Polyhydroxyalkanoate Analysis Essay

This article is about plastics made from renewable biomass. For the information on plastics that are biodegradable, see biodegradable plastic.

Bioplastics are plastics derived from renewable biomass sources, such as vegetable fats and oils, corn starch, or microbiota.[1] Bioplastic can be made from agricultural by-products and also from used plastic bottles and other containers using microorganisms. Common plastics, such as fossil-fuel plastics (also called petrobased polymers), are derived from petroleum or natural gas. Production of such plastics tends to require more fossil fuels and to produce more greenhouse gases than the production of biobased polymers (bioplastics).[citation needed] Most bioplastics biodegrade more readily than commodity fossil-fuel derived plastics. Bioplastics are usually derived from sugar derivatives, including starch, cellulose, lactic acid. As of 2014, bioplastics represented approximately 0.2% of the global polymer market (300 million tons).[2]

IUPAC definition

Biobased polymer derived from the biomass or issued from monomers derived
from the biomass and which, at some stage in its processing into finished
products, can be shaped by flow.

Note 1: Bioplastic is generally used as the opposite of polymer derived from
fossil resources.

Note 2: Bioplastic is misleading because it suggests that any polymer derived
from the biomass is environmentally friendly.

Note 3: The use of the term "bioplastic" is discouraged. Use the expression
"biobased polymer".

Note 4: A biobased polymer similar to a petrobased one does not imply any
superiority with respect to the environment unless the comparison of respective
life cycle assessments is favourable.[3]


Bioplastics are used for disposable items, such as packaging, crockery, cutlery, pots, bowls, and straws.[4] Few commercial applications exist for bioplastics. In principle they could replace many applications for petroleum-derived plastics, however cost and performance remain problematic. Beyond structural materials, electroactive bioplastics are being developed that promise to be used to carry electric current.[5]

Biopolymers are available as coatings for paper rather than the more common petrochemical coatings.[6]


Starch-based plastics[edit]

Thermoplastic starch currently represents the most widely used bioplastic, constituting about 50 percent of the bioplastics market[citation needed]. Simple starch bioplastic can be made at home.[7] Pure starch is able to absorb humidity, and is thus a suitable material for the production of drug capsules by the pharmaceutical sector. Flexibiliser and plasticiser such as sorbitol and glycerine can also be added so the starch can also be processed thermo-plastically. The characteristics of the resulting bioplastic (also called "thermo-plastical starch") can be tailored to specific needs by adjusting the amounts of these additives.

Starch-based bioplastics are often blended with biodegradable polyesters to produce starch/polylactic acid,[8] starch/polycaprolactone[9] or starch/Ecoflex[10] (polybutylene adipate-co-terephthalate produced by BASF[11]). blends. These blends are used for industrial applications and are also compostable. Other producers, such as Roquette, have developed other starch/polyolefin blends. These blends are not biodegradable, but have a lower carbon footprint than petroleum-based plastics used for the same applications.[12]

Cellulose-based plastics[edit]

Cellulose bioplastics are mainly the cellulose esters, (including cellulose acetate and nitrocellulose) and their derivatives, including celluloid.

Protein-based plastics[edit]

Bioplastics can be made from proteins from different sources. For example, wheat gluten and casein show promising properties as a raw material for different biodegradable polymers.[13]

Some aliphatic polyesters[edit]

The aliphatic biopolyesters are mainly polyhydroxyalkanoates (PHAs) like the poly-3-hydroxybutyrate (PHB), polyhydroxyvalerate (PHV) and polyhydroxyhexanoate (PHH).

Polylactic acid (PLA)[edit]

Polylactic acid (PLA) is a transparent plastic produced from corn[14] or dextrose. Superficially, it is similar to conventional petrochemical-based mass plastics like PS. It has the distinct advantage of degrading to nontoxic products. Unfortunately it exhibits inferior impact strength, thermal robustness, and barrier properties (blocking air transport across the membrane).[2] PLA and PLA blends generally come in the form of granulates with various properties, and are used in the plastic processing industry for the production of films, fibers, plastic containers, cups and bottles. PLA is also the most common type of plastic filament used for home fused deposition modeling.

Poly-3-hydroxybutyrate (PHB)[edit]

The biopolymer poly-3-hydroxybutyrate (PHB) is a polyester produced by certain bacteria processing glucose, corn starch[15] or wastewater.[16] Its characteristics are similar to those of the petroplastic polypropylene. PHB production is increasing. The South Americansugar industry, for example, has decided to expand PHB production to an industrial scale. PHB is distinguished primarily by its physical characteristics. It can be processed into a transparent film with a melting point higher than 130 degrees Celsius, and is biodegradable without residue.

Polyhydroxyalkanoates (PHA)[edit]

Polyhydroxyalkanoates are linear polyesters produced in nature by bacterialfermentation of sugar or lipids. They are produced by the bacteria to store carbon and energy. In industrial production, the polyester is extracted and purified from the bacteria by optimizing the conditions for the fermentation of sugar. More than 150 different monomers can be combined within this family to give materials with extremely different properties. PHA is more ductile and less elastic than other plastics, and it is also biodegradable. These plastics are being widely used in the medical industry.

Polyamide 11 (PA 11)[edit]

PA 11 is a biopolymer derived from natural oil. It is also known under the tradename Rilsan B, commercialized by Arkema. PA 11 belongs to the technical polymers family and is not biodegradable. Its properties are similar to those of PA 12, although emissions of greenhouse gases and consumption of nonrenewable resources are reduced during its production. Its thermal resistance is also superior to that of PA 12. It is used in high-performance applications like automotive fuel lines, pneumatic airbrake tubing, electrical cable antitermite sheathing, flexible oil and gas pipes, control fluid umbilicals, sports shoes, electronic device components, and catheters.

A similar plastic is Polyamide 410 (PA 410), derived 70% from castor oil, under the trade name EcoPaXX, commercialized by DSM.[17] PA 410 is a high-performance polyamide that combines the benefits of a high melting point (approx. 250 °C), low moisture absorption and excellent resistance to various chemical substances.

Bio-derived polyethylene[edit]

Main article: Renewable Polyethylene

The basic building block (monomer) of polyethylene is ethylene. Ethylene is chemically similar to, and can be derived from ethanol, which can be produced by fermentation of agricultural feedstocks such as sugar cane or corn. Bio-derived polyethylene is chemically and physically identical to traditional polyethylene – it does not biodegrade but can be recycled. The Brazilian chemicals group Braskem claims that using its method of producing polyethylene from sugar cane ethanol captures (removes from the environment) 2.15 tonnes of CO2 per tonne of Green Polyethylene produced.

Genetically modified bioplastics[edit]

Genetic modification (GM) is also a challenge for the bioplastics industry. None of the currently available bioplastics – which can be considered first generation products – require the use of GM crops, although GM corn is the standard feedstock.

Looking further ahead, some of the second generation bioplastics manufacturing technologies under development employ the "plant factory" model, using genetically modified crops or genetically modified bacteria to optimise efficiency.

Polyhydroxyurethanes (PHU)[edit]

Recently, there have been a large emphasis on producing biobased and isocyanate-free polyurethanes. One such example utilizes a spontaneous reaction between polyamines and cyclic carbonates to produce polyhydroxurethanes.[18] Unlike traditional cross-linked polyurethanes, cross-linked polyhydroxyurethanes have been shown to be capable of recycling and reprocessing through dynamic transcarbamoylation reactions.[19]

Lipid derived polymers[edit]

A number bioplastic classes have been synthesized from plant and animal derived fats and oils.[20]Polyurethanes,[21][22]polyesters,[23]epoxy resins[24] and a number of other types of polymers have been developed with comparable properties to crude oil based materials. The recent development of olefin metathesis has opened a wide variety of feedstocks to economical conversion into biomonomers and polymers.[25] With the growing production of traditional vegetable oils as well as low cost microalgae derived oils,[26] there is huge potential for growth in this area.

Environmental impact[edit]

The environmental impact of bioplastics is often debated, as there are many different metrics for "greenness" (e.g., water use, energy use, deforestation, biodegradation, etc.) and tradeoffs often exist.[27] The debate is also complicated by the fact that many different types of bioplastics exist, each with different environmental strengths and weaknesses, so not all bioplastics can be treated as equal.

The production and use of bioplastics is sometimes regarded as a more sustainable activity when compared with plastic production from petroleum (petroplastic), because it requires less fossil fuel for its production and also introduces fewer, net-new greenhouse emissions if it biodegrades. The use of bioplastics can also result in less hazardous waste than oil-derived plastics, which remain solid for hundreds of years.

Petroleum is often still used as a source of materials and energy in the production of bioplastic. Petroleum is required to power farm machinery, to irrigate crops, to produce fertilisers and pesticides, to transport crops and crop products to processing plants, to process raw materials, and ultimately to produce the bioplastic. However, it is possible to produce bioplastic using renewable energy sources and avoid the use of petroleum.

Italian bioplastic manufacturer Novamont[28] states in its own environmental audit that producing one kilogram of its starch-based product uses 500 g of petroleum and consumes almost 80% of the energy required to produce a traditional polyethylene polymer. Environmental data from NatureWorks, the only commercial manufacturer of PLA (polylactic acid) bioplastic, says that making its plastic material delivers a fossil fuel saving of between 25 and 68 per cent compared with polyethylene, in part due to its purchasing of renewable energy certificates for its manufacturing plant.

A detailed study examining the process of manufacturing a number of common packaging items from traditional plastics and polylactic acid carried out by Franklin Associates and published by the Athena Institute shows that using bioplastic has a lower environmental impact for some products, and a higher environmental impact for others.[29] This study, however, does not factor in the end-of-life environmental impact of these products, including possible methane emissions from landfills due to biodegradable plastics.

While production of most bioplastics results in reduced carbon dioxide emissions compared to traditional alternatives, there is concern that the creation of a global bioeconomy required to produce bioplastic in large quantities could contribute to an accelerated rate of deforestation and soil erosion, and could adversely affect water supplies. Careful management of a global bioeconomy would be required.

Other studies showed that bioplastics result in a 42% reduction in carbon footprint.[30]

On October 21, 2010, a group of scientists reported that corn-based plastic ranked higher in environmental defects than the main products it replaces, such as HDPE, LDPE and PP. In the study, the production of corn-based plastics created more acidification, carcinogens, ecotoxicity, eutrophication, ozone depletion, respiratory effects and smog than the synthetic-based plastics they replaced.[31] However the study also concluded that biopolymers trumped the other plastics for biodegradability, low toxicity, and use of renewable resources.

The American Carbon Registry has also released reports of nitrous oxide caused from corn growing which is 310 times more potent than CO2. Pesticides are also used in growing corn-based plastic.[32]


Further information: Biodegradable plastic

The terminology used in the bioplastics sector is sometimes misleading. Most in the industry use the term bioplastic to mean a plastic produced from a biological source. All (bio- and petroleum-based) plastics are technically biodegradable, meaning they can be degraded by microbes under suitable conditions. However, many degrade so slowly that they are considered non-biodegradable. Some petrochemical-based plastics are considered biodegradable, and may be used as an additive to improve the performance of commercial bioplastics.[33] Non-biodegradable bioplastics are referred to as durable. The biodegradability of bioplastics depends on temperature, polymer stability, and available oxygen content. The European standard EN 13432, published by the International Organization for Standardization, defines how quickly and to what extent a plastic must be degraded under the tightly controlled and aggressive conditions (at or above 140 °F (60 °C)) of an industrial composting unit for it to be considered biodegradable. This standard is recognized in many countries, including all of Europe, Japan and the US. However, it applies only to industrial composting units and does not set out a standard for home composting. Most bioplastics (e.g. PH) only biodegrade quickly in industrial composting units. These materials do not biodegrade quickly in ordinary compost piles or in the soil/water. Starch-based bioplastics are an exception, and will biodegrade in normal composting conditions.[34]

The term "biodegradable plastic" has also been used by producers of specially modified petrochemical-based plastics that appear to biodegrade.[35] Biodegradable plastic bag manufacturers that have misrepresented their product's biodegradability may now face legal action in the US state of California for the misleading use of the terms biodegradable or compostable.[36] Traditional plastics such as polyethylene are degraded by ultra-violet (UV) light and oxygen. To prevent this, process manufacturers add stabilising chemicals. However, with the addition of a degradation initiator to the plastic, it is possible to achieve a controlled UV/oxidation disintegration process. This type of plastic may be referred to as degradable plastic or oxy-degradable plastic or photodegradable plastic because the process is not initiated by microbial action. While some degradable plastics manufacturers argue that degraded plastic residue will be attacked by microbes, these degradable materials do not meet the requirements of the EN13432 commercial composting standard. The bioplastics industry has widely criticized oxo-biodegradable plastics, which the industry association says do not meet its requirements. Oxo-biodegradable plastics – known as "oxos" – are conventional petroleum-based products with some additives that initiate degradation. The ASTM standard for oxo-biodegradables is called the Standard Guide for Exposing and Testing Plastics that Degrade in the Environment by a Combination of Oxidation and Biodegradation (ASTM 6954).[37] Both EN 13432 and ASTM 6400 are specifically designed for PLA and Starch based products and should not be used as a guide for oxos.


Because of the fragmentation in the market and ambiguous definitions it is difficult to describe the total market size for bioplastics, but estimates put global production capacity at 327,000 tonnes.[38] In contrast, global production of polyethylene (PE) and polypropylene (PP), the world’s leading petrochemical derived polyolefins, was estimated at over 150 million tonnes in 2015.[39]

COPA (Committee of Agricultural Organisation in the European Union) and COGEGA (General Committee for the Agricultural Cooperation in the European Union) have made an assessment of the potential of bioplastics in different sectors of the European economy:

Catering products: 450,000 tonnes per year
Organic waste bags: 100,000 tonnes per year
Biodegradable mulch foils: 130,000 tonnes per year
Biodegradable foils for diapers 80,000 tonnes per year
Diapers, 100% biodegradable: 240,000 tonnes per year
Foil packaging: 400,000 tonnes per year
Vegetable packaging: 400,000 tonnes per year
Tyre components: 200,000 tonnes per year
Total: 2,000,000 tonnes per year

In the years 2000 to 2008, worldwide consumption of biodegradable plastics based on starch, sugar, and cellulose – so far the three most important raw materials – has increased by 600%.[40] The NNFCC predicted global annual capacity would grow more than six-fold to 2.1 million tonnes by 2013.[38] BCC Research forecasts the global market for biodegradable polymers to grow at a compound average growth rate of more than 17 percent through 2012. Even so, bioplastics will encompass a small niche of the overall plastic market, which is forecast to reach 500 billion pounds (220 million tonnes) globally by 2010.[41]Ceresana forecasts the world market for bioplastics to reach 5.8 billion US dollars in 2020 - i.e. three times more than in 2014.[42]


At one time bioplastics were too expensive for consideration as a replacement for petroleum-based plastics. However, the lower temperatures needed to process bioplastics and the more stable supply of biomass combined with the increasing cost of crude oil make bioplastics' prices[43] more competitive with regular plastics.

Research and development[edit]

Further information: List of bioplastic producers

  • In the early 1950s, amylomaize (>50% amylose content corn) was successfully bred and commercial bioplastics applications started to be explored.
  • In 2004, NEC developed a flame retardant plastic, polylactic acid, without using toxic chemicals such as halogens and phosphorus compounds.[44]
  • In 2005, Fujitsu became one of the first technology companies to make personal computer cases from bioplastics, which are featured in their FMV-BIBLO NB80K line. Later, the French company Ashelvea (also listed on EU Energy Star registered partners), launched its fully recyclable PC with biodegradable plastic case "Evolutis", reported in "People Inspiring Philips", a series of 3 mini-documentaries to inspire Philips employees with some examples from the civil society.[45][46]
  • In 2007 Braskem of Brazil announced it had developed a route to manufacture high-density polyethylene (HDPE) using ethylene derived from sugar cane.
  • In 2008, a University of Warwick team created a soap-free emulsion polymerization process which makes colloid particles of polymer dispersed in water, and in a one step process adds nanometre sized silica-based particles to the mix. The newly developed technology might be most applicable to multi-layered biodegradable packaging, which could gain more robustness and water barrier characteristics through the addition of a nano-particle coating.[47]

Testing procedures[edit]

Industrial compostability – EN 13432, ASTM D6400[edit]

The EN 13432 industrial standard is arguably the most international in scope. This standard must be met in order to claim that a plastic product is compostable in the European marketplace. In summary, it requires biodegradation of 90% of the materials in a lab within 90 days. The ASTM 6400 standard is the regulatory framework for the United States and sets a less stringent threshold of 60% biodegradation within 180 days for non-homopolymers, and 90% biodegradation of homopolymers within industrial composting conditions (temperatures at or above 140F). Municipal compost facilities do not see above 130F.[citation needed]

Many starch-based plastics, PLA-based plastics and certain aliphatic-aromatic co-polyester compounds, such as succinates and adipates, have obtained these certificates. Additive-based bioplastics sold as photodegradable or Oxo Biodegradable do not comply with these standards in their current form.

Compostability – ASTM D6002[edit]

The ASTM D 6002 method for determining the compostability of a plastic defined the word compostable as follows:

that which is capable of undergoing biological decomposition in a compost site such that the material is not visually distinguishable and breaks down into carbon dioxide, water, inorganic compounds and biomass at a rate consistent with known compostable materials.[48]

This definition drew much criticism because, contrary to the way the word is traditionally defined, it completely divorces the process of "composting" from the necessity of it leading to humus/compost as the end product. The only criterion this standard does describe is that a compostable plastic must look to be going away as fast as something else one has already established to be compostable under the traditional definition.

Withdrawal of ASTM D 6002[edit]

In January 2011, the ASTM withdrew standard ASTM D 6002, which had provided plastic manufacturers with the legal credibility to label a plastic as compostable. Its description is as follows:

This guide covered suggested criteria, procedures, and a general approach to establish the compostability of environmentally degradable plastics.[49]

The ASTM has yet to replace this standard.

Biobased – ASTM D6866[edit]

The ASTM D6866 method has been developed to certify the biologically derived content of bioplastics. Cosmic rays colliding with the atmosphere mean that some of the carbon is the radioactive isotope carbon-14. CO2 from the atmosphere is used by plants in photosynthesis, so new plant material will contain both carbon-14 and carbon-12. Under the right conditions, and over geological timescales, the remains of living organisms can be transformed into fossil fuels. After ~100,000 years all the carbon-14 present in the original organic material will have undergone radioactive decay leaving only carbon-12. A product made from biomass will have a relatively high level of carbon-14, while a product made from petrochemicals will have no carbon-14. The percentage of renewable carbon in a material (solid or liquid) can be measured with an accelerator mass spectrometer.[50][51]

There is an important difference between biodegradability and biobased content. A bioplastic such as high-density polyethylene (HDPE)[52] can be 100% biobased (i.e. contain 100% renewable carbon), yet be non-biodegradable. These bioplastics such as HDPE nonetheless play an important role in greenhouse gas abatement, particularly when they are combusted for energy production. The biobased component of these bioplastics is considered carbon-neutral since their origin is from biomass.

Anaerobic biodegradability – ASTM D5511-02 and ASTM D5526[edit]

The ASTM D5511-12 and ASTM D5526-12 are testing methods that comply with international standards such as the ISO DIS 15985 for the biodegradability of plastic.

See also[edit]


  1. ^Hong Chua; Peter H. F. Yu & Chee K. Ma (March 1999). "Accumulation of biopolymers in activated sludge biomass". Applied Biochemistry and Biotechnology. Humana Press Inc. 78: 389–399. doi:10.1385/ABAB:78:1-3:389. ISSN 0273-2289. Retrieved 2009-11-24. 
  2. ^ abAndreas Künkel, Johannes Becker, Lars Börger, Jens Hamprecht, Sebastian Koltzenburg, Robert Loos, Michael Bernhard Schick, Katharina Schlegel, Carsten Sinkel, Gabriel Skupin and Motonori Yamamoto (2016). "Polymers, Biodegradable". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.n21_n01.pub2. 
  3. ^Vert, Michel (2012). "Terminology for biorelated polymers and applications (IUPAC Recommendations 2012)"(PDF). Pure and Applied Chemistry. 84 (2): 377–410. doi:10.1351/PAC-REC-10-12-04. 
  4. ^Chen, G.; Patel, M. (2012). "Plastics derived from biological sources: Present and future: P technical and environmental review". Chemical Reviews. 112 (4): 2082–2099. doi:10.1021/cr200162d. 
  5. ^Suszkiw, Jan (December 2005). "Electroactive Bioplastics Flex Their Industrial Muscle". News & Events. USDA Agricultural Research Service. Retrieved 2011-11-28. 
  6. ^Khwaldia, Khaoula; Elmira Arab-Tehrany; Stephane Desobry (2010). "Biopolymer Coatings on Paper Packaging Materials". Comprehensive Reviews in Food Science and Food Safety. 9 (1): 82–91. doi:10.1111/j.1541-4337.2009.00095.x. Retrieved 9 Mar 2015. 
  7. ^Make Potato Plastic!. Instructables.com (2007-07-26). Retrieved on 2011-08-14.
  8. ^Khalid, Saud; Yu, Long; Meng, Linghan; Liu, Hongsheng; Ali, Amjad; Chen, Ling (2017). "Poly(lactic acid)/starch composites: Effect of microstructure and morphology of starch granules on performance". Journal of Applied Polymer Science. 134: n/a–n/a. doi:10.1002/app.45504. Retrieved 25 July 2017. 
  9. ^"Starch based Bioplastic Manufacturers and Suppliers". Archived from the original on August 14, 2011. 
  10. ^"Enhancing biopolymers: additives are needed for toughness, heat resistance & processability.(biopolymers)(Cover story)". Archived from the original on 2012-07-13. 
  11. ^"BASF announces major bioplastics production expansion". 
  12. ^"Roquette, nouvel acteur sur le marché des plastiques, lance GAÏALENE® : une gamme innovante de plastique végétal". 
  13. ^Song, J. H.; Murphy, R. J.; Narayan, R.; Davies, G. B. H. (2009-07-27). "Biodegradable and compostable alternatives to conventional plastics". Philosophical Transactions of the Royal Society B: Biological Sciences. 364 (1526): 2127–2139. doi:10.1098/rstb.2008.0289. ISSN 0962-8436. PMC 2873018. PMID 19528060. 
  14. ^"History, Travel, Arts, Science, People, Places - Smithsonian". smithsonianmag.com. 
  15. ^"Mirel: PHAs grades for Rigid Sheet and Thermoforming". 
  16. ^"Micromidas is using carefully constructed populations of bacteria to convert organic waste into bio-degradable plastics". Archived from the original on October 23, 2011. 
  17. ^"Home". dsm.com. 
  18. ^Nohra, Bassam; Laure Candy; Jean-Francois Blanco; Celine Guerin; Yann Raoul; Zephirin Mouloungui (2013). "From Petrochemical Polyurethanes to Biobased Polyhydroxyurethanes". Macromolecules. 46: 3771–3792. doi:10.1021/ma400197c. 
  19. ^Fortman, David J.; Jacob P. Brutman; Christopher J. Cramer; Marc A. Hillmyer; William R. Dichtel (2015). "Mechanically Activated, Catalyst-Free Polyhydroxyurethane Vitrimers". Journal of the American Chemical Society. 137: 14019–14022. doi:10.1021/jacs.5b08084. PMID 26495769. 
  20. ^Meier, Michael A. R.; Metzger, Jürgen O.; Schubert, Ulrich S. (2007-10-02). "Plant oil renewable resources as green alternatives in polymer science". Chemical Society Reviews. 36 (11): 1788. doi:10.1039/b703294c. ISSN 1460-4744. 
  21. ^Floros, Michael; Hojabri, Leila; Abraham, Eldho; Jose, Jesmy; Thomas, Sabu; Pothan, Laly; Leao, Alcides Lopes; Narine, Suresh (2012). "Enhancement of thermal stability, strength and extensibility of lipid-based polyurethanes with cellulose-based nanofibers". Polymer Degradation and Stability. 97 (10): 1970–1978. doi:10.1016/j.polymdegradstab.2012.02.016. 
  22. ^Pillai, Prasanth K. S.; Floros, Michael C.; Narine, Suresh S. (2017-07-03). "Elastomers from Renewable Metathesized Palm Oil Polyols". ACS Sustainable Chemistry & Engineering. 5 (7): 5793–5799. doi
Plastics packaging made from bioplastics and other biodegradable plastics
Jar made of PLA-blend bio-flex, a bioplastic
Packaging air pillow made of PLA-blend bio-flex
Tea bags made of polylactide (PLA), (peppermint tea)
Bioplastics Development Center - University of Massachusetts Lowell
A pen made with bioplastics (Polylactide, PLA)
A bioplastic shampoo bottle made of PLA-blend bio-flex

2.1.1. Saccharophagus Degradans

Saccharophagus degradans, formerly known as Microbulbifer degradans, refers to a species of marine bacteria capable of degrading complex marine polysaccharides, such as cellulose of algal origin and higher plant material [11,12,13]. So far, the only strain reported is S. degradans 2-40, that was isolated from decaying marsh cord grass Spartina alterniflora, in the Chesapeake Bay watershed [14]. It is a Gram-negative, aerobic, rod-shaped, and motile γ-proteobacterium and is able to use a variety of different complex polysaccharides as its sole carbon and energy source, including agar, alginate, cellulose, chitin, β-glucan, laminarin, pectin, pullulan, starch, and xylan [11,13,15,16,17].

The key enzymes involved in PHA biosynthesis—β-ketothiolase, acetoacetyl-CoA reductase, and PHA synthase—have been identified in the genome of S. degradans 2-40 [12,18]. Preliminary studies have been performed in order to evaluate the feasibility of S. degradans to produce PHAs from d-glucose and d-cellobiose as the sole carbon source in minimal media comprised of sea salts [19]. In addition, the authors evaluated the capability of the strain to degrade lignocellulosic material in the form of tequila bagasse (A. tequilana). According to the results obtained, it was shown that S. degradans successfully degraded and utilized cellulose as the primary carbon source to grow and produce PHB. However, PHB yields were not reported, so as to evaluate the efficiency of the process, but it became evident that prior hydrolysis of the lignocellulosic material is not required. This is considered positive since it can contribute to up-stream processing cost reduction and thus encourage further research employing the certain strain. In another study, Gonzalez-Garcia et al. [20], investigated PHA production using glucose as the sole carbon source and a culture medium designed according to bacterial biomass and seawater composition. Experiments were performed using a two-step batch strategy, where in the first step bacterial growth was performed under balanced conditions for 24 h, whereas in the second step cells were aseptically transferred to a fresh nitrogen deficient medium and incubated for 48 h. Under these conditions PHA content reached up to 17.2 ± 2.7% of the cell dry weight (CDW).

PHB biosynthesis from raw starch in fed-batch mode was also investigated and the results were compared to the ones obtained using glucose as the carbon source under the same conditions [21]. When starch was used PHA yield, content, and productivity reached up to 0.14 ± 0.02 g/g, 17.5 ± 2.7% of CDW and 0.06 ± 0.01 g/L h, respectively. In the case where glucose was fed the respective values were higher but still low compared to other PHA producers. However, only a few microorganisms have been reported to directly utilize raw starch for PHA production [21,22]. During the experiments, the authors observed the simultaneous production of organic acids and exopolymers and this was the main reason for the low PHB accumulation. Higher PHA efficiency could be achieved by optimizing cultivation parameters to drive carbon flux towards PHA biosynthesis and also by applying genetic engineering to knock out genes responsible for the production of side products such as exopolymers.

PHA production in aquarium salt medium supplemented with 1% of different types of cellulosic substrates such as α-cellulose, avicel PH101, sigmacell 101, carboxymethyl cellulose (CMC), and cellobiose have also been studied [23]. In flask experiments, PHB production was 11.8, 14.6, 13.7, and 12.8% of the DCW respectively. Fed-batch cultivation strategy resulted in increased PHB contents reaching up to 52.8% and 19.2% of the DCW using glucose and avicel respectively, as carbon sources.

Recently, another approach towards PHA biosynthesis from S. degradans was proposed. During their experiments, Sawant et al. [24] observed that Bacillus cereus (KF801505) was growing together with S. degradans 2-40 as a contaminant and had the ability of producing high amounts of PHAs [25]. In addition, the viability and agar degradation potential of S. degradans increased with the presence of B. cereus. Taking those into account, they further investigated the ability of co-cultures of S. degradans and B. cereus to produce PHAs using 2% w/v agarose and xylan without any prior treatment. PHA contents obtained from agarose and xylan were 19.7% and 34.5% of the DCW respectively when co-cultures were used compared to 18.1% and 22.7% achieved by pure cultures of S. degradans. This study reported for the first time the production of PHAs using agarose. Moreover, according to the results, the highest PHA content from xylan was obtained using a natural isolate. So far, only recombinant Escherichia coli has been reported to produce 1.1% PHA from xylan, which increased to 30.3% and 40.4% upon supplementation of arabinose and xylose, respectively [26].

These unique features of S. degradans open up the possibility to use it as a source of carbohydrases in order to saccharify lignocellulosic materials. Thus, coupled hydrolysis and fermentation is a promising alternative for the production of PHAs using carbon sources that may derive from biomass residues of different origin (Table 2). However, saccharification and coupled PHA production need to be studied in detail in order to understand their potential and find ways to increase the rates of their processes.

2.1.2. Caldimonas Taiwanensis

Caldimonas taiwanensis is a bacterial strain isolated from hot spring water in southern Taiwan in 2004 [27]. Researchers had been searching for thermophilic amylase-producing bacteria since those enzymes are of high industrial importance for the food and pharmaceutical sector. In addition, since starch hydrolysis is known to be faster at relatively high temperatures, thermophilic amylases are usually preferred [28]. Upon morphological and physiological characterization, it was shown that this Gram-negative, aerobic, rod shaped bacteria can form PHB granules.

A few years later, Sheu et al. [22] investigated PHA production from a wide variety of carbon sources. At first cultivation of C. taiwanensis on a three-fold diluted Luria-Bertani medium supplemented with sodium gluconate, fructose, maltose, and glycerol as the sole carbon sources under optimal nitrogen limiting conditions, C/N = 30 was performed. PHB contents reached up to 70, 62, 60, and 52% of the CDW at 55 °C in shake flask experiments. In the next step, fatty acids were tested as sole carbon sources for growth and PHA production. It was observed that the strain did not grow at a temperature between 45 °C or 55 °C while no PHA was formed. Nevertheless, when mixtures of gluconate and valerate were provided bacterial growth was feasible and PHA cellular content reached up to 51% of its CDW. The presence of valerate induced the presence of HV units in the polymer resulting in the production of a PHBV copolymer constituting of 10–95 mol % HV depending on the relative valerate concentration in the mixture. Moreover, mixtures of commercially available starches and valerate were evaluated for PHA production at 50 °C. The carbon source mixture consisted of 1.5% starch and 0.05% valerate. Starch types examined were cassava, corn, potato, sweet potato, and wheat starch. After 32 h of cultivation PHBV copolymer was produced in all cases, composed of approximately 10 mol % HV. PHA contents of 67, 65, 55, 52, and 42% of its CDW were achieved respectively.

Despite the fact that biotechnological process using thermophilic bacteria need to be performed at high temperatures, they reduce the risk of contamination. Another advantage is the fact that thermophiles grow faster compared to mesophiles, therefore less time is needed to achieve maximum PHA accumulation [22]. Moreover, employing C. taiwanensis for PHA production using starch-based raw materials is extremely beneficial, in economic terms, since no prior saccharification is required. On the other hand, as mentioned before, C. taiwanensis


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