Isolation and evaluation of autochthonous microalgae strains for biodiesel production and wastewater treatment.

Abstract

The current depletion of global fossil fuel reserves and increasing CO2 emission are generating global climate change concerns hence, the increasing calls for the development of renewable biofuels with low CO2 emission. Microalgae has been touted as the most suitable feedstock of renewable fuels due to a number of reasons including high biomass productivity, fast growth rates, cost-effective cultivation systems and alleviation of food security concerns associated with use of other feedstock for biofuel production. Bioprospecting could yet reveal microalgae with high lipid contents for biodiesel production and novel characteristics such as production of unique bioactive compounds. With an attractive climate and plenty of sunlight all year round, South Africa is in a unique position to take advantage of these organisms to guarantee her future energy needs. This research thus aims to bioprospect for indigenous strains of microalgae in aquatic habitats and wastewater treatment plants in KwaZulu-Natal province of South Africa and evaluate their potential for biofuel production and wastewater remediation. DGGE and T-RFLP fingerprinting techniques were adapted to give an overview of the diversity of microalgae in various aquatic habitats including wastewater treatment plants (WWTP), rivers, estuaries and marine ecosystems in order to identify areas for potential bioprospecting. Ten (10) microalgae species from these aquatic environments were isolated, identified and evaluated for lipid, total carbohydrate and protein accumulation using well defined procedures. The lipid profile, biodiesel characteristics as well as effect of starvation on lipid accumulation were also determined. The biomass production and simultaneous phycoremediation potential of two autochthonous isolates Asterarcys quadricellulare and Neochloris aquatica were also investigated. Microalgae classes; Trebouxiophyceae and Chlorophyceae were observed to be dominant in fresh water habitat, while those belonging to the class Ulvophycea (Oltmannseillopsis spp.) dominated the salt water environment. At the brackish water habitat, microalgae belonging to the classes Trebouxiophyceae (Chlorella minutissima) and Chlorophyceae (M. sturmi) were dominant. A shift in community structure was observed at the maturation ponds of WWTPs and in rivers over time. The growth rates of 10 isolated microalgae ranged from 0.219 ± 0.003 to 0.175 ± 0.023 gL-1day-1 while dry weight ranged from 0.433 ± 0.208 to 1.167 ± 0.153 gL-1. Chlorococcum LM1 showed high accumulation of lipid (11.93 ± 0.76 mg/L). The microalgae isolated in this study accumulated high carbohydrate content ranging from 25 to 61 % of their dry weight while protein content ranged from 1.06 ± 0.1 mg/L to 1.39 ± 0.1 mg/L accounting for 21 to 28% dry weight. Lipid accumulation also varied under nutrient limitation condition. Lipid accumulation was enhanced in some of the isolates such as Chlorococcum sp. LM1 (17.2%) and C. sorokiniana NWS5 (24%) while a decrease was observed in others such as C. minutissima TS9 (16%), N. aquatica Toti4 (3.5%) and Chlorococcum sp. LM2 (6.5%). Response to enhanced lipid accumulation via starvation seem to be unique to each algal strain irrespective of species. The lipid profile consisted mainly of saturated fatty acid such as oleic acid (C18:1), palmitic acid (C16:0) and stearic acid (C18:0) with low amounts of polyunsaturated fatty acids such as linoleic acid (C18:2 n-6). Characteristic of the biodiesel based on the lipid profile reveal a low viscosity and density. The biodiesel was determined to be of good quality with high oxidation stability, low viscosity and conformed to the ASTM guidelines. Low total phosphorus concentration in the wastewater resulted in an unbalanced N:P ratio of 44 at the Northern wastewater treatment works (NWWTW) and 4 at the Umbilo wastewater treatment works (UWWTW). Asterarcys quadricellulare utilized the wastewater for growth and reduced the COD of the wastewater effluent from the UWWTW by 12.4% in contrast to Neochloris aquatica which did not show any growth. Asterarcys quadricellulare was able to utilise the wastewater achieving a growth rate up to 0.18 day−1 in sterilized wastewater from the NWWTW and 0.17 day−1 in the unsterilized wastewater from UWWTW. Asterarcys quadricellulare accumulated high biomass of 460 mg/L compared to 180 mg/L in Neochloris aquatica. Total nitrogen (TN) and Phosphorus (TP) were reduced by 48% and 50% respectively by Asterarcys quadricellulare cultivated in sterile wastewater from NWWTW while, Neochloris reduced the TP by 37% and TN by 29%. At the UWWTP, TP and TN were reduced by 32% and 44% respectively by Asterarcys quadricellulare cultivated in sterile wastewater while 29% and 19% reduction were recorded in Neochloris aquatica. The study showed the diversity and community structure of microalgae in aquatic ecosystems in the study area. Autochthonous microalgae were rich in lipid, carbohydrate and protein and could be applied for biofuel production. Wastewater effluent can be used to generate biomass for biodiesel production while treating wastewater. However, optimization of the N:P ratio and carbon source are necessary to improve remediation and biomass productivity for future commercial scale production.