Red shift in the spectrum of a chlorophyll species is essential for the drought-induced dissipation of excess light energy in a poikilohydric moss, Bryum argenteum

Accumulation of quenched LHCII around PSI in Chlamydomonas cells in state2 revealed by cryo-fluorescence lifetime imaging

Fluorescence-spectral microscope observations of photosynthetic organisms at cryogenic temperatures have the ability to spectrally resolve the two photosystems (PSs) and thus provide a powerful tool to elucidate the functional analysis of photosynthesis in vivo. In the present study, a measurement channel of the fluorescence lifetime at 680 nm was added to the cryo-microscope system previously developed by the authors. This provides access to information on the functional state of the light-harvesting system in living cells during regulation by a mechanism called state transitions. The observations of state1-locked and state2-locked Chlamydomonas cells at 80 K enabled us to identify a component showing rapidly decaying fluorescence with a lifetime of ca. 3 ps and emitting at around 676 nm. The component was assigned to the light-harvesting complex II (LHCII) that is isolated from both PSs and in a quenched state, probably due to the formation of aggregates. Simultaneous spectral observations revealed the accumulation of this free LHCII in the photosystem I (PSI)-enriched region within each state2-locked cell. To the best of our knowledge, this is the first in-vivo observation which suggests the localization of the quenched LHCII aggregates.

Ultrafast laser filament-induced fluorescence for detecting uranium stress in Chlamydomonas reinhardtii

Plants and other photosynthetic organisms have been suggested as potential pervasive biosensors for nuclear nonproliferation monitoring. We demonstrate that ultrafast laser filament-induced fluorescence of chlorophyll in the green alga Chlamydomonas reinhardtii is a promising method for remote, in-field detection of stress from exposure to nuclear materials. This method holds an advantage over broad-area surveillance, such as solar-induced fluorescence monitoring, when targeting excitation of a specific plant would improve the detectability, for example when local biota density is low. After exposing C. reinhardtii to uranium, we find that the concentration of chlorophyll a, chlorophyll fluorescence lifetime, and carotenoid content increase. The increased fluorescence lifetime signifies a decrease in non-photochemical quenching. The simultaneous increase in carotenoid content implies oxidative stress, further confirmed by the production of radical oxygen species evidence in the steady-state absorption spectrum. This is potentially a unique signature of uranium, as previous work finds that heavy metal stress generally increases non-photochemical quenching. We identify the temporal profile of the chlorophyll fluorescence to be a distinguishing feature between uranium-exposed and unexposed algae. Discrimination of uranium-exposed samples is possible at a distance of [Formula: see text]35 m with a single laser shot and a modest collection system, as determined through a combination of experiment and simulation of distance-scaled uncertainty in discriminating the temporal profiles. Illustrating the potential for remote detection, detection over 125 m would require 100 laser shots, commensurate with the detection time on the order of 1 s.

Redox-state dependent blinking of single photosystem I trimers at around liquid-nitrogen temperature

Efficient light harvesting in a photosynthetic antenna system is disturbed by a ragged and fluctuating energy landscape of the antenna pigments in response to the conformation dynamics of the protein. This situation is especially pronounced in Photosystem I (PSI) containing red shifted chlorophylls (red Chls) with the excitation energy much lower than the primary donor. The present study was conducted to clarify light-harvesting dynamics of PSI isolated from Synechocystis sp. PCC6803 by using single-molecule spectroscopy at liquid‑nitrogen temperatures. Fluorescence emission at around 720 nm from the red Chls in single PSI trimers was monitored at 80-100 K. Intermittent variations in the emission intensities, so-called blinking, were frequently observed. Its time scale lay in several tens of seconds. The blinking amplitude depended on the redox state of the phylloquinone (A₁). Electrochromic shifts of Chls induced by the negative charge on A₁ were calculated based on the X-ray crystallographic structure. A Chl molecule, Chl-A839 (numbering according to PDB 5OY0), bound near A₁ was found to have a large electrochromic shift. This Chl has strong exciton coupling with neighboring Chl (A838) whose site energy was predicted to be determined by interaction with an arginine residue (ArgF84) [Adolphs et al., 2010]. A possible scenario of the blinking was proposed. Conformational fluctuations of ArgF84 seesaw the excitation-energy of Chl-A838, which perturbs the branching ratio of excitation-energy between the red Chl and the cationic form of P700 as a quencher. The electrochromic shift of Chl-A839 enhances the effect of the conformation dynamics of ArgF84.