Effects of growth phase and irradiance on phytoplankton pigment ratios: implications for chemotaxonomy in coastal waters

Pigment (chlorophylls and carotenoids) concentrations were studied in cultures of two dinophyceae (Amphidinium carterae and Gymnodinium catenatum), two bacillariophyceae (Cylindrotheca closterium and Phaeodactylum tricornutum), one chlorophyceae (Dunaliella salina), one prasinophyceae (Tetraselmis suecica) and one cryptophyceae (Rhodomonas sp.), isolated from Portuguese coastal waters. Consistently higher pigment to chlorophyll a (Chl a) ratios were observed at stationary compared to exponential growth phase. The ratios most affected were fucoxanthin/Chl a in C. closterium and zeaxanthin/Chl a in D. salina, which were 2.3- and 10.5-fold higher in the stationary phase, respectively. Most marker pigments to Chl a ratios were not dependent on growth irradiance as light-harvesting photosynthetic pigments tend to co-vary with Chl a. However, fucoxanthin/Chl a ratios decreased with irradiance for C. closterium, while lutein/Chl a doubled from low to high light in T. suecica. Ratios based on photo-protective pigments (diatoxanthin, zeaxanthin and antheraxanthin) to Chl a increased with irradiance. The effects of growth phase and irradiance on phytoplankton pigment/Chl a ratios, as well as the interspecific differences within the same phytoplankton group, underline the importance of choosing the most abundant species in the study area when applying chemotaxonomic methods to phytoplankton natural communities.