Inherent optical properties of Phaeocystis globosa colonies in the China Seas

Phaeocystis globosa (P. globosa), a distinctive harmful algal bloom (HAB)-forming species, is capable of 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), particularly in the form of intact colonies. However, these properties remain poorly understood for the giant colonies of P. globosa, given that measurements of them are challenging. Here, by modifying existing methods for IOPs measurements, we successfully obtained the absorption and scattering coefficients of intact P. globosa colonies up to 25 mm in size. Six strains were isolated from different regions of the China Seas: one collected in situ, and the others grown in culture. Our results show that the IOPs of intact colonies are similar to those of free-living cells, regardless of colony size (1-25 mm). A prominent absorption peak at 468-472 nm was consistently observed across all five colonial strains and the free-living cell strain, corresponding to chlorophyll-c₃ (Chl-c₃) absorption. Notably, although intracolonial fluid contains colored dissolved organic matter and the colony envelope consists of optically active particulate matter, their contributions to light absorption and scattering by colonies in the spectral range of 400-750 nm are negligible when the colony remains non-collapsed. Interestingly, broken colonies exhibit reduced chlorophyll-specific absorption compared to intact ones, suggesting that colony structure enhances light absorption. This result, combined with the fact that measurements of broken colonies via the traditional quantitative filter technique are compromised by uneven pad distribution, underscores the importance of measuring IOPs in intact P. globosa colonies rather than in filtered samples containing broken colonies. These findings will undoubtedly aid in the future development of bio-optical models for P. globosa colonies and enhance remote sensing algorithms for detecting P. globosa blooms.