Interspecies differences in the response of fluorescence transients to changes in growth irradiance reflect different photoacclimation strategies in two diatom species

Phytoplankton respond to changes in their light environment by altering the composition of the photosynthetic electron transport (PET) chain. This process, called photoacclimation, balances light absorption with metabolic energy needs and is necessary for cells to maximize growth rate under changing light conditions. PET chain structure affects the shape of fluorescence curves (i.e. transients), and thus fluorescence may contain information about phytoplankton photoacclimation state. In this study, we examined whether the shape of the fluorescence transient measured in the marine diatoms Thalassiosira weissflogii and Phaeodactylum tricornutum changes in response to photoacclimation to different growth irradiances. In addition, we utilized various inhibitors of photosynthesis (DBMIB, methyl viologen, nigericin, and propyl gallate) to test whether the same PET compo-nents were controlling transient shape in both species. The overall shape of the fluorescence transient signifi-cantly differed between T. weissflogii and P. tricornutum, but in both species the transient shape changed predictably in response to increasing irradiance. However, the effect of various inhibitors differed between the two species, suggesting significant differences in the composition and activity of PET chain components. We hypothesize that the differences in transient shape and the effects of PET chain inhibitors in T. weissflogii and P. tricornutum reflect differences in photoacclimation strategy due to evolution in different marine environments.