An optimized method for high quality DNA extraction from microalga Prototheca wickerhamii for genome sequencing

BACKGROUND: The complex cell wall structure of algae often precludes efficient extraction of their genetic material. The purpose of this study was to design a next-generation sequencing-suitable DNA isolation method for unicellular, achlorophyllous, yeast-like microalgae of the genus Prototheca, the only known plant pathogens of both humans and animals. The effectiveness of the newly proposed scheme was compared with five other, previously described methods, commonly used for DNA isolation from plants and/or yeasts, available either as laboratory-developed, in-house assays, based on liquid nitrogen grinding or different enzymatic digestion, or as commercially manufactured kits. RESULTS: All five, previously described, isolation assays yielded DNA concentrations lower than those obtained with the new method, averaging 16.15 ± 25.39 vs 74.2 ± 0.56 ng/µL, respectively. The new method was also superior in terms of DNA purity, as measured by A260/A280 (−0.41 ± 4.26 vs 2.02 ± 0.03), and A260/A230 (1.20 ± 1.12 vs 1.97 ± 0.07) ratios. Only the liquid nitrogen-based method yielded DNA of comparable quantity (60.96 ± 0.16 ng/µL) and quality (A260/A280 = 2.08 ± 0.02; A260/A230 = 2.23 ± 0.26). Still, the new method showed higher integrity, which was best illustrated upon electrophoretic analysis. Genomic DNA of Prototheca wickerhamii POL-1 strain isolated with the protocol herein proposed was successfully sequenced on the Illumina MiSeq platform. CONCLUSIONS: A new method for DNA isolation from Prototheca algae is described. The method, whose protocol involves glass beads pulverization and cesium chloride (CsCl) density gradient centrifugation, was demonstrated superior over the other common assays in terms of DNA quantity and quality. The method is also the first to offer the possibility of preparation of DNA template suitable for whole genome sequencing of Prototheca spp.