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

Macroalgal 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 dose (µM N week⁻¹), amplitude (µM N) and duration (hours) of fertilization, affects the dynamics of nitrogen content and biomass production of Ulva sp. 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 these experimental data, we developed a biomass productivity model that predicts nitrogen and biomass dynamics temporally over three weeks of cultivation. This study highlights efficient fertilizing regimes and enables the development of a comprehensive understanding of the dynamic relationship between external N, internal N and biomass production of Ulva sp. 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 leading to an improved dynamic cultivation model, enabling better control of nutrient application and biomass production in macroalgal farming for a sustainable marine bioeconomy.