Ever since awareness about the toxicity of commercially available non-biodegradable plastics has been discovered, the consumers wish to opt for products packaged in Eco-friendly or bio-plastic labels. However, just because the consumers demand it, the products do not appear magically. Therefore, the scientific community in academia and industries is still striving towards creating truly ‘biodegradable’ plastic.
In fact, currently, biodegradable plastics are simply not available in the market. So what do we purchase that is labeled under ‘bioplastic?’ In a nutshell, the commercial bio-plastic is more eco-friendly than the conventional kind; however, it is not entirely biodegradable. Bioplastic is made by combining bio-based polymers such as starch with smaller entities of non-degradable plastic to create a product that is susceptible to microbial attack. The microorganisms can attack the biological constituents of the bio-based plastic to break it down into smaller fragments and decreasing the half-life of these plastics. Since they leave behind a non-biodegradable entity, they cannot be considered eco-friendly or biodegradable.
Accordingly, is there even a completely biodegradable plastic, and if there is, what is it? To address this question, I would have to bring up the basic principle of biodegradation. Only the compounds that are naturally synthesized can be universally degraded by bacteria simply because they are used to it. Consequently, numerous bacteria, algae, and plants exist in nature that capability to synthesize this biodegradable plastic, known as Polyhydroxyalkanoates (PHAs).
The simplest PHA is Polyhydroxy butyrate (PHB) which is formed after polymerization of 3-hydroxy-butyric acid molecules by organisms of several genera. These polymers aggregate together and form large PHA granules within the cells, which can be extracted by bursting the bacterial cells. The molecular weight of biopolymers is similar to polypropylene, making their properties comparable to that of polypropylene (plastic). Due to the versatile nature of PHAs, they can also be used for different applications.
Ecological significance Of PHAs
Why do the bacteria need to produce PHAs? Considering that bacteria grow in the wild, with uncertain environmental conditions, it needs to prepare for ‘starvation’ conditions. PHAs are, therefore, produced for storing food for adverse times. Bacteria can skillfully store energy sources like glucose, fructose, sucrose, etc., in the form of PHA aggregates for later use. Similar to how we store fat as an energy reserve, PHA is also stored for ‘starvation’ conditions. Bacteria metabolize the stored PHA to prevent from starving after the nutritional resources become limited or exhaust completely.
Advantages of replacing plastic with PHAs
Similar to plastics, PHA is also resistant to moisture, oxygen, and chemicals. The polymers can remain stable even under UV radiations, and optical purity makes them suitable for making carry bags. Besides, they possess a highly desirable quality of biodegradability, which makes them exceptionally valuable. PHAs can be completely degraded into carbon dioxide and water by microorganisms without leaving any undesirable residues. Currently, PHAs are commercially produced for biomedical purposes, including the production of stents, drug-delivery systems, etc., since PHAs are biocompatible with mammalian cells.
Moreover, the PHAs are produced from renewable resources such as food crops, waste feed, etc., unlike conventional plastic, which is made from the hydrocarbons obtained from non-renewable resources like petroleum. Therefore, the existence of PHAs is not dependent on the availability of fossil fuels which will become extinct sooner than later.
Microbial biosynthesis pathways are very complex, which can cause differences in the polymer quality obtained from different batches. This will cause fluctuation in the quality of the product, thereby affecting the commercial market.
PHA packaging cannot replace plastic for providing longer shelf life due to its biodegradable nature. Therefore, major efforts are targeted towards using PHAs for the replacement of single-use plastic.
Traditional PHB (or PHBV) is produced from biological resources like food crops since bacteria use simple sugars as the substrate. However, this increases the cost of production, which seriously limits their commercial applications. For example, C. necator, a traditional PHB producing organism, utilizes glucose or fructose as a carbon source, and therefore, the estimated production cost of 1 Kg PHB is about US$ 15-30, almost 30 times higher than the cost of polypropylene (US$ 0.7- 1.0/Kg).
Primary research needs to be targeted towards implementing strategies for reducing the cost of PHA production. For example, developing recombinant microbes by introducing PHA-producing genes in industrially important microbes like E. coli can improve the process efficiency.
Replacement of traditional substrates with cheaper sources such as glycerol, molasses, etc., can also reduce production costs. In addition, attempts have been made to culture microorganisms using organic feed from the waste discharge for PHB production; however, these methods have limited commercial applications.
It is also interesting that methane – an abundant greenhouse gas, can also be transformed into PHAs by methane-utilizing organisms such as Methylobacterium, Methylosinus, etc. However, their commercial implementation is even more lacking due to the substrate properties. Methane is a gaseous molecule with a solubility of about 22 mg/l in pure water at STP conditions. How much bacterial cell density can this concentration support, and how much can actually be converted to PHA? Even though it could be a cheaper and cleaner solution, its potential has not been realized.
The PHA production process has to be improved to identify other areas where their applications could be instrumental. In addition, acceptable strategies need to be developed and implemented for establishing a sustainable process since PHAs are our only available biodegradable substitute for plastics.