This study introduces a novel method for monitoring microalgae growth and evaluating mass transfer efficiency in photobioreactors, specifically under non-limiting and controlled growth conditions. By leveraging data on elemental composition, gas transfer rates, and oxygen production rate, the method estimates biomass growth and assesses gas-liquid mass transfer coefficients. The approach uses indirect measurements to infer critical parameters, including biomass concentration, total inorganic carbon, and nitrogen levels. Results demonstrate accurate predictions of biomass growth and carbon dynamics, along with effective characterization of mass transfer coefficients. This method offers a robust tool for optimizing photobioreactor performance and enhancing process control.