Phosphorus availability is a master regulator of microalgal metabolism, yet its dose-dependent effects on the simultaneous optimization of biomass, pigment, protein, and lipid accumulation remain poorly resolved for many industrially relevant species. This study systematically evaluated the effects of a phosphate gradient (0, 2.02, 4.09, 8.18, and 17.14 mg L⁻¹) on growth, biochemical composition, and lipid productivity in Monoraphidium braunii. M. braunii was cultivated in modified BG-11 medium under controlled laboratory conditions using a batch system for five days. Phosphate deprivation severely suppressed growth and biomass yield, while progressive supplementation supported sustained exponential proliferation and drove a 123% increase in protein content, peaking at 37.5% DW at 8.18 mg L⁻¹. Photosynthetic pigments exhibited a characteristic non-linear response, maximizing at 4.09 mg L⁻¹ before declining at higher concentrations. Lipid content followed an inverse trajectory, declining by 24% from deprivation to full repletion, yet lipid productivity peaked at 4.09 mg L⁻¹ (31.5 mg L⁻¹ d⁻¹), a 70% increase over phosphate-deprived cultures revealing that severe nutrient stress elevates cellular lipid fraction at the cost of volumetric output. Collectively, these findings demonstrate that phosphorus availability governs carbon partitioning in M. braunii, imposing a fundamental trade-off between lipid enrichment and biomass productivity. Moderate phosphate supply (4.09 mg L⁻¹) simultaneously maximized pigment accumulation and lipid productivity, representing the strategic optimum for integrated biorefinery applications targeting biofuel, nutraceutical, and aquaculture feed co-production.