Statistical evaluation of nutritional components impacting phycocyanin production in Synechocystis SP

MICROBIAL PHYSIOLOGY

Statistical evaluation of nutritional components impacting phycocyanin production in Synechocystis SP.

Devendra V. Deshmukh; Pravin R. Puranik^*

School of Life Sciences, North Maharashtra University, P.B. No. 80, Jalgaon 425 001, MS, India

ABSTRACT

Alkaliphilic cyanobacterial cultures were isolated from Lonar lake (MS, India). Among the set of cultures, Synechocystis sp, was studied for phycocyanin production. A maximum yield was obtained in BG-11 medium at optimized conditions (pH 10 and 16 h light). In order to increase the phycocyanin yield media optimization based on the eight media components a Plackett-Burman design of the 12 experimental trials was used. As per the analysis CaCl_2.2H₂O and Na₂CO₃ have been found to be the most influencing media components at 95% significance. Further the optimum concentrations of these components were estimated following a Box Wilson Central Composite Design (CCD) with four star points and five replicates at the center points for each of two factors was adopted for optimization of these two media components. The results indicated that there was an interlinked influence of CaCl_2.2H₂O and Na₂CO₃ on 98% significance. The maximum yield of phycocyanin (12% of dry wt) could be obtained at 0.058 g/l and 0.115 g/l of CaCl_2.2H₂O and Na₂CO₃, respectively.

Key words:Synechocystis, Lonar lake, Phycocyanin, Plackett-Burman, Central composite design

INTRODUCTION

The alkaline lake of Lonar (Dist. Buldhana, MS, India, lat 19° 58', lag 76° 34') is one few alkaline lakes on the earth. Meteoritic impact around 50-60 thousand years ago formed this crater. This is a unique crater in basaltic rock (9). Craters formed by such hypervelocity impact are considered to be a unique ecological habitat (14). The lake water remains alkaline throughout the year owing to the high content of sodium carbonate. Soda lakes are characterized by the presence of large amount of sodium carbonate (or complexes of these salts) formed by evaporative concentration under particular condition of geology, geography, and climate (7). Soda lakes are the most stable and productive naturally occurring alkaline environments in the world, with pH values greater than 10 (9).

Along with Spirulina, Synechocystis sp., a unicellular cyanobacterium gives the water a dull blue-green color. Cyanobacteria has four basic types of biliproteins i.e.; phycocyanin, allophycocyanin, phycoerythrin and phycoerythrocyanin (16). These chromophores, called bilins, are covalently attached to the cystine residues of the apoproteins. Allophycocyanin and C-phycocyanin have a bilin called as phycocyanobilin, and C-phycoerythrin has a bilin called phycoerythrobilin (15). The absorbance coefficient of phycobiliproteins is markedly higher than those of most commercial dyes (6). This is the main reason that phycobiliproteins are used as highly- fluorescent substance in Fluorescence Activated Cell Sorting (FACS), immunochemistry, detection of reactive oxygen species, and amino acid sequencing (1, 2). Cyanobacterial phycobiliproteins have gained importance in the commercial sector, as they have several applications. The primary potential of these molecules are as natural dyes but a number of investigations have shown their health-promoting properties and broad range of pharmaceutical applications (16). They are used as the colorants in chewing gum, ice sherbets, popsicles, candies, soft drinks, dairy products and cosmetics like lipstick and eyeliners. In addition, phycobiliproteins are widely used in clinical and immunological research laboratories (20).

The viability of product produced from micro-organism depends on cost of production and net yield of the product. Media composition and growth conditions influence culture growth rate and in this case phycocyanin production. In order to avoid conventional optimization, statistical models Fractional Factorial Design (FFD) and Central Composite Design (CCD) have been proved to be effective (8, 22). Here we attempt to increase the production of phycocyanin by Synechocystis sp, isolated from Lonar Lake, using methods of FFD and CCD. Media components affecting phycocyanin production were evaluated by the application of two-level factorial Plackett-Burman design (4, 13).

MATERIALS AND METHODS

Organism and Culture Conditions

The unicellular cyanobacterium Synechocystis sp. used in this study was isolated from the water sample of Lake Lonar. The identification was done as per Desikachary (5). The cyanobacterium was maintained on BG-11 media (11). The pH of the media was adjusted with NaOH at 10.36 (lake water pH). In 250 ml capacity Erlenmeyer flasks 100 ml culture medium was taken and the initial cell density was maintained at 10⁶ cells/ml. The flasks were exposed to a light intensity of 75 µmol photons.m^-2.s^-1 with a light/dark cycle of 16/8 h at 24 ± 2°C. The irradiance was measured under the white fluorescent tube light in the culture racks using a luxmeter.

Extraction of Phycocyanin

After incubation of 12 days the cells were harvested by centrifugation at 5000 × g for 10 min at 5°C (Sorvall, USA, Model RC2). The pellet was washed with distilled water and resuspended in 5 ml of 0.05 M phosphate buffer. The content was sonicated (Sonics and Materials Inc., USA) with 30 Hz frequency at a pulse of 10/5 min. Freezing-thawing of the sonicated content was done at least for two times and cell rupture was confirmed microscopically. The extract was centrifuged at 10000 × g for 10 min at 5°C. The supernatant was collected and the concentration of phycocyanin was measured spectrophotometrically at 615 and 652 nm using following Equation (1) with 0.05 M phosphate buffer as blank (11).

Phycocyanin (PC) mg/ml = {A₆₁₅ - (0.474 × A₆₅₂)}/5.34 (1)

Effect of pH and Light Exposure on Phycocyanin Production

In order to check the effect of pH and light exposure on phycocyanin production two sets of Erlenmeyer flasks containing 100 ml BG-11 medium at varying pH (5 to 12) and light exposure (8L:16D, 12L:12D, 16L:8D and 24L:0D h) were inoculated with Synechocystis sp. The pH of the medium was adjusted using 0.1 M sodium hydroxide and incubated at 24 ± 2°C for 12 days. After incubation phycocyanin content was measured.

Statistical Analysis

A 12-run Plackett-Burman design (13) was used to screen the eight major nutrients in BG-11 growth medium that could influence the production of phycobiliproteins. The level of micronutrients was kept constant in all experiments. A method of least squares was used to analyze the experimental response fit using the following first-order model, Equation (2):

Ŷ = β₀ + β₁X₁ + β₂X₂ + β₃X₃ + β₄X₄ + β₅X₅ + β₆X₆ + β₇X₇ + β₈X₈ (2)

where, Ŷ was the predicted response (phycobiliproteins yield), β₀, β₁, β₂, β₃,β₄, β₅, β₆, β₇ and β₈ were the regression coefficients, and X₁, X₂, X₃, X₄, X₅, X₆, X₇ and X₈ were the coded levels of the independent variables. Based on the first-order model equation obtained by the Plackett–Burman design, a second-order polynomial model i.e. a central composition design (CCD) with five coded levels was performed. The quadratic model for predicting the optimal point was expressed according to the following Equation (3):

Ŷ = β₀ + β₁X₁ + β₂X₂ + β₁₁X₁² + β₁₂X₁X₂ + β₂₂X₂² (3)

where, Ŷ was the predicted response (phycobiliprotein yield), β₀, β₁, β₂, β₁₁, β₁₂ and β₂₂ were the regression coefficients, and X ₁ and X ₂ were the coded levels of the independent variables. All statistical analyses were done using statistical software MINITAB 13.31, USA, Inc.

RESULTS AND DISCUSSION

The alkaliphiles are unique microorganisms, with great potential for microbiology and biotechnological exploitation. In present study cyanobacterial culture, Synechocystis sp. was isolated from the soda lake of Lonar, which is a unique habitat in India, rich source of naturally occurring alkaliphilic microbes. Fig. 1 shows the effect of medium pH on phycocyanin production by Synechocystis sp. Culture could not grow at pH 5 and 6. Maximum phycocyanin production of 14.5 mg/l was obtained at pH 10. The phycocyanin production by Synechocystis sp. was also assessed at varying light exposures (Fig. 2). Light exposure of 16 h was found optimum for phycocyanin production.

Cyanotech is developing micro-algae based technologies and producing phycobiliproteins, currently range in price from US $ 5,000-33,000 g^-1 (16). While Dainippon Ink Corporations have been the market leaders for many years in production of commercial products from algae and cyanobacteria (23). Statistical design is an aid to reach a statistical valid outcome with minimum number of observations (8,10). Response Surface Methodology (RSM) is a sequential procedure with an initial objective of leading the experimenter rapidly and efficiently to the general vicinity of the optimum and since the location of the optimum is unknown prior to running RSM experiments; it makes sense to have a design that provides equal precision of estimation in all directions employed. Based on the eight media components a Plackett-Burman design of the 12 experimental trials was used as in Table 1. Each row represents the 12 different experiments to evaluate their effect on phycocyanin production and each column represents a different variable. Each independent variable was investigated at a high (+1) and a low (-1) level which represents two different nutrient concentrations. The significant levels of each media variable were determined by using t test and regression analysis and are shown in Table 2.

The experimental yields of phycocyanin at varied levels of medium components as per Plackett-Burman design are given in Table 1. The analysis showed that CaCl_2.2H₂O and Na₂CO₃ had a significant influence (95% significant level) on phycocyanin production, while other factors had insignificant influence on phycocyanin production from Synechocystis sp. On the basis of statistical testing and the F-test applied for the analysis of variance (ANOVA) the F-value and p-value were 5.70 and 0.090, respectively. The coefficient of determination (R²) obtained was 0.938 had indicating that the model explained 93.8%.of variability in the response.

Though the Plackett-Burman design pointed out the factors influencing phycocyanin production one cannot determine the exact optimum values of the individual factors. This has necessitated using a second order experimental design. A central composite design (CCD) was applied. A steepest ascent experiment was performed before the second-order polynomial model. The design and the steepest ascent experiment are shown in Table 3. In CCD the centre point could be taken as a level that had shown maximum response in the steepest ascent experiment (21). It can be seen from Table 3 that the maximum phycocyanin production was obtained at 0.052 g/l of CaCl_2.2H₂O and 0.10 g/l of Na₂CO₃. On this basis a Box Wilson CCD (22) with four star points and five replicates at the center points for each of two factors was adopted for optimization of these two media components. Table 4 shows the observed and predicted values of phycocyanin yield obtained using CCD Equation (3).

The real values of significant variables X₄ and X₈ used in the Box Wilson model are shown in Table 5. The significant levels of each variable were determined by t-test (Table 6). The factors, X₄^.X₄ and X₄ were found to be significant with a probability of 99% and 96.3%, respectively. The interaction between X₄ and X₈ had an interlinked influence on phycocyanin production at 98% significance.

The equation for phycocyanin yield for the second order model could be written as:

Ŷ = 23.6 + 1.574 X₄ + 0.564 X₈ – 5.483 X₄^.X₄ + 2.528 X₄^.X₈ – 1.381X₈^.X₈ (4)

Statistical testing of the experimental data was carried out using Fisher's test for analysis of variance (ANOVA) using MINITAB 13.31. The F- and p-values were 17.54 and 0.001, respectively. The quality of the fit of the quadratic regression model equation was expressed by R²_, which indicated 92.6% of the variability of the response could be explained by this model. The value of the Adj R² (87.3%) also indicated the high significance of the model. Thus these results indicated that the response equation provided a suitable model for the CCD experiment. The response surface plot for the CCD used for phycocyanin production is depicted in Fig. 3. It can be seen that maximum yield of phycocyanin (23.8 mg/l, i.e. 12% of cell dry weight) could be obtained at 0.058 g/l and 0.115 g/l of X₄, CaCl_2.2H₂O and X₈, Na₂CO₃, respectively.

In order to confirm the second order model, Synechocystis sp. was grown using optimized media constituents and yielded 22.6 mg/l of phycocyanin. The yield of phycocyanin obtained with Synechocystis sp. isolated from Lonar lake was comparable with earlier report on Synechocystis sp. PCC 6701 (25 mg/l) (8). Nitrate and phosphate were identified as the most significant factors.

The production of phycocyanin not only depends upon the media components but also some operational factors such as light and pH (8). The pH of the medium also influences the production of phycobiliproteins. Gloeotrichia natans, a nitrogen fixing cyanobacterium has been studied as a possible source of phycobiliproteins (3). Under optimal growth conditions (38°C, pH 8.0, with no carbon enrichment), the specific growth rate of the rice-field isolate of Gloeotrichia natans was 0.076 h^-1. The pH of the medium (between 6.5 and 9.0) did not influence the growth rate, but it did affect phycobiliprotein content, as reflected by a change of the color of the cultures. At pH 7.0, the culture was green-brown with phycobiliproteins constituting up to 10% of the total protein, while at pH 9.0 the culture was brownish-black and the content of these pigments was as high as 28% of the total protein. In the current study the optimum pH level for the production of phycocyanin was found to be pH 10 (Fig. 1). Phycocyanin production from cyanobacterial cultures has been reported to be dependent on various factors viz. culture conditions, particularly nitrogen and carbon sources. Supply of 3% carbon dioxide and 97% nitrogen in gaseous form supported 20.1% of C-phycocyanin yield in a laboratory culture of Nostoc sp. (19). A screening process indicated that the content of C-phycocyanin was 17% of dry weight in some strains of Anabaena and Nostoc and 10% of in some Nostoc sp. (12). A commercial process of open-tank mass cultivation of a marine cyanobacterium Phormidium valderianum BDU 30501 was also developed for producing phycocyanin (17). The average pigment yield was 20% on dry weight basis. Optimization of mass cultivation media for the production of biomass and natural colourants from marine cyanobacteria by a mathematical design of experiments were also performed (18). Kathiresan et al. (10) reported effect of major media constituents of Porphyridium spp. studied using response surface methodology (RSM) on biomass yield, total phycobiliprotein and the production of phycoerythrin. The independent variables such as concentration of NaCl, MgSO₄, NaNO₃ and K₂HPO₄ influenced the total phycobiliproteins and phycoerythrin production.

The production of phycobiliproteins by cyanobacteria has been observed as a pH dependent phenomenon (8). The production of phycobiliproteins increases with increase in alkaline conditions. The soda lake habitats often have a combination of Ca²⁺ and carbonate levels (7). The Synechocystis sp. was isolated from Lonar Lake, a saline alkaline condition. The alkaline conditions are mainly favorable for primary producers such as cyanobacteria in the lake.

ACKNOWLEDGEMENTS

The author Devendra Deshmukh is thankful to University Grants Commission, New Delhi for providing Junior Research Fellowship under Research Fellowship in Science for Meritorious Students (RFSMS) Scheme. The financial assistance towards instrumentation facility at School of Life Sciences from UGC, New Delhi under SAP-DRS program and DST, New Delhi under FIST program is duly acknowledged.

Submitted: August 26, 2010; Approved: January 16, 2012.

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Publication Dates

• Publication in this collection
  02 May 2012
• Date of issue
  Mar 2012

History

• Received
  26 Aug 2010
• Accepted
  16 Jan 2012