Light spectra shape population structure and the allocation of fucoxanthin and fatty acids with secondary phytohormone modulation in Tisochrysis lutea

) and endpoint cell productivity, approximately 1.5-fold and 1.4-fold higher than under white and blue light, respectively. Red light also yielded the highest fucoxanthin content (1.43-1.62% DW), total fatty acid content (maximum 14.60% DW), and strongly single-cell-dominant populations (96.72-98.46%) with minimal aggregation. Auxiliary morphometric analysis showed that red-light cultures converged toward a smaller-cell, lower-biovolume single-cell state, whereas white and blue light retained larger dividing and aggregated fractions. Fatty acid class composition shifted with light spectrum, indicating altered lipid allocation and omega-3 partitioning rather than uniform expansion of the fatty acid pool. Phytohormones modified selected endpoints within each spectral regime but did not override the spectrum-driven pattern. These results show that biochemical measurements combined with event-based phenotyping improve interpretation of cultivation responses and support condition screening for coordinated fucoxanthin and fatty acid production in T. lutea.