Growth, photophysiology and photosynthesis of two marine diatoms grown under predictable intra-diel light fluctuations

Natural diatom populations experience variability in irradiance because of several physical processes. Laboratory studies often neglect this and use a static light environment. Consequently, the way dynamic light environments impact growth and photophysiology of diatoms is poorly understood. To address this, several aspects of photophysiology were measured in the diatoms Thalassiosira pseudonana and Phaeodactylum tricornutum when grown under square-wave (SQ), sinusoidal (SI), and low (LF) and high (HF) amplitude light fluctuations of a 1-hour period each with the same light dose. Acclimation to increasing light fluctuation amplitude was found to be functionally similar to high light acclimation. Chlorophyll-a specific light absorption coefficients, maximum photosystem II electron transport rates (ETR) and the capacity for non-photochemical quenching (NPQ) all increased from SI to HF cultures. In both species increasing light fluctuation amplitude reduced growth rate. This reduction was greater for T. pseudonana than P. tricornutum. In HF cultures growth rates were 50% and 62% of those in SQ cultures respectively for the two species. Similar daily ETR between SI and LF cultures of P. tricornutum suggested that differences in the photosynthetic efficiency of light utilisation were a poor explanation for the lower growth rates in LF cultures. Lower growth rate under fluctuating light was instead hypothesised to be caused by greater photodamage and energy investment in photoprotection. Higher NPQ in P. tricornutum reduced net photodamage compared with T. pseudonana. However, light harvesting and ETR in P. tricornutum appeared to be adapted to a lower light environment than T. pseudonana. Higher amplitude light fluctuations also decreased intradiel variability in photoacclimation. This response was greater in P. tricornutum and was responsible for this species ability to maintain a consistent daily ETR between SI and LF regimes. This is thought to give P. tricornutum a competitive advantage in more dynamic light environments.