Mapping the Energy Transfer Network in the Diatom Cyclotella meneghiniana by Picosecond Time-Resolved Fluorescence Spectroscopy down to Cryogenic Temperatures

Diatoms are unicellular photosynthetic microalgae that contribute nearly a quarter of the global primary production. Knowledge of the mechanisms, networks, and time scales of the excitation transfer is crucial for a deeper understanding of their photosynthetic machinery. Unfortunately, the short excited-state lifetime of photosystem I at room temperature and research focus on isolated structures have limited the development of a complete excitation transfer scheme in intact cells. Here, we address this gap through picosecond time-resolved fluorescence measurements on intact Cyclotella meneghiniana cells obtained from 16 to 130 K and room temperature. Analysis of the data in terms of evolution-associated spectra reveals a complex interplay of excitation transfer, population of fluorescing trapping states, and excitation quenching at the whole-cell level. We show rapid depopulation of all antenna complexes, on the order of 30 ps even at 16 K, indicating a more efficient energy-transfer network in diatoms than in higher plants.