disintegrated by microbes' action, thus possess a lesser threat to the ecosystem. Factors like polymer molecular weight, functional group, and crystallinity affect plastic's degradation process.8 Exoenzymes released by microbe break down these complex molecules into simpler ones using aerobic and anaerobic processes.6,9 Fungi and bacteria degrade plastics finally into carbon dioxide, and water through various metabolic activities, e.g., strains like bacillus and Brevibacillus produce proteases with aid in the degradation of plastics.10 Fungi are natural decomposers that produce laccase enzymes for the degradation of plastics.11 Under the anaerobic situation, bacteria use oxygen as an electron donor to synthesize smaller organic molecules. While under anaerobic conditions in the absence of oxygen, other molecules like nitrate, sulfate, and manganese are used as electron acceptors.12 Figure 1: Types of bioplastics.7 Production of PHB and PHA from microbial sources PHA are linear polyesters that comprise of molecular weight more than 60000 Daltons, which is commonly observed to get accumulated in a variety of microbes, including gram harmful bacteria, gram-positive bacteria, and some archaea as intracellular granules under unfavorable growth conditions.13-15 PHB is the principal subsidiary of PHA. Under limited nutrient conditions with surplus carbon, various bacteria such as Alcaligene eutrophus, Azotobacter vinelandii, Bacillus, Streptomyces accumulate PHA as stored food material.16-18 The optimized strain of Rhizobium elti E1 and Pseudomonas stutzeri E114 improves the yield of PHBs when given mannitol as carbon and ammonium sulfate as a nitrogen source at PH of 7. Optimum PHB production conditions were 48 h, temperature 30 oC under incubation period of 48 hrs during the highest yield.19 164 NEUROPHARMAC JOURNAL 6 (2021) 162-168 PHBs are biodegradable polyesters that have great potential to develop fully biodegradable plastics. The cheapest source for its production is agricultural products like cane molasses and steep corn liquor as carbon and nitrogen sources.20 Fungi and bacteria utilize PHB, a thermoplastic polyester in intracellular carbon, and accumulate in the cell as an inclusion body.21 Obtaining a cheap carbon source is a significant hurdle in the production of PHA and PHBs. It is estimated that one-tonne polymer 3 tons of glucose are needed, which is pretty costly. PHB can be obtained using other cheap sources like agro-residues, dairy waste, molasses, corn steep liquor.22,23 Microalgae have appeared as rich sources for bioplastic production.7 For bioplastic production, Chlorella and Spirulina species are the most promising microalgae.7 Biomass produced by microalgae is potentially high, and it is not dependent upon food sources as substrate and has potential for high lipid accumulation.24,25 Bioplastic from microalgae offers a more sustainable approach and contributes to the bioeconomy.26,27 Growing microalgae in a closed cultivation system involve much lower cost with higher production.28 Strain improvement and optimization of culture A gram-positive bacterium SRKP-3, which resembled Bacillus megaterium, was identified from brackish water showing an excellent capability to accumulate PHAs. Optimization using response surface methodology resulted in an enhanced yield of 6.37 g/L of PHB dry weight at pH 9.22 Optimization of Alcaligenes sp. in batch culture was performed for PHB production using cane molasses and urea as a nutrient source. Strain selection was made using Nile blue A staining; parameter optimization showed 34.5 °C temperature, 6.54 pH, 3.13 Hz agitation enhanced peed to result in PHB yield of 76.80% of dry cell mass.29 Yang et al., 2010 showed enhanced PHA production in Ralstonia eutropha H16 by using an optimal acetate, propionate, and butyrate dose. With increasing butyrate production, PHA production is enhanced by 1.5-fold, and further optimizes phosphate buffer increased PHA production by fourfold.30 Bacterium Cupriavidus necator is known to accumulate PHB in a large amount. Using the Plackett-Burman design, crucial factors affecting the accumulation of PHB granules were agitation speed, fructose, KH2PO4, and initial pH, which yielded 7.46 g/L of PHB.31 Similarly, for Botryococcus braunii applying RSM, the optimal parameters were pH in the range of 4-11, temperature 30-50 0C, and sewage water as a substrate for maximum yield of PHB was studied.32 Likewise, various statistical optimization was employed for obtaining a maximum yield of PHB in Bacillus subtilis NG05, Burkholderia sacchari, Bacillus sphaericus NCIM 5149, with a variety of substrate.33,34 Genetic approaches used in enhancing bioplastic production Several archaea and eubacteria accumulate PHA within their cells to meet energy needs and carbon storage. The alteration in genetic and metabolic pathways for PHA metabolism has been achieved in these bacteria. However, finding a cost-effective production methodology remains a hurdle that could be sorted out using genetic engineering these microbes to produce PHAs in a high amount.35 Earlier in 1990, Pseudomonas saccharophila and Alcaligenes eutrophus were engineered to uptake galactose and lactose, but they resulted in low PHB accumulation within the cell.36 At the same time, incorporation of lacZ, lacl, and lacO genes from E. coli to C. necator allowed better lactose utilization resulting in higher accumulation.37 Likewise, by introducing Mannheimia succiniciproducens MBEL55E sacC gene to R. eutropha for su