Authors: Dr. Rajni Singh and Neha Sharma
Biopolymers or organic plastics are polymers derived from biological sources unlike fossil-fuel plastics which are being derived from petroleum. These biopolymers are important for sustainable development as they help in conservation of fossil resources and reduction in CO2 emissions. Algal based plastics have been a recent trend in the era of bioplastics compared to conventional plastics. Algae provide the advantage such as high yield and the ability to grow in a range of environments. Algal biomass along with yeast is capable of fermenting to form fermented products (ethanol) and ethanol is further converted to ethylene which forms biopolymer.
Types of algal bioplastics:
Bioplastics are manufactured from biopolymers by using two routes:
- From living organism: these are typically made from cellulose, soy protein and starch.
- Polymerizable Molecules: these are typically made from lactic acid and triglycerides, (renewable natural resources), and can be polymerized to biodegradable plastics.
Hybrid Plastics: These plastics are made by adding denatured algal biomass to petroleum based polymers like polyurethane and polyethylene as fillers. It provides the plastics with very desirable properties including biodegradability. Filamentous green algae of the order Cladophorales are used in the hybrids.
Cellulose-based Plastics: These are cheap and low quality bioplastics derived from cellulose. 30% of the biomass produced after extraction of algal oil is cellulose. Valonia and Cladophora strains are used for the production of cellulose-based bioplastics.
Poly-Lactic Acid (PLA): Lactic acid is produced by fermentation of algal biomass is polymerized to produce polylactic acid. Lactic acid and its polymer poly-lactic acid (PLA) are used as a biodegradable alternative and are economically viable alternatives on a large scale. Majorly brown algae strains are used for PLA production.
Bio-Polyethylene: The monomer (ethylene) used in the production of polyethylene is converted from ethanol produced by bacterial digestion of algal biomass or directly from algae. It is not economically feasible since algae derived ethanol is costlier than petroleum derived ethanol. Normally Oscillatoria and Batrochospermum are used for biopolyethylene production.
Algae biopolymer production:
Cultivation of the algae requires a nutrient medium containing nitrogen and other mineral nutrients and micronutrients, a source of assimilable carbon (Carbon dioxide in the case of the obligate photoautotrophs and organic carbon source in case of photoheterotrophic), illumination with light energy, and favorable conditions of temperature, pH, and salinity.
The cultivation of algae biomass and production of biopolymers are carried out in two stages: first stage in which algae growth is initiated and a second stage in which biopolymer is carried to completion. In the first stage, growth of algae biomass in a culture medium is accomplished in a continuous mode (continuous culture) in which fresh nitrogen-containing nutrient medium is supplied to the culture. A portion of the culture medium is transferred from the first stage to the second stage. The supply of nitrogen is limited in the second stage which shifts the culture to a senescent phase to enhance biopolymer production. Culture withdrawn from the first stage is transferred sequentially to each of the second stage reaction chambers such that biopolymer production occurs in several second stage chambers and the cells are produced in the first stage reaction chamber simultaneously.
Algae biopolymers may be synthesized by microalgae from the divisions Chlorophyta, Cyanophyta, and Rhodophyta. Genera include: Chlorophyta-- Chlorella, Ulva, Chlamydomonas, Scenedesmus, and Stichococcus; Cyanophyta--Anabaena; and species include are: Chlorella stigmataphora, Chlorella vulgaris, Chlorella pyrenoidosa, Chlamydomonas mexicana, Ulva lactuca, Scenedesmus obliquus, Scenedesmus braziliensis, Stichococcus bacillaris, Anabaena flos-aquae, Porphyridium aerugineum, and Porphyridium cruentum .
Various two stages process for the production of algal biopolymers:
- In the first stage, the algae culture (containing sodium nitrate and sodium glycerophosphate) is subjected continuously to artificial illumination under conditions in which certain radiant energy parameters are controlled for a period of time that growth of the alga and synthesis of the biopolymer begin. In the second stage, artificial illumination is terminated and the culture is subjected to diurnal cycles of solar radiation and darkness provided by natural outdoor illumination to continue alga growth and synthesis of the biopolymer.
- Two-stage synthesis procedure in which a nitrogen deficiency is created to enhance biopolymer production. There are two main points at which carbon is removed from the photosynthetic carbon reduction cycle, as 3-phosphoglyceric acid and fructose-6-phosphate. When the nitrogen is sufficient, carbon is removed from the cycle as 3-phosphoglyceric acid which in turn is aminated directly to amino acids or which enters the tricarboxylic acid cycle and the products then transaminated to amino acids. The amino acids are converted to specific proteins which are structural and regulatory in nature and new cells are made. In case of nitrogen deficiency, carbon is removed from the synthetic carbon reduction cycle as fructose-6-phosphate and converted to glucose-6-phosphate which in turn is converted to carbohydrates, in the case of Porphyridium aerugineum--floridean starch and anionic biopolymer.
Growth Conditions of Algal species used in manufacturing biopolymers:
Nutrient medium: Cultivation of algae requires a nutrient medium containing nitrogen and other nutrients, illumination with light energy, optimal temperature, pH and salinity.
Carbon: 2.5-5% Carbon dioxide is required.
Illumination : In the first stage the illumination required is in the range of 2.0 to 2.5 Einsteins per day per liter of culture. In the second stage, the illumination rate is increased to a level within the range of 3.0 to 4.5 Einsteins per day per liter. Porphyridium aerugineum is enhanced by using illumination of energy content predominantly in the region of 600-700 nanometers.
Optimum temperatures: for algae growth normally will fall within the range of 18° to 25° C.
Optimum pH: within the range of 6 to 8
Salinity: It varies from distilled water to a total dissolved salt content of about 3 weight percent for certain species.
Applications of algal biopolymers
The advantages of biopolymers over traditional plastics are numerous. The practical side of the use of biopolymers is the economical advantage for industries and municipal works.
- Thickening agents for mobility control in waterflood oil recovery
- Food additives
- Flocculants useful in waste water treatment
- Soil conditioning
- Drilling mud extenders
- Pet food
- Farm feed stabilizers
- Culture media from specialty crops like orchids
- Brewery fining
References:
• Bassham et al.,(1964).Photosynthesis of Amino Acids,Biochim, Biophys. Acta, 90, 553-562
• Mitsufumi Matsumoto et al ,Ethanol from algae,Department of Biotechnology,Tokyo University of Agriculture and Technology,184-8588.
• Burlew, et al (1978) "Algal Culture from Laboratory to Pilot Plant, Conditions for Growth of Algae”Carnegie Institution of Washington Publication 600, Washington, D.C
About Author / Additional Info:
I am working as Additional Director and Head in Amity Institute of Microbial Biotechnology, Amity University Uttar Pradesh, Noida, India. I have 13 patents, executed 6 different projects and authored, co-authored or presented over 48 scientific papers, articles, book and chapters and received different grants from Govt. organization to present paper at different international platforms.