Alkaline microalgae from Yellowstone National Park: physiological and genomic characterization for biofuel production

Abstract

Alternatives are needed to avoid future economic and environmental impacts from continued exploration, harvesting transport, and combustion of conventional hydrocarbons resulting in a rise of atmospheric CO 2. Microalgae, including diatoms, are eukaryotic photoautotrophs that can utilize inorganic carbon (e.g., CO 2) as a carbon source and sunlight as an energy source, and many microalgae can store carbon and energy in the form of neutral lipids. In addition to accumulating useful precursors for biofuels and chemical feed-stocks, the use of autotrophic microorganisms can further contribute to reduced CO 2 emissions through utilization of atmospheric CO 2. Most microalgal biofuel research has focused on green algae. However, there are good reasons to consider diatoms for biofuel research. Diatoms are responsible for approximately 40% of marine primary productivity, are important in freshwater systems, and are known to assimilate 20% of global CO 2. Identification and implementation of factors that can contribute to rapid growth will minimize inputs and production costs, thus improving algal biofuel viability. Nine green algae strains that were isolated from Witch Creek, Yellowstone National Park, were compared to two culture collection strains (PC-3 and UTEX395) for growth rates, dry cell weights and lipid accumulation. The strains exhibiting the fastest growth rates were WC-5, WC-1 and WC-2b. The culture collection strain was the best biomass producer and WC-5 and UTEX395 were the most productive for lipid. Based on the growth rates and lipid content, the best strains for biodiesel production were WC-1 and WC-5. In addition to the green algae strains, diatom strain, RGd-1 has previously been found to accumulate 30-40% (w/w) triacylglycerol and 70-80% (w/w) fatty acid methyl esters that can be transesterified into biodiesel. The RGd-1 was sequenced via Illumina 2x50 and PacBio RSII reads and genome comparisons revealed that the RGd-1 genome is significantly divergent from other publicly available genome sequences. RGd-1 was found to have nearly complete metabolic pathways for fatty acid elongation using acetyl-CoA in the mitochondrion or malonyl-CoA in the cytoplasm. The ability to switch between two different starting substrates may confer an advantage for fatty acid and neutral lipid biosynthesis. Further, RGd-1 was found to use the glyoxylate shunt as part of its central carbon metabolism. This carbon conservation pathway may potentially explain why RGd-1 is able to produce high concentrations of lipids. Using Illumina R MiSeq sequencing it was possible to obtain thorough community analysis of bacteria associated with RGd-1 in culture. Nine primary taxa were identified and further research will elucidate their roles as potential phycosphere bacteria that may have specific functional roles that contribute to RGd-1 health. With long-range PacBio reads, RGd-1 was found to have a potential bacterial symbiont, Brevundimonas sp.