Use of Marine Microalgae for Biofuels Production: Reduction in Ash Content for Potential Improvements in Downstream Processing

Many species of microalgae have shown potential as feedstocks for production of algal biofuels. Freshwater species have been chosen because of they have demonstrated relatively greater neutral lipid biosynthesis in mass culture. Freshwater has become relatively scarce and there are competing uses making freshwater species less attractive for biofuels applications. Marine microalgae do not require freshwater and can be grown in mass culture on land that is less suitable for other uses. It is also more favorable to isolate local marine species for any mass culture endeavor due to possible accidental release to the environment. Two groups of marine microalgae, diatoms and chlorophytes, have shown great potential for biofuels production. Diatoms often have greater rates of synthesis of biofuel lipids. However, the silica content of diatom frustules can be problematic for downstream processing and lipid extraction. For these reasons, as part of a U.S. Department of Energy Algal Biofuels Consortium, we conducted a study that included pre-screening of 35 strains for biofuel suitability and further testing at a demonstration scale facility in Hawaii. Cultures were grown in f/2 medium with treatments of 100, 75, 50 and 25% of f/2 Si. Some species showed greater biomass with decreased Si. Some species demonstrated enhanced lipid content with lower Si. The best performing 18 species of diatoms and 6 species of chlorophytes were grown at reduced Si content in the medium (for diatoms) or reduced trace metals in the medium (for chlorophytes). Treatments were 100, 50, 25, 12.5 and 0% f/2 Si or f/20 trace metal mix. Five of the diatoms were from culture collections with the others isolated from coastal Hawaiian waters. All of the chlorophytes were isolated from Hawaiian waters. The results showed that ash content of the diatoms was generally <5-10% of DW for diatoms, but that there was no reduction in ash content with reduced trace metals for chlorophytes.