Evaluation of carbon sequestration ability by Chlorella vulgaris in water with different salinity

Evaluation of carbon sequestration ability by Chlorella vulgaris in water with different salinity
Expanded Abstract
Introduction
Nowadays, cultivation of microalgae in order to reduce CO2 has gained attention all around the world. One of the outstanding features is that photosynthetic efficiency of microalgae is greater than that of terrestrial plants. Carbon dioxide (CO2) has been known as one of the most important greenhouse gases. Global warming is specially caused by Anthropogenic CO2 emissions from fossil fuel utilization especially from coal combustion. Co2 emissions are expected to rise in the coming years because of energy needs increasing in the developing worlds. So many attempts have been made to reduce atmospheric CO2 including chemical absorption, physicochemical adsorption, membrane, cryogenics, chemical looping combustion (CLC) and biotechnology (e.g., terrestrial vegetation or hydroponic algae). Bio fixation via microalgae has been known as a potential and new method for CO2 capture and storage. So one of the most understudied methods is carbon biosequestration whereby autotrophic organisms and plants convert this CO2 into organic carbon through photosynthesis producing large amounts of biomass. Photosynthesis is the original process that created the fixed carbon present in today’s fossil fuels, and microalgae are the origin of these fuels. They are among the fastest growing photosynthetic organisms, using CO2 as their main building blocks. Environmental factors, particularly light, temperature, nutrient status, and salinity, affect photosynthesis and productivity biomass. In order to assess the potential of a microalgae system for directly removing CO2, biomass measurement or growth rate evaluations are necessary. However most studies have focused on culturing microalgae in fresh water according to water quality in most part of Iran, in the present study, we tried to sequestrate CO2 by Chlorella vulgaris under salinity water.
Materials and methods
Pure stock culture of Chlorella Vulgaris was obtained from National Inland Water Aquaculture Institute Bandar-e Anzali, Iran and cultivated in Bold's Basal Medium (BBM). Culture of Chlorella vulgaris were individually cultivated in three (9.3 liter working volume) flat plate reactors(40*40*40) under three different EC; artificial seawater (EC34000 µS/cm) , distilled water (EC 3 µS/cm),natural water in study area (EC 1500 µS/cm).(measured by EC meter, Istek Model 915 PDC). The cultures were maintaining under a 12h dark/light photoperiod with 3500 Lx of illumination for 8 days. The average pH of natural water was 7.83 and hardness of 1000mg/l, the amount of carbonate and bicarbonate were 4 mg/l 339.7 and its electrical conductivity was 1500 µs/cm. The amount of initial dissolved CO2 was 8 ppm .Agitation and aeration were accomplished using air from a compressor with pressure of 0.12 MPa. All tests were taken on laboratory conditions and under ambient temperature so the temperature were 25-31°C with an average temperature 28°C. Direct microscopic cellcount by Thoma haemocytometer was performed in this study using optical microscope. Microalgae dry weight (g /L) was measured by centrifuging 10 ml of each sample at 4500 RPM for 30 minutes and then washed with deionized water. Finally dried at105 °C for 40 minutes. After measuring the dry weight, the amount of biomass productivity (P), growth rate (µ) and carbon biofixation rate (R) would be achieved by using equation 1-3.

Poverall (g /L/d= ((xt-x0)/ (tt- t0) (1)
µ (day-1) = (ln (xt/x0))/ (tt- t0) (2)
Rco2 (gCO2 L-1 d-1) =Cc P (mCO2 mC−1) (3)
Where:
Where xt and x0 were the biomass (g/ L) on days tt and t0, respectively . Cc is the carbon content of the microalgae cell (%, w/w) measured with elemental analyzer; mCO2 is the molecular weight of CO2; and mc is the molecular weight of carbon.
Statistical analysis
In the present study, the raw data were stored in Ms Excel and then the relationship among biomass production, specific growth rate and bio-sequestration rate of CO2 with the different level of salinity were interpreted by Tukey analysis using SPSS (version 17) software
Results and discussion
Nowadays The use of micro-algae cultivation to reduce carbon dioxide has attracted a lot of intrests around the world, the higher rate of photosynthesis in these organisms rather than plants and crops is one of their and distinctive features. In the present study the pure stock culture of Chlorella vulgaris cultivated in Bold's Basal Medium was used. The microalgae were cultivated in 3 treatments and 3 replications containing pure water, artificial sea water and natural for 8 days. Lighting conditions provided for this test were periods of 12 h dark / light with light intensity of 3500 Lux.

The results of maximum concentration of biomass, maximum specific growth rate and maximum CO2 fixation for the cultures under different EC are presented in Table 1.

Table 1- the parameters of productivity, specific growth rate and carbon fixation rate of chlorella vulgaris during 8 days.
Culture Biomass productivity (P)
(g L-1 d-1 ) Specific growth rate (µ)
(d-1) Carbon fixation rate (R)
(gCO2 L-1 d-1 )
1-4 4-6 6-8 1-8 1-4 4-6 6-8 1-8 1-4 4-6 6-8 1-8
Pure water
)EC= 3 µs/cm) 0.090 0.020 0.028 0.057 0.266 0.035 0.045 0.15 0.148 0.033 0.045 0.093
Artificial sea water
)EC= 34000 µs/cm ) 0.033 0.010 0.025 0.025 0.025 0.007 0.018 0.019 0.053 0.016 0.041 0.041
Natural water
)EC=1500 µs/cm ) 0.078 0.050 0.065 0.068 0.182 0.077 0.085 0.131 0.127 0.082 0.107 0.111

As table 1 shows the biomass productivity of Chlorella sp. In pure water, artificial sea water and natural water was 0.057, 0.025 and 0068 g/L/d respectively that presented higher biomass productivity in natural water. Specific growth rate of 3 culture was 0.15, 0.019, and 0.13 (day-1) respectively, which the lowest one belongs to artificial sea water (high salinity). Carbon fixation rate of 0.093, 0.041 and 0.111 g/L/d was observed in pure water, artificial sea water and natural water.

Table 2- Average cell numbers of Chlorella vulgaris per liter counted during 8 days of cultivation

8th day 6th day 4th day 1st day Cell counted
(per Liter)
Culture media
10*107 8*107 7*107 3*107 Pure wter
6*107 5*107 5*107 3*107 Artificial sea water
12*107 9*107 6*107 3*107 Natural water

According to table 2 the most number of cells were counted in natural waters at the end of the eighth day. In the artificial sea water because of the high salinity and inappropriate environment the cell growth and proliferation was slow, so that it shows the ability of carbon sequestration in the environment with high salinity was low.
Tukey statistical results showed that there is significant differences at p≤0.05 between biomass production, growth rate, atmospheric carbon fixation rate, and also the number of cells in different salinity treatments. So amounts of salinity have resulted in different effects on the growth rate and carbon fixation rate.

Conclusion
Due to lack of enough fresh water sources, hot, dry climate of study area and inappropriate plants growing conditions, using microalgae is one the best solutions in order to stabilize greenhouse gases (carbon dioxide) and other applications such as the production of renewable fuels and medicinal uses. According to the results of present study, the maximum biomass and cell growth was observed in natural water able environment for freshwater microalgae, so the carbon sequestration potential of microalgae in the culture medium with high salinity is low. Despite being a fresh water microalgae, chlorella vulgaris presented high growth and fixation rate in natural water (EC=1500), so it could be cultivated in study area (South Khorasan, Birjand).

Keywords: Birjand, Carbon sequestration, Chlorella vulgaris, greenhouse gases, Salinity water