Indigenous Cyanobacteria and Microalgae as a Source of Biofuels

Abstract

cyanobacteria and microalgae are photosynthetic organisms and are well known for their variety of metabolites. Considering this, they have been extensively exploited to get many industrially important compounds. Shifting from fossil fuels to biofuels is the need of today, and there is continuous research around the globe to explore the options for biological fuel production. Cyanobacteria and microalgae are considered as an excellent source of different types of biofuels e.g. biodiesel, biohydrogen and bioethanol etc. The present research deals with the investigation of microalgae and cyanobacteria from different habitats of Pakistan and their potential to produce three types of biofuels i.e., biodiesel, biohydrogen and bioethanol. Total 65 strains were isolated and purified i.e., 32 prokaryotic cyanobacteria (further categorized as 19 filamentous; 13 unicellular forms) and 33 eukaryotic microalgae from different sites. The quantity of total lipids from the aforementioned isolates was measured. The isolates showed up to 70 % of lipids per their dry biomass (Oscillatoria sp. strain SM-CFF-6) in BG11 media supplemented with 0.5 % nitrate. Lipids were subjected to transesterification and amount of biodiesel from the isolates was estimated. Biodiesel production under varying nitrate concentrations was observed. The maximum biodiesel productivity (120 mg L-1 day-1) was observed in the (Chlorococcum sp. strain SM-MFUM-16) strain in nitrate less BG11 media. Ability of selected isolates to produce biodiesel under lab and natural environment was compared. In general, the isolates produced significantly higher biodiesel under natural environment (t-test; p<0.05). The maximum biodiesel productivity shown under natural environment is i.e. 667 mgL-1 day-1 (Leptolyngbya sp. strain SM-CFF-18). Lipid profiling of the biodiesel produced from the isolates was performed by gas chro matography. Out of 14 investigated fatty acids, biodiesel from majority of the isolates. contains major portion of unsaturated fatty acids (C16:1, C18:1, C18:2, C18:3 and C22:1), which is considered as a desirable character. Confocal laser scanning microscope (CLSM) and fluorimetry technique was used for the localization and measurement of intracellular lipid droplets by using two lipophilic dyes i.e., Nile red and BODIPY. Single and multiple lipid droplets of size 8.18 μm3 (Lyngbya sp. strain SM-CFF-8), 1.76 μm3 (Microcystis sp. strain SM-CUF-45) and 7.61 μm3 (Gloeocystis sp. strain SM-MFUM-4) were visualized in the cyanobacterial and microalgae cells. Biodiesel associated functional groups especially ester group in the biodiesel obtained from the isolates was detected by Fourier transform infra-red technique (FTIR) analysis. Biohydrogen production potential of the isolates was investigated under eight different culturing conditions (nitrate effect, dark and light effect, glucose supplementation) to get the maximum hydrogen. Generally, the unicellular eukaryotic microalgae produced signifi cantly higher amount of biohydrogen as compared to other groups. Maximum biohydrogen production by isolates under any tested condition was 300 µmol mg Chl-a-1 h-1, 121 µmol mg Chl-a-1 h-1 and 357 µmol mg Chl-a-1 h-1 by filamentous cyanobacteria (Leptolyngbya sp. strain SM-CFF-17), unicellular cyanobacteria (Synechococcus sp. strain SM-CUF-20) and microalgae (Chlorella sp. strain SM-MFUM-22), respectively. Biohydrogen production in prokaryotic cyanobacteria under dark was significantly higher as compared to H2 produc tion in light (t-test; p<0.05). Isolates were subjected to UV light to get mutants that can have better biohydrogen production potential. UV treatment in Dunaliella sp. strain SM MFUM-41 enhanced 380 % H2 production as compared to the respective untreated strain. Many cyanobacteria and microalgae have potential to directly secrete bioethanol in to the medium. Ethanol production by the isolates was measured under various conditions (nitrate effect, dark and light effect, glucose supplementation). Mostly, the unicellular cyanobacteria produced significantly higher amount of bioethanol as compared to other groups. Maximum bioethanol secretion by isolates under any tested condition was 403 mg L-1, 2,221 mg L-1 and 482 mg L-1 by filamentous cyanobacteria (Leptolyngbya sp. strain SM CFF-18), unicellular cyanobacteria (Synechocystis sp. strain SM-CUF-44) and microalgae SUMMARY xi (Chlorella sp. strain SM-MFUM-22), respectively. Bioethanol production in unicellular cyanobacteria under light was significantly higher as compared to bioethanol production in light (t-test; p<0.05). The efficiency of the UV mutated isolated to produce ethanol was en hanced as compared to the unmutated cultures (maximum up to 69 folds in Leptolyngbya sp. strain SM-CFF-15). Hence, the isolates showed varying ability to produce the three types of tested biofuels under different conditions. Strains; Oscillatoria sp. strains SM-CFF-6 and SM-CFF-11, Cyanothece sp. strain SM-CUF-3, Synechococcus sp. strain SM-CUF-36, Chlorococcum sp. strains SM-MFUM-1 and Chlorococcum sp. strain SM-MFUM-19, Dunaliella sp. strain SM-MFUM-24 and Chlamydomonas sp. strain SM-MFUM-42 were equally efficient and recommended to produce all three biofuels. These strains can be used to produce biofuel of choice by manipulating the conditions.