Freshwater filamentous green algae (FFGA)
The primary source of cellulose in the world today is woody biomass. Of the 290 million metric tons per year of wood and wood fiber produced in the USA, about half is consumed as bioenergy fuel, 30% as lumber and furniture, and 15% (43.5 million metric tons) as paper and paperboard (Knoshaug et al 2012). Essentially all cellulose fibers are consumed in the paper and paperboard industry.
A second source of cellulose, although just a small fraction of woody biomass in quantity, is non-wood biomass; agricultural residues, bagasse, and purposely grown biomass (e.g., switch grass, miscanthus). Cotton linter is used as fibrous cellulose pulp for conversion into viscose from which rayon is made. Development of cellulosic biofuel industry would greatly increase demand for plant biomass containing cellulose.
A third potential source cellulose is algae. Assuming conservative algal biomass production of 20 dry grams m-2 d-1, 10% cellulose yield and 90 % conversion rate, 3,190 mi2, an algal cultivation area approximately equal to states of Delaware and Rhode Island combined would be needed to supply 25% of 43.5 million metric tons of cellulose fibers currently produced with wood and woody biomass. Assuming higher production of 30 dry grams m-2 d-1, 30% cellulose yield and 95 % conversion rate, 1,346 mi2 would be required (area of Rhode Island is 1,545 mi2). This simplistic analysis indicates that reliance on algae as a source of cellulose fibers has merit. In fact, algal cellulose could be a significant supply of feedstock for cellulosic ethanol plants. Because the cost of producing algae cellulose fibers is estimated to be lower than cellulose fibers from wood and woody biomass (simpler production and harvesting, lower cost of storage and transportation, less energy and chemicals for processing, lower waste disposal cost, potential revenues from non-cellulose fraction), demand for large quantity of algal cellulose may develop.
Our cellulose target for the present is in the application of crystalline cellulose and crystalline nanocellulose. The quantity of crystalline cellulose required is unknown, but it is likely to be a fraction of the 43.5 million metric tons of cellulose fibers that is directed to paper and paperboard. Woody and plant biomass are potential sources of crystalline cellulose and crystalline nanocellulose. For the same reason that algal cellulose fiber is lower in cost to produce than woody and plant based cellulose fibers, the cost of algal crystalline cellulo0se is expected to less than woody and plant based crystalline cellulose.
Among the algae, cellulose is produced by at least some species of glaucophytes, haptophytes, dinoflagellates, stramenopiles (including xanthophyte and brown algae), red algae, and green algae (Graham et al. 2009). Whether derived from woody or plant biomass or algae, cellulose is a chemically homogeneous linear polymer of thousands of glucose molecules linked by β-1,4 bonds, with each glucose tilted 180o toward those on either side. Extensive intermolecular hydrogen bonding between cellulose chains generates crystalline structure, with degree of crystallinity varying among natural celluloses. Natural celluloses occur as mixtures of two crystalline forms, cellulose Iα and cellulose Iβ (Atalla and Vanderhart 1984). Form Iα has a triclinic unit structure that is thermodynamically more stable than form Iβ, which occurs in a metastable monoclinic unit that can be converted to the Iβ form by high-temperature treatment (reviewed by Habibi et al. 2010). Form Iβ is the dominant form in celluloses produced by streptophytes (embryophytic land plants and closely related charophycean green algae). Cellulose Iα is more abundant in the celluloses of bacteria and non-streptophyte algae. These biochemical differences mean that woody and plant and (non-streptophyte) algal celluloses respond differently to chemical and microbial hydrolysis, which affects carbon sequestration and fossilization potential (Graham et al. 2013a) as well as industrial utility of cellulose (Mihranyan 2010, Hoover et al. 2011).
Cellulose is a key precursor for many current and innovative industrial applications (Habibi et al. 2010, Mihranyan 2010, Endler and Persson 2011, Lalia et al 2012), though the industrial utilization of plant cellulose is complicated by the presence of lignin, which is difficult to remove (Vanholme et al. 2013). Algal celluloses are considered of high industrial importance because lignin is typically absent, and because algal celluloses have distinctive physical properties (Mihranyan 2010). FFGA have been proposed as industrial cellulosic feedstock sources because they produce a high proportion of cellulose on a dry weight basis, and are large enough to harvest efficiently (Hoover et al. 2011, Zulkifly et al. 2013). Production of cellulose-rich cell walls comprising 20% or more of dry biomass is an important aspect of the ecology of these filamentous algae because the crystalline cellulose they produce is resistant to microbial degradation and thereby contributes to carbon sequestration (Zulkifly et al. 2013). AlgaXperts’ past work reveals that these filamentous algae grow profusely in high-nutrient (eutrophic) waters such as wastewater effluent, and can be readily harvested to simultaneously improve water quality. Their cellulose is easily extracted for use in industrial applications (Hoover et al. 2011). Phylogeny indicates that FFGA likely have cellulose similar to that of land plants, but not lignified, eliminating expensive lignin removal steps.
Some algal species are better sources of cellulose than others. Based on our knowledge of algae, we have determined that FFGA overall represents the best source of algal cellulose. In addition to benefits of growing FFGA as described above, other reasons for growing FFGA include:
- Cultivation is possible in low cost open ponds.
- Exhibits high resistance to biological contamination and herbivory.
- Can be cultivated as single or mixed species and naturally integrated with periphyton community.
- Cultivation responds to process variables that can be easily engineered.
- Some FFGA yield exceptionally high content of cellulose while still yielding lipids and amorphous carbohydrates.
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