Phototrophs trade growth for defense through graded regulatory states under escalating oxidative stress

Oxygenic phototrophs face a paradox: the reactions powering growth generate oxidative stress, whose damage or costly defense reduces fitness. In Synechococcus elongatus PCC 7942, we profiled steady-state growth, photophysiology, and transcription across a dissolved oxygen gradient (0–374% air saturation). Independent component analysis of these transcriptomes within a 407-sample compendium resolved 78 regulatory modules (iModulons) explaining 72.3% of variance. Growth rate followed a graded, sigmoidal dose-response, which we divide into low, moderate, and severe stress regimes (14% and 47% decline, then arrest). Stress-responsive module activities shifted across these regimes: low stress activated SufR and PerR at minimal growth cost; moderate stress modulated three RpaB states, then ManR, CopR, and Rre1; and severe stress triggered a dual-control checkpoint with a fourth RpaB state linking redox sensing to the stringent response and growth arrest. Protein oxidation, pigments, and energy transfer tracked these transitions, suggesting cells trade growth for defense through discrete regulatory states.