Modeling and optimization of astaxanthin accumulation in Haematococcus pluvialis cultivated in outdoor five-row parallel horizontal tubular photobioreactors.

Outdoor cultivation is challenged by natural environmental instability, especially daily and seasonal variations in light and temperature, which critically affect astaxanthin accumulation in Haematococcus pluvialis. Thus, developing robust models to clarify the relationship between these parameters and astaxanthin yield is essential for improving production. Unlike traditional models that assume constant light and temperature, FFHT-ap (productivity model) and FFHT-ac (content model) were developed for astaxanthin accumulation in heterotrophically pre-cultured H. pluvialis during outdoor photoinduction in a horizontal tubular photobioreactor (HTPBR), explicitly incorporating daily fluctuations of light and temperature. Their predictive accuracy was validated against experimental data from a five-row parallel HTPBR (15.6 m3 volume) operated at various inclination angles. Guided by these validated models and coupled with real-time temperature and light intensity, the initial inoculum density, geometric structure, and placement orientation of the HTPBR were optimized via an orthogonal experimental design. The results demonstrated that the optimal HTPBR configuration for maximizing annual astaxanthin productivity per unit area was determined to be: tube diameter of 0.059m, northeast orientation (45°), inclination of 45°, row spacing of 1m, initial inoculum density of 0.9gL-1, and tube spacing of 0.0648m. By implementing the optimized design and operational parameters, the annual astaxanthin productivity per unit area of the five-row parallel HTPBR reached 22.90gm-2 year-1, a value that surpasses previously reported benchmarks. This study provides a robust theoretical and experimental basis for scaling up outdoor production of astaxanthin from H. pluvialis, offering actionable insights for designing cost-effective, high-yield photobioreactors.