Speaker
Description
Photosystem I (PSI-LHCI) is a multi-subunit pigment-protein complex responsible for light-driven electron transfer in oxygenic photosynthesis. It consists of a core reaction center (RC), where charge separation occurs at the primary electron donor, P700, and a peripheral light-harvesting complex (LHCI) that enhances light absorption capacity to drive photochemical reactions. P700 absorbs light around 700 nm, which is lower in energy than bulk chlorophylls (Chls), whose average absorption peaks around 680 nm. In general, PSI complexes contain red chlorophyll forms (Chls RF); in cyanobacteria, these are predominantly associated with the core, whereas in higher plants, they are localized in the outer antenna LHCI, extending absorption beyond 700 nm, reaching up to 750 nm. These Chls RF improve energy utilization under limiting light conditions enriched in near infrared (NIR) light (https://doi.org/10.1007/s11120-013-9838-x). However, in higher plants, Chls RF presents both advantages and limitations: while they enhance light-harvesting capacity in specific natural environments, their absorption at lower energy with respect to P700 (700 nm) can hinder efficient energy transfer to the RC, imposing limitations on energy trapping. (https://doi.org/10.1016/j.bbabio.2013.03.008, https://doi.org/10.1007/s11120-020-00717-y).
In this context, the seagrass Posidonia oceanica, a higher sea plant endemic to the Mediterranean Sea, evolved from terrestrial ancestors to thrive in underwater environments at depths of up to 40 meters. As an adaptive mechanism to support efficient photosynthesis in marine conditions where the solar radiation spectrum is firmly restricted to higher energy wavelength (NIR light above 700nm is absent) P. oceanica has lost Chls RF typical of higher land plants. This loss was evidenced by the blue-shifted emission spectrum of the PSI-LHCI complex, which could prevent any potential limitation on charge separation efficiency. While these adaptations have become known recently in biology, the P. oceanica PSI-LHCI supercomplex a comprehensive study of the ultrafast energy transfer (ET) mechanisms in this species is still completely lacking.
Specifically, this study required high temporal and spectral resolution since ET mechanisms are in the order of hundreds of femtoseconds up to few picoseconds. Two-dimensional electronic spectroscopy (2DES) is a prefect tool for this aim since it provides time-resolved excitation/detection maps allowing to gain high spectral excitation selectivity still preserving high temporal resolution (15fs) (https://doi.org/10.1063/1.4902938). In this work, we combine pump probe and 2DES to study the ET mechanisms that are taking place in P. oceanica PSI-LHCI by covering a spectral range from 580nm to 720nm.
Our results show a downhill ET from Chls bulk to the RC Chls (absorption peak at 680-690nm) and P700 in <500fs which is four time faster with respect the downhill process observed for higher plants (>2ps) (https://doi.org/10.1021/acs.jpcb.1c01498). This suggests that the absence of the Chls RF accelerate the downhill ET mechanism without imposing limitation on the energy trapping which we characterize around 6ps (twice faster with respect to other higher plants).
In conclusion, we provide for the first time, temporal and spectral characterization of the ET mechanisms in P. oceanica, which adapts their photosynthetic complexes to a natural environment where the NIR light cannot be absorbed. Furthermore, these results give the possibility to deeply understand how the presence of Chls RF impacts the photosynthetic efficiency, providing crucial insight for bioengineering enhanced light-harvesting complexes.
