Photophysiological Characterization of Phytoplankton by Measuring Pigment Production Rates: A Description of Detail Method and a Case Study

Each phytoplankton species has intrinsic pigments, which result in different photophysiological characteristics in response to natural light conditions. Therefore, phytoplankton pigments provide important information on the photosynthetic activity that produces the basic food source for marine ecosystems. This study addresses the challenge of accurately measuring pigment production rates in phytoplankton communities. Two strategies are proposed for improving measurement sensitivity. Firstly, increasing the injection of ¹³C substrate into incubation bottles up to 15% of the total dissolved inorganic carbon is recommended, with minimal impact on pigment production rate determinations. Secondly, optimizing sample injection volume for high-performance liquid chromatography balances analysis time and dilution effects. The in situ field experiments conducted in this study for pigment production measurements revealed diminished activity of photoprotective mechanisms involving zeaxanthin and diatoxanthin during the study period. Furthermore, the results showed that the notable production rates of chl-<i>b</i> (0.069–0.105 ng C L^–1 h^–1, 74–89% of total accessary pigment production rates), an accessory pigment mainly attributed to prasinophytes, potentially due to restricted light availability. Prioritization of chl-<i>b</i> production over primary production (negative correlation between primary and chl-<i>b</i> production; R² = 0.6662) highlights the potential impact of compensatory pigment-related activities on overall phytoplankton productivity. In conclusion, this study underscores the significance of directly quantifying pigment production rates to enhance our comprehension of phytoplankton photophysiology and the production mechanisms specific to various pigments.