Genome sequencing and variant analysis reveal high-impact mutations in key starch biosynthesis genes in a UV-induced mutant of Desmodesmus sp. with enhanced lipid production

Abstract Random mutagenesis combined with genome-scale analysis represents a powerful approach to uncover metabolic reprogramming in non-model microalgae. In this study we investigated the strain Desmodesmus sp. Petrobras/FURG, a thermotolerant and contamination-resistant microalga with potential for large-scale cultivation. UV mutagenesis followed by iodine-vapor screening yielded the starch-deficient mutant tN-30, which maintained wild-type growth but exhibited 2.3-fold higher neutral lipid fluorescence, 25% higher total lipid content, and 18% fewer carbohydrates. Whole-genome sequencing revealed eleven high-impact mutations affecting starch-related genes, including lesions in UGPase, plastidial PGM, AGPase-LSU, SSI, SSIII, SSIV, SBEII, and starch phosphorylase, while AGPase-SSU, GBSS, and SBEI remained intact. Structural analysis showed that the BT1-like adenylate translocator is truncated and non-functional, and the plastidial PGM mutation blocks the conversion of imported G6P into G1P, disrupting the canonical route to ADP-glucose. The residual starch observed in tN-30 is therefore best explained by a combination of enzyme redundancy and potential alternative transport mechanisms, possibly involving direct G1P import through an as-yet-unidentified plastidial transporter, as proposed in Arabidopsis . These results reveal a distributed attenuation of the starch biosynthetic network in Desmodesmus , where partial enzyme function and alternative precursor routes sustain basal starch formation while redirecting carbon toward lipids and proteins. The tN-30 mutant exemplifies how classical mutagenesis coupled with high-resolution genomics can expose the network-level flexibility that underlies the metabolic resilience of green microalgae.