A pilotscale floating closed culture system for t

Authors: Masakazu Toyoshima Shimpei Aikawa Takahiro Yamagishi Akihiko Kondo Hiroshi Kawai

Publish Date: 2014/12/08

Volume: 27, Issue: 6, Pages: 2191-2202

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Abstract

Microalgae are considered to be efficient bioresources for biofuels and biobased chemicals because they generally have high productivity The filamentous cyanobacterium Arthrospira Spirulina platensis has been widely used for food feed and nutrient supplements and is usually cultivated in open ponds In order to extend the surface area for growing this alga we designed a pilotscale floating closed culture system for cultivating A platensis on open water and compared the growth and quality of the alga harvested at both subtropical and temperate regions The biomass productivity of A platensis NIES39 was ca 9 g dry biomass m−2 day−1 in summer at Awaji Island warm temperature region and ca 10 and 6 g dry biomass m−2 day−1 in autumn and winter respectively at Ishigaki Island subtropical region in Japan If seawater can be used for culture media culture cost can be reduced therefore we examined the influence of seawater salt concentrations on the growth of A platensis NIES39 Growth rates of A platensis NIES39 in diluted seawater with enrichment of 25 g L−1 NaNO3 001 g L−1 FeSO4·7H2O and 008 g L−1 Na2EDTA were considerably lower than SOT medium but the biomass productivity dry weight was comparable to SOT medium This is explained by the heavier cell weight of the alga grown in modified seawater media compared to the alga grown in SOT medium Furthermore A platensis grown in modified seawaterbased medium exhibited selfflocculation and had more loosely coiled trichomesDevelopment of clean and sustainable biofuels has gained significant support owing to global climate change the shortage of energy and petroleum supplies Photosynthetic microorganisms including eukaryotic microalgae and prokaryotic cyanobacteria are considered to be generally more efficient than land plants in converting solar energy and recycling CO2 into fuels Dismukes et al 2008 In order to avoid conflict with the food industry for arable land it is desirable to use sites where agricultural use is difficult eg desert ocean surface etc However in order to achieve largescale commercial uses of photosynthetic microorganisms it is important to reduce the production cost by developing lowcost culture facilities and culture media and efficient harvesting methods Strains of the cyanobacterium Arthrospira Spirulina platensis originally isolated from an African alkaline lake have seen wide commercial use because they grow vigorously in tropical and subtropical climatic conditions Kim et al 2007 Lodi et al 2005 Ogbonda et al 2007 and have high tolerance to alkaline and high salt concentrations Zeng and Vonshak 1998 A platensis has been widely used for foods feeds and nutrient supplements Anupama and Ravindra 2000 Spolaore et al 2006 owing to its high nutritional value Converti et al 2006 Furthermore it has great potential as a source of bioenergy because its photosynthetic storage product ie glycogen is an effective substrate for alcohol fermentation Aikawa et al 2012 2013The wholegenome DNA sequence of A platensis NIES39 has been determined Fujisawa et al 2010 On the basis of this genome sequence gene manipulation through genetic engineering is anticipated to improve growth performance and produce useful bioproductsOpen ponds and raceway methods have been widely used for the mass culture of photosynthetic microorganisms but only a few species can be stably maintained in such systems because diverse contaminating organisms eg algae bacteria fungi protozoa reduce production of the targeted alga To avoid contamination species that can grow under extreme conditions eg high temperature alkaline or acidic conditions high salt concentrations etc that inhibit growth of contaminants have been selected for cultures in open systems A platensis is one of the most widely cultured commercial photosynthetic microorganisms in traditional open systems because of its preference for alkaline conditions Vonshak and Richmond 1988 Lu et al 2011 that do not allow the growth of most contaminantsThe growth and biomass productivity of A platensis depend on several factors such as solar irradiance in the photosynthetically active range PAR pH quantities of contaminants temperature dissolved oxygen concentration salinity amount of agitation or aeration and nutrient availability Colla et al 2004 Ogbonna et al 1995 Vonshak 1997 In the traditional open culture systems of A platensis light intensity temperature and dissolved oxygen concentration have been considered to be most important for determining growth Chaumount 1993 Borowitzka and Moheimani 2013For use of transformed strains of A platensis in largerscale cultures it is essential to establish a closed culture system to avoid the risk of the strains escaping to natural habitats Fully closed photobioreactors also allow better control of the culture conditions than open systems However closed photobioreactors generally cost more than open systems due to higher construction operation and maintenance costs The production cost is especially important when the price of the products is not high as in the case of bioenergy production Also both traditional open systems and fully closed photobioreactors require land for their operation Considering the abovementioned aspects we aimed to establish a lowcost continuous/semicontinuous closed culture system using open water surfacesA platensis is a planktonic multicellular cyanobacterium with helical filamentous form It has relatively large individual cells of 6–16 μm in diameter 30–70 μm in helix diameter 12–72 μm helix pitch Pelosi et al 1982 and strong motility by the rotation of the helical trichome These characters are beneficial in mass cultures because the trichomes tend to remain suspended in the culture media and are rather easy to harvest using fine mesh screens Energy and costs for pretreatment of the harvested algae eg drying cell breakage extraction etc are important elements of the production cost However in A platensis the harvested cells can be used for fermentation processes without pretreatment or enzymatic hydrolysis Aikawa et al 2013 For mass cultures use of the sea surface of subtropical/tropical coasts or open oceans provides high solar irradiances and stable growth temperatures because the temperature range of the tropical surface water 20–30 °C matches the optimum growth range of this species A platensis is usually cultured utilizing a chemically defined medium and a significant portion of the production cost is contributed by these chemicals Hence the use of lowcost media such as seawater media will reduce the production costs In studies using seawater as an alternative medium pretreatment to remove Ca2+ and Mg2+ Faucher et al 1979 Leema et al 2010 and supplementation with specific nutrients have been employed under laboratory conditions Materassi et al 1984 or in outdoor raceways Tredici et al 1986 Wu et al 1993Here we discuss the design and operation of a pilotscale floating closed culture system and the use of lowcost media modified SOT media and modified seawater media for the continuous culture of A platensis using outdoor water surfaces such as the sea and ponds

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