Balanced nutrient replenishment sustains Arthrospira platensis production during long-term medium recycling.

Medium recycling is crucial for semi-continuous cultivation of Arthrospira. Small discrepancies between the nutrients replenished and those consumed can progressively accumulate over repeated medium recycling, leading to compositional shifts that potentially inhibit Arthrospira growth. This study revealed that the nutrient composition shifted markedly after five cycles of medium recycling in a semi-continuous Arthrospira culture, for example, phosphorus (P) decreased from -85.01 % to -95.30 % (with standard deviations of ±2.71 % and ±2.17 %, respectively), potassium (K) increased from 3.5 % to 77.36 % (with standard deviations of ±2.83 % and ±3.06 %, respectively), while the microelement molybdenum (Mo) increased from 75.01 % to 383.33 % (with standard deviations of ±18.26 % and ±23.12 %, respectively), in comparison to its initial levels. To mitigate this drift, an iterative strategy was implemented, wherein the nutrient replenishment formula was progressively corrected based on the compositional feedback from each recycling cycle. When the replenishment formula was upgraded to Gen2, the elemental shifts, the accompanied 7 % reduction in biomass yield, the 2 to 4 percentage point decrease in protein content were less pronounced, and the biomass productivity at the final cycle reached 3.36 ± 0.25 g L-1, which was not significantly different from that at the first cycle (P = 0.172). These findings highlight the importance of precision nutrient management for the development of efficient medium recycling technologies for Arthrospira production and suggest intelligent control to boost efficiency, reducing medium replacement and enabling long‑term recycling.