The growth of Scenedesmus sp. attachment on different materials surface
© Chen et al.; licensee BioMed Central Ltd. 2014
Received: 17 April 2014
Accepted: 19 September 2014
Published: 1 October 2014
Microalgae has been concerned as a potential source of biodiesel in the recent years. However, it is costly to harvest microalgae as it is commonly cultured in water and the cells are too small to harvest. In order to reduce the cost of cultivation and harvesting, it is important to improve the biomass productivity of microalgae. Here, we utilized the attachment method to culture microalgae to cut off the cost of culture and harvest.
In this paper, various supporting surface with different hydrophility including polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polysulfone (PS), which are not easy to be degraded in the culture medium, were used for microalgae culture by the attachment method. The results showed that PVDF supporting cloth was suitable for the algae growth, and its average biomass productivity was to 4.0 g/m2/day. Furthermore, a series of PVDF concentrations were tested, and cloth treated with 3% or 5% PVDF solution was better for the algae culture. In addition, Polyvinylpyrrolidone (PVP) with different molecular weight was added to the PVDF solution as porogens to produce rough surface. And the addition of PVP resulted in better growth with 6.0 g/m2/day of average biomass productivity.
This attachment method makes the harvest of microalgae easy and energy-saving, because the microalgae grow on the supporting material and is easy to be scraped. The results indicate that the PVDF-treated cloth is a potential alternative for the microalgae attachment culture.
The world has been confronted with an energy crisis in recent decades, associated with irreversible depletion of traditional fossil fuels. Their use as major form of energy is indeed unsustainable, further to accumulation of greenhouse gases in the atmosphere that brings about global warming . Compared to conventional diesel, biodiesel generally contains a higher level of oxygen and lower levels of sulfur and nitrogen and therefore, less SOx, NOx, CO, benzene and toluene are released upon combustion. ,. Currently, the most widely available form of biodiesel comes from oil crops such as palm, oilseed rape, soybean and microalgae. For all the biodiesel sources, microalgae has attracted interest as a source of high-lipid materials to produce biofuel, since photosynthetic conversion is an efficient and alternative process and avoids any competition with food crops .
However, to date, the commercial viability of producing biodiesel from microalgae is still marginal due to high production cost. This mainly includes cultivation cost, harvesting cost and lipids extraction cost. In order to cut off the cost of biodiesel production, the above three cost must be saved. Firstly, to reduce the cultivation and harvesting cost, it is important to improve the biomass productivity of microalgae.
Microalgae cultivation is typically performed in open ponds or enclosed photo bioreactors in which algal cells are grown in suspension. The biomass concentration in the culture solution is common in the range of 0.1-1 g/L  . This directly causes high harvesting cost. In addition to the above culturing methods, an attachment algal culture system such as an algal turf scrubber (ATS) is another culture configuration in which benthic algae grow on the surface of solid supporter. This culture system has been successfully used for growing filamentous microalgae for removing nutrients from animal wastewater and growing Chlorella sp. on different materials  ,. However, the productivity of Chlorella sp. attachment on supporting surface of polystyrene foam was very low . If this was as the biodiesel feedstock, the productivity should be further improved. Therefore, in this paper, we adopted the attachment algal growth concept to grow Scenedesmus sp. on different supporting surface with the purpose of finding good material to grow microalgae and improve the productivity by attachment culture.
Results and discussion
The growth of Scenedesmus sp. on different supporting materials
The Sw of different supporting materials
The effects of different concentrations of PVDF solution on the growth of Scenedesmus sp.
The effects of addition of PVP porogens with different molecular weight on the growth of Scenedesmus sp.
This paper introduced attachment porous supporting material to cultivate Scenedesmus sp.. And the results showed that PVDF-PVP-8000 was good for the growth of Scenedesmus sp.. In addition, in our lab, other algae such as Spirulina platensis and Tribonema sp. have been successfully cultivated (not published). However, it need fatherly study whether this supporting material or the treatment method was fit for other kinds of microalgae such as diatoms and dinoflagellates that are sensitive to shear stresses. In spite of this, it is important step for the cultivation of microalgae, because most of algae was fixed on the supporting material and only medium was circulated, which was beneficial for the incubation and harvest of microalgae. Therefore, it was suitable for the commercialization because the incubation and harvest was energy-saved. In addition, the culture system was different from open ponds and enclosed photobioreactors including thin-layer photobioreactor which has been used more than 20 years used for commercial production of algal biomass in The Institute of Microbiology, Czech Republic  . This system avoided high energy consumption of separation cells from culture mixture because most of the cells grew on the supporting material and the harvesting step only need scrapping tool, but the microalgae cultured in the other photobioreactors still need centrifugation, ultrafiltration or other harvesting methods to separate cells from the suspension mixture.
In this paper, we adopted the attachment algal growth concept to grow Scenedesmus sp. on different supporting surface with the purpose of finding good material to grow microalgae and improving the productivity. The results showed that cloth treated with 3% or 5% PVDF solution was appropriate material for the algae growth. And the porogen of PVP with lower molecular weight added to the PVDF solution resulted in better growth than that of non-addition group (about 6.0 g/m2/day of average biomass productivity via about 4.0 g/m2/day of average biomass productivity). The biomass productivity is higher than the reported attachment culture (0.26 g/m2/day) , which indicated that PVDF treated cloth is an good attachment material for microorganism culture because of its good attachment and desirable physical properties.
Algae strain and subculture
The used alga was Scenedesmus sp. donated from Arizona State University. This green microalgae was chosen as a test organism because it is fast growing, easy culture and high oil production  ,. Algae cells were maintained in BG11 culture medium(NaNO3 10 ml/L stock solution 15.0 g/L dH2O; K2HPO4 10 ml/L stock solution 10.0 g/L dH2O; MgSO4.7H2O 10 ml/L stock solution 7.5 g/L dH2O; CaCl2.2H2O 10 ml/L stock solution 3.6 g/L dH2O; Citric acid 10 ml/L stock solution 0.6 g/L dH2O; Ferric ammonium citrate 10 ml/L stock solution 0.6 g/L dH2O; EDTANa2 10 ml/L stock solution 0.1 g/L dH2O; Na2CO3 10 ml/L stock solution 2.0 g/L dH2O; H3BO3 1 ml/L stock solution 2.86 g/L; MnCl2.4H2O 1 ml/L stock solution 1.86 g/L; ZnSO4.7H2O 1 ml/L stock solution 0.22 g/L; Na2MoO4.2H2O 1 ml/L stock solution 0.39 g/L; CuSO4.5H2O 1 ml/L stock solution 0.08 g/L; Co(NO3)2.6H2O 1 ml/L stock solution 0.05 g/L) before inoculums on the supporting surface.
Attached algae culture
When the attached algae cells formed a thick “mat” on the surface, it was harvested by a glass blade. After harvesting, the substrate was placed into the growth chamber and the medium in the chamber was replaced with fresh BG11 culture medium. The remaining algal colonies on the surface can serve as inoculums for the next cycle of growth .
In which the 0.0025 represents the area of the supporting materials. “t” represents cultivated time (d).
The preparation of different supporting materials
Different materials were tested as supporting materials for algae attachment, including polyvinylidene fluoride(PVDF), polyacrylonitrile (PAN), polysulfone (PS), which were not easy to degrade in the culture medium, and they have different hydrophilicity.
For this experiment, the casting solution was prepared by dissolving the above materials and polyvinylpyrrolidone (PVP) in N, N-dimethylformamide (DMF). The solution was over-coated on the surface of dried cotton cloth with a roller, and vaporized for 10 minutes at room temperature. Then, the composite materials were placed into de-ionized water for 30 minutes in order to get rid of water-soluble components. Finally, they were dried at 50 °C for 24 hours.
Where W and W0 denote the weights of the treated supporting material with absorbed water and the dry one, respectively. The time duration for swelling was 24 hours.
Scanning electron microscopy (SEM) was used to study the morphology of the surface of the cotton cloth and the surface of treated cotton cloth in order to evaluate the supporting materials. In the measurement, the supporting material was fractured in liquid nitrogen and then the fractured part was coated with a conductive layer of a sputtered gold. The surface of the supporting material was investigated using HITACHI S-4800.
Statistical analysis was carried out using SPSS 11.0 software (SPSS Inc, Chicago, USA). ANOVA was performed to evaluate significance of individual differences with a probability threshold of 0.05, followed by a Post-Hoc Tukey test.
This work was supported by the Project 21206182 of National Natural Science Foundation of China, and the commonweal item of State Oceanic Adminixtration People’s Republic of China(201405038-2).
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