Characterization of Amphora sp., a newly isolated diatom wild strain, potentially usable for biodiesel production.

Microalgae as feedstock for biofuel production have attracted serious consideration as an important sustainable source of energy. For biodiesel production with microalgae, a series of consecutive processes should be performed as selection of adequate microalgal strains, mass culture, cell harvesting, oil extraction and transesterification. The aim of this study was to investigate the growth and lipid accumulation of a new isolated marine microalgal strain by optimizing culture medium composition and applying different stressful culture conditions. Microalga CTM 20023 was isolated from the evaporating salt-ponds at Sfax, Tunisia, using serial-dilution technique from enriched cultures. Phylogenetic analysis based on SSU rDNA and rbcL-3P sequences attributed this isolate to a new species of the Amphora genus. This wild strain possesses rapid gravity sedimentation of 2.91 m h(-1), suitable for an easy and low-cost biomass harvest. The optimization of the composition of the culture medium through statistical experimental designs improved the specific growth rate of Amphora sp. from 0.149 to 0.262 day(-1) and increased its 15-day culture biomass production from 465 to 2200 mg L(-1) (dw) and its lipid content from 140 to 370 mg g(-1) (dw). Highest biomass productivity of 178 mg L(-1) day(-1) was achieved at the 10th day of culture. Highest lipid content of 530 mg g(-1) (dw) was obtained under phosphorus starvation and 64.34% of these lipids were saturated fatty acids. A first growth stage, in optimized condition, would thus offer the maximum productivity for an algal biomass feed stream, followed by second stressful stage for lipid accumulation, thus suitable for biodiesel production.

Optimisation of the critical medium components for better growth of Picochlorum sp. and the role of stressful environments for higher lipid production.

BACKGROUND: Coastal countries that suffer from a scarcity of water, such as Tunisia, have to cultivate marine microalgae on non-arable land in order to produce feedstock and overcome their demands of nutrition and energy. In this framework, a green microalga, CTM 20019, was isolated, identified as Picochlorum sp. and tested for its lipid production. RESULTS: The dry weight of Picochlorum sp. is composed of 163 g kg(-1) lipids, 225 g kg(-1) total sugars, 440 g kg(-1) proteins and 112 g kg(-1) ash rich in potassium, calcium, iron, magnesium and zinc. Gas chromatography-mass spectrometry analysis showed that the main fatty acids were palmitic acid (29%), linolenic acid (26.5%), linoleic acid (23.5%), hexadecatrienoic acid (11%) and hexadecadienoic acid (8.5%). As it is known that culture conditions greatly influence the composition of microalgae, the experiments were designed to optimise the composition of the medium in order to increase Picochlorum sp. growth from OD680nm = 0.53 to OD680nm = 2.2 and lipid accumulation from 163 g kg(-1) to 190 g kg(-1) . The highest lipid contents of 570 and 585 g kg(-1) were achieved under phosphate starvation and sodium carbonate supplementation, respectively. Under these conditions, the fatty acid profile is dominated by mono-unsaturated and polyunsaturated acids, and is therefore suitable for aqua-culture feeding. However, under high salinity, growth and lipid synthesis are inhibited, and the fatty acids are saturate, and the product is therefore suitable for biodiesel. CONCLUSION: This high lipid content rich in essential fatty acids, omega-6 and omega-3, endorses this wild strain of Picochlorum sp. as a promising feedstock for aqua-culture and human nutrition or for the production of biodiesel. © 2013 Society of Chemical Industry.

Salinity stress increases lipid, secondary metabolites and enzyme activity in Amphora subtropica and Dunaliella sp. for biodiesel production.

Amphora subtropica and Dunaliella sp. isolated from Tunisian biotopes were retained for their high lipid contents. Respective optimized parameters for rapid growth were: pH 9 and 10, light period 21 and 24h and temperature 31 and 34°C, respectively. After optimization, Amphora subtropica growth rate increased from 0.2 to 0.5day(-1) and Dunaliella sp. growth rate increased from 0.38 to 0.7day(-1). Amphora subtropica biomass production, productivity and lipid content increased from 0.3 to 0.7gL(-1)(dw), 69-100mgL(-1)d(-1)(dw) and 150-190gkg(-1)(dw), respectively, and Dunaliella sp. from 0.5 to 1.4gL(-1)(dw), 124-200mgL(-1)d(-1) (dw) and 190-280gkg(-1)(dw), respectively. Often to overcome trade-off between microalgae rapid growth and high lipid content which are often conflicting and very difficult to obtain at the same time, separation in a growth stage and a lipid accumulation stage is obvious. Salinity stress in a single stage of culture was studied. Compared to the optimal concentration of growth, excess or deficiency of NaCl engendered the same cellular responses by implication of oxidative stress systems and reactivation of defense and storage systems. Indeed, increasing salinity from 1M to 2M for Amphora subtropica or decreasing salinity from 3M to 2M for Dunaliella sp. have both increased lipids content from (220 and 280) to (350 and 430)gkg(-1), carotenoids from (1.8 and 2.4) to (2.3 and 3.7)pgcell(-1), TBARS amount from (10.4 and 5.3) to (12.1 and 10.7)nmolmg(-1) proteins and SOD activity from of (46.6 and 61.8) to (71.6 and 79.4)Umg(-1) proteins, respectively. With further improved fatty acids profile, the microalgae strains could be potent candidates for biofuel production.