Phaeocystis globosa (P. globosa), a unique causative species of harmful algal blooms (HABs), exhibits a complex polymorphic life cycle alternating between free-living cells and gelatinous colonies. Developing optical techniques for the rapid detection of this HAB-causing species requires knowledge of its inherent optical properties (IOPs). However, these properties remain poorly understood in the China seas, where P. globosa forms giant colonies (up to 30 mm in diameter), making IOPs measurements particularly challenging. Here, by modifying the existing methods for light absorption measurements, we for the first time successfully obtained the absorption coefficients (a_ph) of P. globosa in the form of colonies. Samples of six strains were isolated from different regions of the China Sea, one of which was collected in situ and the other was grown in culture. A prominent absorption peak in the range from 468 nm to 472 nm was consistently observed in five strains with colonies of varying sizes (ranging from 1 mm to 25 mm), similar to the results from the North Sea, where relatively small (up to 3 mm in diameter) colonies formed, corresponding to the absorption of chlorophyll-c₃ (Chl-c₃). However, the free-living cell strain appears distinctively different from the colony samples, showing an absorption maximum at 465 nm and with no Chl-c₃ detected. Thus, it is suggested that intra-species differences are determined by pigments regardless of colony size. In contrast, the strain isolated in situ displayed a significant reduction in a_ph when the giant colonies were disrupted. This suggests that, in addition to pigments, colony structure, particularly in giant colonies, may also contribute to intra-species differences in absorption properties. It also highlights the importance of a_ph measurements of P. globosa in the original form of colonies rather than in filtered samples with broken colonies. These findings will aid the future development of bio-optical models for P. globosa colonies and enhance remote sensing algorithms for detecting P. globosa blooms.

Harmful Algal Blooms (HABs),p. globosa,IOPs,absorption