Adaptive laboratory evolution under acetate stress alleviates the trade-off between growth and astaxanthin production in Haematococcus pluvialis.

The industrial cultivation of Haematococcus pluvialis for astaxanthin is constrained by a fundamental trade-off: stress promotes astaxanthin accumulation but suppresses growth. To overcome this, we applied adaptive laboratory evolution (ALE) under progressive high acetate stress (2.5-5-7.5  g/L NaAc). This approach generated robust strains, AC50 and AC75, with survival rates of 81% and 44%-over fourfold higher than the wild-type (WT). In the green stage, AC50 under 5 g/L NaAc increased cell density, biomass, and astaxanthin yield by 44%, 55%, and 80%, respectively, relative to WT under 1 g/L NaAc. These advantages extended in the red stage, where AC50 also showed substantial improvements in biomass and astaxanthin yield. Transcriptomic analysis revealed a dual strategy: ALE restored global stress-response networks (plant hormone signaling, transporters, photosystem, and lipid metabolism) toward homeostasis while selectively upregulating key pathways for astaxanthin biosynthesis and glycolysis. Our work provides both a superior strain (AC50) and a mechanistic framework for alleviating the classical growth-production dichotomy, advancing efficient acetate-based astaxanthin production.