Abstract
Economizing microalgal cultivation is a considerable milestone targeted by efforts put into microalgal biorefineries. In light of that, the present study was aimed to explore the potential of using anaerobic liquid digestate (ALD) as culture media to grow microalgae and compared it with three different synthetic media (i.e., N8, BBM, and M8) in terms of biomass yield, fatty acid composition, and nutrient utilization/recovery. Moreover, a mixed culture of wild-type microalgae was employed in this study owing to the ability of mixed cultures to survive extreme conditions, eliminating the risk of losing the culture easily, as it mostly happens with pure cultures. The highest nutrient yield coefficients were achieved when the mixed microalgae culture was cultivated in ALD, where the yield coefficient for nitrogen (YN) and yield coefficient for phosphorus (YP) were 10.7 mg biomass mg-1 N and 98 mg biomass mg-1 P, respectively. The highest lipid content (34%) and the highest concentrations of C16:0 (114 mg L-1) and C18:0 (60.9 mg L-1) were also recorded when the mixed microalgae culture was cultivated in ALD. Furthermore, the polyunsaturated fatty acids (PUFA) content also increased significantly in ALD, a beneficial phenomenon as PUFAs in microalgae allow them to adapt more effectively to extreme conditions. Based on the microbial community analysis performed using the multi-marker metabarcoding approach, Diphylleia rotans, Synechocystis PCC-6803, Cyanobium gracile PCC 6307, and Chlorella sorokiniana were identified as the most abundant species in the ALD growth. Overall, based on the findings of the present study, ALD could be used as a promising cultivation medium for microalgae, offering a process integration approach to combine anaerobic digestion and algae cultivation as an effective way to simultaneously treat the high-strength dark-colored ALD and valorize it into profitable byproducts.
Highlights
Economizing microalgal cultivation is a considerable milestone targeted by efforts put into microalgal biorefineries
The reduced level of Chl-a observed in the synthetic media was due to increased light intensity after adaptation to the dark-colored wastewater
The low growth of wild-type mixed microalgae on synthetic media shows how well microalgae adapted to dark-colored anaerobic liquid digestate (ALD) and were probably inhibited/limited by high illumination and/or trace elements when inoculated in the synthetic media
Summary
Multi-marker metabarcoding approach was applied through 16S rDNA, 18S rDNA, 23S rDNA, and tufA marker analyses of the microbial composition in mixed cultures. PCR amplification and sequence analyses of 16S rDNA, 18S rDNA, 23S rDNA, and tufA regions were performed as described in our previous study (Ermis et al, 2020). Next-generation sequencing was performed in an Illumina MiSeq platform (2×300 paired-end reads). Denoising and generation of amplicon sequence variants (ASVs) were performed using the ‘qiime dada denoise-single’ command. Taxonomic assignments of resulting sequences were done by aligning individual reads against reference databases. The rarified number of reads are 49357, 46094, 52553, and 6757 for 16S rDNA, 18S rDNA, 23S rDNA, and tufA analysis, respectively (Table S1). Principal coordinates analysis (PCoA) plots based on the Bray-Curtis distance matrix were generated to analyze beta diversity (similarity between individual microbial communities) (Fig. S3). Phylogenetic analysis was performed with the command ‘qiime phylogeny align-to-tree-mafft-fasttree’ for each marker analysis. Phylogenetic trees with the most abundant ASVs (minimum total feature frequency of 100) were visualized using the Interactive Tree of Life (iTOL) tool (Letunic and Bork, 2016) (Fig. S4)
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