Stress-Induced Metabolic Reprogramming in the Green Microalga Chlorella sp. SLA-04.

Microalgae thrive in diverse and often challenging environments by coordinating metabolic and physiological responses to environmental stress, yet the molecular mechanisms underlying these adaptations remain incompletely understood. Here, we combined physiological, biochemical, and transcriptomic analyses to investigate the response of the green microalga Chlorella sp. SLA-04 to nitrogen depletion and high-alkalinity growth. Nitrogen depletion redirected carbon toward carbohydrate and lipid storage and was accompanied by coordinated transcriptional changes that suppressed photosynthesis while activating TAG biosynthetic and degradative pathways. The coordinated induction of lipid biosynthetic and degradative pathways is consistent with a model in which TAG synthesis and turnover help maintain redox homeostasis during nutrient stress. In contrast, high-alkalinity growth altered fatty-acid composition and was associated with transcriptional changes involving ion transport, membrane remodeling, and osmotic adaptation. Alkaline growth also altered the expression of mobile genetic elements and genes associated with RNA-mediated genome surveillance, suggesting that adaptation to high pH extends beyond central metabolism. Together, these findings provide a system-level view of how coordinated physiological and transcriptional responses enable Chlorella sp. SLA-04 to adapt to nutrient limitations and high-alkalinity growth.