Modeling of growth of Ulva sp. macroalgae in a controlled photobioreactor based on nitrogen accumulation dynamics

Macroalgae biomass production models that capture nutrient dynamics, temperature, light, and salinity are important for the design and operation of large-scale farms. The goal of this study is to understand how the nitrogen fertilizing regime, relating to fertilizing dose (μM N week-1), amplitude (μM N), and duration (hours), affects the dynamics of nitrogen content and biomass production of the Ulva sp. macroalgae. We hypothesize that the nitrogen fertilizing regime controls the Ulva Nitrogen Use Efficiency (NUE), defined here as the fraction of fertilizer nitrogen that is utilized and allocated to yield N, and, accordingly, also nitrogen assimilation in the biomass and the growth rate. We test this hypothesis by measuring internal nitrogen and biomass weight and by calculating NUE under various fertilization regimes in controlled photobioreactors. Based on this experimental data, we developed a biomass productivity model that predicts nitrogen and biomass dynamics in time over three weeks of cultivation. This study points out efficient fertilizing regimes and enables the development of a comprehensive understanding of the dynamic relationship between external N, internal N, and biomass production of the Ulva sp. macroalgae under varying external N levels, which is important for real-world agricultural applications. This study provides a better understanding of the external N-internal N-biomass triangle followed by an improved dynamic cultivation model, enabling better control of nutrient application and biomass production in macroalgae farming for a sustainable marine bioeconomy. ### Competing Interest Statement The authors have declared no competing interest. * NUE : Nitrogen Use Efficiency MPBR : Macroalgae Photobioreactor FW : Fresh Weight DGR : Daily Growth Rate RMSRE : Root Mean Square Relative Error