Effects of Fluctuating Environments on the Selection of High Yielding Microalgae

Abstract

Microalgae have the potential of producing biomass with a high content of lipids at high productivities using seawater or saline ground water resources. Microalgal lipids are similar to vegetable oils and suitable for processing to liquid fuels. Engineering Icost analysis studies have concluded that, at a favorable site, microalgae cultivation for fuel production could be economically viable. The major uncertainties involve the microalgae themselves: biomass and lipid productivity and culture stability.The mass culture of microalgae for low cost lipids production requires a detailed understanding of both the physiology of lipid biosynthesis by the selected algal strains and of the factors making these strains competitive in outdoor ponds. Mass culture systems will, because of climatic variability, the day-night cycle, and inherent design factors, exhibit fluctuations in key parameters affecting algal growth, productivity and competitiveness: temperature, pH, po?, pC02' light, and nutrient availability. This project has for ~ts overall objective to quantitate the effects of such fluctuations on culture productivity and species competitiveness. A second objective has been the development of basic information on algal physiology as it relates to lipid productivity.Species competition experiments with several strains of algae isolated by the SERI/DOE Aquatic Species Program, some of which were previously used in outdoor mass cultures, were carried out. In these experiments certain parameters proved to be more important then others in determining species succession: fluctuating p02 was more important than fluctuating temperature, which was more important than fluctuating pH. These results must be extended to other species and conditions to determine how far these findings can be generalized. Future work will concentrate on comparing unialgal to mixed culture experiments and on elucidating the specific environmental factors that impact on species productivity and competitiveness.To help guide this work, a computer model was developed capable of predicting the amplitude and frequency of the environmental parameters (p02' pH, PC02, temperature, and light intensity) over the diurnal cycle throughout the year at any location for which climatic data is available. This model must still be validated with data collected from operating ponds.Using one algal strain, "Nanno Q" (Nannochloropsis sp.), as a test organism we also have investigated strategies for maximizing lipid productivity. Our data suggest a two stage process: the first stage is operated at the highest density permitting maximal productivity and the second stage at a higher light input (lower standing biomass or cell density) and under conditions of nitrogen starvation. Such a two stage system is more productive than a continuous nitrogen limited culture process. Future work in this area should utilize algal strains that have previously been selected for their competitiveness under outdoor conditions.

Algae Grown on Dairy and Municipal Wastewater for Simultaneous Nutrient Removal and Lipid Production for Biofuel Feedstock

“…The lipid content of the VSS was analyzed gravimetrically by a procedure adapted from Bligh and Dyer (1959) by Benemann and Tillett (1987). The method consisted of solvent-based extraction to isolate both polar and nonpolar lipids from cell biomass and water.…”

“…However, nitrogen limitation decreases growth rate, which can lead to decreased overall lipid productivity (Shiffrin and Chisolm 1981). Benemann and Tillett (1987) investigated this problem, but maximizing lipid productivity remains an outstanding problem.…”

Life cycle analyses of CO2, energy, and cost for four different routes of microalgal bioenergy conversion

“…Nutrients in forms of urea (0.15 g/L) and diammonium phosphate (DAP, 0.01 g/L) (Darzins et al, 2010) are used as the nitrogen and phosphorous sources. CO 2 is supplied at a ratio of 2.2 ton/ton algae and assumed to be obtained from a nearby power plant with the concentration of 15 vol.% (Benemann and Tillet, 1987). Water is supplied continuously using recycled and industrial water.…”

“…The concentration of alum coagulant is 0.74 mg/L (Molina et al, 2003) and final biomass concentration and recovery yield were assumed to be 30 g/L and 97% on mass basis, respectively (Benemann and Tillet, 1987). For further dewatering, a self-discharged disc-stack centrifuge with the recovery yield of 85% (Heasman et al, 2000;Molina et al, 2003;Darzins et al, 2010) is used to obtain a final concentration of 150 g/L algal biomass.…”

“…Although a complete recycle of water was implemented in the design, a huge amount of water was needed to compensate for the cultivation of microalgae. It is known that the large area of the raceway pond is prone to high water loss due to evaporation (Benemann and Tillet, 1987;Weismann and Goebel, 1987). In the analysis, it was observed that 5.66 wt.% of the daily pond water was lost due to evaporation.…”