Abstract
Microalgal Chlorella has been demonstrated to process wastewater efficiently from piggery industry, yet optimization through genetic engineering of such a bio-treatment is currently challenging, largely due to the limited data and knowledge in genomics. In this study, we first investigated the differential growth rates among three wastewater-processing Chlorella strains: Chlorella sorokiniana BD09, Chlorella sorokiniana BD08 and Chlorella sp. Dachan, and the previously published Chlorella sorokiniana UTEX 1602, showing us that BD09 maintains the best tolerance in synthetic wastewater. We then performed genome sequencing and analysis, resulting in a high-quality assembly for each genome with scaffold N50 > 2 Mb and genomic completeness ≥91%, as well as genome annotation with 9,668, 10,240, 9,821 high-confidence gene models predicted for BD09, BD08, and Dachan, respectively. Comparative genomics study unravels that metabolic pathways, which are involved in nitrogen and phosphorus assimilation, were enriched in the faster-growing strains. We found that gene structural variation and genomic rearrangement might contribute to differential capabilities in wastewater tolerance among the strains, as indicated by gene copy number variation, domain reshuffling of orthologs involved, as well as a ~1 Mb-length chromosomal inversion we observed in BD08 and Dachan. In addition, we speculated that an associated bacterium, Microbacterium chocolatum, which was identified within Dachan, play a possible role in synergizing nutrient removal. Our three newly sequenced Chlorella genomes provide a fundamental foundation to understand the molecular basis of abiotic stress tolerance in wastewater treatment, which is essential for future genetic engineering and strain improvement.
Highlights
Increasing demand on meat consumption has triggered rapid growth of the piggery industry throughout the world, especially in China[1], which is accompanied by increasing wastewater production and potential environmental problems
In order to comprehensively compare the performance of different Chlorella in response to wastewater with rich carbon, nitrogen and phosphorus compounds in the lab, as well as to explore the underlying molecular mechanism responsible for this capability, we made one-to-one growth rate comparisons among Chlorella strains in synthetic wastewater with various concentrations of carbon, nitrogen and phosphorus
In addition to BD08, BD09 and Dachan, the strain Chlorella sorokiniana UTEX 1602, the draft genome of which was reported recently[9], was included into the growth assay as a control to facilitate further comparative genomic analyses (Fig. 1)
Summary
Increasing demand on meat consumption has triggered rapid growth of the piggery industry throughout the world, especially in China[1], which is accompanied by increasing wastewater production and potential environmental problems. Some Chlorella strains do show good tolerance of certain wastewater environment in nature[5,6], driving the industry to continuously seek better strains for harsher conditions, especially for extremely high nutrient imbalance and salt concentrations, and to satisfy a growing market need for more efficient production of high-value metabolites. No genomic comparison between various Chlorella strains has been done to associate a genomic variation with a target phenotype, such as organic molecule clearance and stress-tolerance. Such a lack of knowledge in the genetics of Chlorella necessitates more efforts to obtain genomic blueprints of desirable Chlorella strains, together with various experiments on wastewater treatment
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