Physiological–Biochemical Characteristics and Omics Landscape of Drought Tolerance in Chlorella sp. from the Taklamakan Desert of China

Microalgae endemic to desert ecosystems possess exceptional stress-resistance traits; however, the physiological and molecular drought-adaptation mechanisms of desert-dwelling Chlorella remain largely uncharacterized. Herein, we combined physiological–biochemical assays, ultrastructural observation, transcriptomics and metabolomics to dissect drought responses in Taklamakan-desert-origin Chlorella sp. under polyethylene glycol 6000 (PEG-6000)-simulated mild (10%), moderate (20%), and severe (30%) drought stress, with mechanistic focus on moderate-drought conditions. Moderate drought repressed algal biomass (−26.32%), chlorophyll a (−24.40%), maximum photosystem II quantum yield (Fv/Fm, −16.55%) and effective quantum yield of photosystem II (ΦPSII, −24.35%). Malondialdehyde (MDA) accumulated (+50.69%), alongside elevated activities of superoxide dismutase (SOD, +156.74%) and catalase (CAT, +11.61%). Compatible solutes, including proline, soluble sugars, total protein and total lipid, increased significantly by 39.00%, 39.36%, 26.35% and 23.06%, respectively (p < 0.05). Moderate drought triggered cell swelling, partial cell-wall rupture and prominent enlargement of intracellular starch granules and lipid droplets. Transcriptomic profiling demonstrated up-regulation of transcription–translation and DNA-repair machinery, ATP-binding cassette (ABC) transporters, abscisic acid (ABA)/jasmonic acid (JA) signaling, and biosynthetic pathways for protective metabolites, whereas catabolic pathways and mitogen-activated protein kinase (MAPK) signaling were suppressed. Untargeted metabolomics further validated accumulation of trehalose, stress-protective amino acids, vitamins and functional lipids, while chloroplast-membrane-related metabolites and catabolic intermediates declined. This multi-omics study systematically reveals physiological and molecular regulatory cascades underlying drought tolerance in desert Chlorella sp., providing candidate genes and metabolites to support microalgal resource exploitation for arid-zone applications.