Biomass and lipid production from marine microalgae using light emitting diodes (LEDs) and two phase culture system by manipulation of different culture conditions

Abstract

There is a rising concern on the utilization of fossil fuel nowadays due to the deleterious consequences they exert on the environment. Since the beginning of the industrial age, the environment has witnessed increasing anthropogenic perturbations which have caused increasing carbon emissions from the combustion of fossil fuel. One of the possible alternatives to fossil fuels gaining much attention is the use of microalgae to capture carbon dioxide, while accumulating biomass rich in lipids and other metabolites. These lipids can be converted into biodiesel or used as feeds for commercial aquaculture. The lipid produced from microalgae is around 20–50% of the cell dry weight, and can be as high as 80% depending on the culture conditions. About 50.000 species of microalgae have been identified but not all have been exploited to ascertain the composition or possible applications. Also, the use of microalgae for the extraction of lipids is still slow in commercialization due to its high production cost. It is therefore necessary to explore all the available species with the potentials to accumulate enough biomass for lipids production within a short period. To achieve this, studies are needed to ascertain the optimal culture conditions practically possible to accumulate these metabolites in high amounts. The first microalgae species we examined was Chlamydomonas hedleyi. Chlamydomonas hedleyi, is a marine green alga from class chlorophyceae. This species was chosen because it belongs to the well-studied genera but very little is known about the species. The effects of nitrate concentration (80, 160, 240, 320mg/L) and the actual light source suitable for stress was determined. Also we determined the effects of LED wavelengths (purple (400 nm), blue (465 nm), green (520 nm), red (625nm) and fluorescent light), photoperiod (12:12-h, 18:6-h, 24:0-h light/dark cycles), light intensity (100, 200, 300, 400, 500, 600 µmol/m2/s) on the species cultured in 2-L flasks. The findings show that, C. hedleyi grows better and accumulate high biomass under red (625nm) light emmitting diodes. LED were prefered to fluoresecent light for the cultivation of C.hedleyi. C. hedleyi accumulates high amounts of lipid when exposed in continous red (625nm) wavelengths under normal conditions. However 18:6-h light/dark cycle produces much higher lipids in the presence of green LED stress. The highest biomass obtained in this study was 1.45 g dcw/L under red LED with 300µmol/m2/s and the highest lipid content was 60.4% (w/w) obtained from samples cultured under red LED with 300µmol/m2/s and exposed to green LED for 2 days. The effects of combined red and blue wavelengths on Isochrysis galbana cultured in a 14-L photobioreactors were studied for the production of biomass, lipids and the determination of fatty acids composition. The microalgae were cultured at 20 ± 1 °C, with an aeration rate of 0.5 vvm (vessel volume per minute) and 100 rpm. Based on previous results, a ratio of 50:50 mixed blue (465 nm) and red (640 nm) wavelength was used. A 12:12-h light/dark cycle was used to investigate the effect of light intensity (100, 200, 300, 400, 500 µmol/m2/s) on the biomass and lipid production. Then, the optimal light intensity to monitor the performance of the microalgae in terms of the biomass and lipid production cultured within three different photoperiod cycles (12:12-h, 18:6-h, 24:0-h) in a 14-L photo bioreactor were evaluated. The findings showed the optimal light intensity was 400 µmol/m2/s giving a maximum cell biomass of 1.05 g dcw/L and total lipid content of 65.2% (w/w) cultured under 12:12-h L/D cycle. The optimal light intensity of 400 µmol/m2/s was applied at different L/D cycles, the maximum cell biomass (1.25 g dcw/L) and lipid content (71.1% w/w) were obtained at 18:6-h L/D cycle. Stearic acid was the main fatty acid ranging from 42.91 (500 µmol/m2/s) to 65.57% w/w (100 µmol/m2/s) and 53.84 (18:6 h) to 65.44 % w/w (24:0 h). The final study was on Nannochloris atomus, where the main aim was to identify other stress options for possible lipid production. The LED wavelength mixing ratio, the photoperiod, single and combined stress were the parameters investigated in this study. The findings show that N. atomus grows better as the proportion of red wavelength increases and produces more lipids under single wavelengths as compared to mixed wavelengths. Cultures treated with 24:0 L/D produced highest biomass of 1.15g dcw/L while those exposed in 20:4 L/D produced highest lipids of 37.3% (w/w). Salt concentrations of 1.41M gives highest lipids production (52.5%w/w) under single stress and under combined stress of green LED and salt concentration of 1.23M in 2 days produces 73.6% (w/w) of lipid content which is the highest in this study. Generally, combined stress induced the production of more lipids than single stress. Polyunsaturated fatty acids dominated the fatty acids composition with linolenic acid (ω-3 fatty acid) as the most dominant fatty acid composition making it more suitable for aquaculture.