Acoustic signatures of microalgal growth: ultrasonic velocity and attenuation as quantitative biomass indicators in Spirulina maxima

• Ultrasonic velocity and attenuation quantify Spirulina maxima biomass non-invasively at 1 MHz. • Sound velocity increased by ∼13.5% and attenuation by >100% over 14-day cultivation. • Strong correlations with OD 680 were achieved (velocity r = 0.93; attenuation r = 0.97). • Concentration-resolved measurements (0–100% v/v) validated biomass sensitivity. • Ultrasound enables real-time, in situ biomass monitoring for photobioreactor control. Accurate, real-time measurement of microalgal biomass is vital for optimizing photobioreactor performance and advancing bioprocess monitoring technologies. This study systematically investigated the acoustic signatures of Spirulina maxima ( S. maxima ) by measuring sound velocity and attenuation at 1 MHz over a 14-day cultivation period and across controlled biomass dilutions (0–100% v/v). Ultrasonic time-of-flight and amplitude measurements were used to determine acoustic parameters, which were subsequently correlated with optical density (OD 680 ) to develop predictive models for biomass estimation. Sound velocity increased from approximately 1507 to 1710 m/s during cultivation, exhibiting nonlinear, concentration-dependent behavior ( R 2 = 0.96), while attenuation rose from 0.0035 to 0.0074 dB/cm and exhibited near-linear dependence at early stages, with pronounced nonlinearity at higher biomass levels ( R 2 = 0.93). Concentration-resolved measurements confirmed consistent trends: sound velocity increased steadily across all biomass concentrations, whereas attenuation increased sharply beyond roughly 60% v/v due to greater scattering and viscous–thermal losses. Temporal OD 680 profiles revealed distinct lag, quasi-stationary, and secondary growth phases, each reflected in characteristic shifts in acoustic response. Strong correlations between ultrasonic parameters and OD 680 (velocity: r = 0.93; attenuation: r = 0.97) demonstrate the capability of acoustic sensing to detect both mechanical and structural changes in microalgal cultures. Overall, these findings establish low-power ultrasound as a robust, sensitive, and non-invasive technique for real-time biomass monitoring, with significant potential for integration into automated photobioreactors and environmental monitoring systems.