Oxygen-evolving photosynthetic organisms possess nonphotochemical quenching (NPQ) pathways that protect against photoinduced damage. The majority of NPQ in plants is regulated on a rapid timescale by changes in the pH of the thylakoid lumen. In order to quantify the rapidly reversible component of NPQ, called qE, we developed a mathematical model of pH-dependent quenching of chlorophyll excitations in Photosystem II. Our expression for qE depends on the protonation of PsbS and the deepoxidation of violaxanthin by violaxanthin deepoxidase. The model is able to simulate the kinetics of qE at low and high light intensities. The simulations suggest that the pH of the lumen, which activates qE, is not itself affected by qE. Our model provides a framework for testing hypothesized qE mechanisms and for assessing the role of qE in improving plant fitness in variable light intensity.regulation of photosynthesis | nonlinear differential equations | biological feedback | chlorophyll fluorescence | photoprotection P hotosynthetic organisms are highly efficient at absorbing photons and transferring energy to a reaction center, where charge separation takes place. However, when the rate of energy consumption by the reaction center is slower than the rate of energy transfer to the reaction center, long-lived chlorophyll excited states build up in the Photosystem II (PSII) antenna. These longlived states present a significant hazard to the organism because the energy contained in excited chlorophyll is sufficient to generate singlet oxygen, which is highly reactive and can break bonds in the proteins essential for photosynthesis (1). Because sufficient light harvesting is necessary for fueling growth, but too much is harmful, plants face a challenge in balancing light harvesting and photoprotection, especially when light intensity rapidly fluctuates between levels that limit photosynthesis and levels that exceed the plant's capacity for photosynthesis (2).The mechanisms of regulated dissipation of excess absorbed energy in the PSII antenna are collectively known as nonphotochemical quenching (NPQ) (3). NPQ mechanisms dissipate excitation energy harmlessly as heat, reducing the extent of photoinhibition (4). There are multiple mechanisms for NPQ and these mechanisms respond on different timescales (3). The most rapid component of NPQ is called qE, and it responds to fluctuations in light intensity on the timescale of seconds to minutes (5, 6).The qE quenching pathway is activated by a decrease in the pH of the thylakoid lumen (3). The low pH of the lumen activates qE by protonating the proteins PsbS (7) and violaxanthin deepoxidase (VDE) (8, 9), and possibly other light harvesting complexes (10, 11). VDE goes on to convert the carotenoid violaxanthin to zeaxanthin in the xanthophyll cycle, which includes the intermediate antheraxanthin (12). The presence of zeaxanthin and the xanthophyll lutein, along with PsbS, is necessary for full expression of qE in vivo. In addition to the protonation of PsbS and the formation of zeaxanthin...

Photosynthetic organisms are capable of harvesting solar energy with near unity quantum efficiency. Even more impressively, this efficiency can be regulated in response to the demands of photosynthetic reactions and the fluctuating light-levels of natural environments. We discuss the distinctive design principles through which photosynthetic light-harvesting functions. These emergent properties of photosynthesis appear both within individual pigment-protein complexes and in how these complexes integrate to produce a functional, regulated apparatus that drives downstream photochemistry. One important property is how the strong interactions and resultant quantum coherence, produced by the dense packing of photosynthetic pigments, provide a tool to optimize for ultrafast, directed energy transfer. We also describe how excess energy is quenched to prevent photodamage under high-light conditions, which we investigate through theory and experiment. We conclude with comments on the potential of using these features to improve solar energy devices.

Energy-dependent quenching (qE) in photosystem II (PSII) is a pH-dependent response that enables plants to regulate light harvesting in response to rapid fluctuations in light intensity. In this review, we aim to provide a physical picture for understanding the interplay between the triggering of qE by a pH gradient across the thylakoid membrane and subsequent changes in PSII. We discuss how these changes alter the energy transfer network of chlorophyll in the grana membrane and allow it to switch between an unquenched and quenched state. Within this conceptual framework, we describe the biochemical and spectroscopic measurements and models that have been used to understand the mechanism of qE in plants with a focus on measurements of samples that perform qE in response to light. In addition, we address the outstanding questions and challenges in the field. One of the current challenges in gaining a full understanding of qE is the difficulty in simultaneously measuring both the photophysical mechanism of quenching and the physiological state of the thylakoid membrane. We suggest that new experimental and modeling efforts that can monitor the many processes that occur on multiple timescales and length scales will be important for elucidating the quantitative details of the mechanism of qE.

One mechanism used by plants to protect against damage from excess sunlight is called nonphotochemical quenching (NPQ). Triggered by low pH in the thylakoid lumen, NPQ leads to conversion of excess excitation energy in the antenna system to heat before it can initiate production of harmful chemical species by photosynthetic reaction centers. Here we report a synthetic hexad molecule that functionally mimics the role of the antenna in NPQ. When the hexad is dissolved in an organic solvent, five zinc porphyrin antenna moieties absorb light, exchange excitation energy, and ultimately decay by normal photophysical processes. Their excited-state lifetimes are long enough to permit harvesting of the excitation energy for photoinduced charge separation or other work. However, when acid is added, a pH-sensitive dye moiety is converted to a form that rapidly quenches the first excited singlet states of all five porphyrins, converting the excitation energy to heat and rendering the porphyrins kinetically incompetent to readily perform useful photochemistry.

We describe a technique to measure the fluorescence decay profiles of intact leaves during adaptation to high light and subsequent relaxation to dark conditions. We show how to ensure that photosystem II reaction centers are closed and compare data for wild type Arabidopsis thaliana with conventional pulse-amplitude modulated (PAM) fluorescence measurements. Unlike PAM measurements, the lifetime measurements are not sensitive to photobleaching or chloroplast shielding, and the form of the fluorescence decay provides additional information to test quantitative models of excitation dynamics in intact leaves.

Photosynthetic organisms avoid photodamage to photosystem II (PSII) in variable light conditions via a suite of photoprotective mechanisms called nonphotochemical quenching (NPQ), in which excess absorbed light is dissipated harmlessly. To quantify the contributions of different quenching mechanisms to NPQ, we have devised a technique to measure the changes in chlorophyll fluorescence lifetime as photosynthetic organisms adapt to varying light conditions. We applied this technique to measure the fluorescence lifetimes responsible for the predominant, rapidly reversible component of NPQ, qE, in living cells of Chlamydomonas reinhardtii. Application of high light to dark-adapted cells of C. reinhardtii led to an increase in the amplitudes of 65 ps and 305 ps chlorophyll fluorescence lifetime components that was reversed after the high light was turned off. Removal of the pH gradient across the thylakoid membrane linked the changes in the amplitudes of the two components to qE quenching. The rise times of the amplitudes of the two components were significantly different, suggesting that the changes are due to two different qE mechanisms. We tentatively suggest that the changes in the 65 ps component are due to charge-transfer quenching in the minor light-harvesting complexes and that the changes in the 305 ps component are due to aggregated light-harvesting complex II trimers that have detached from PSII. We anticipate that this technique will be useful for resolving the various mechanisms of NPQ and for quantifying the timescales associated with these mechanisms.photosynthesis | in vivo spectroscopy | time-resolved fluorescence | feedback de-excitation | pH-dependent regulation

Abstract. Secondary organic aerosol (SOA) constitutes a large fraction of atmospheric aerosol. To assess its impacts on cli-mate and air pollution, knowledge of the number of phases in internal mixtures of different SOA types is required. Atmospheric models often assumed that different SOA types form a single phase when mixed. Here, we present visual observations of the number of phases formed after mixing different anthropogenic and biogenic SOA types. Mixing SOA types generated in environmental chambers with oxygen-to-carbon (O / C) ratios between 0.34 to 1.05, we found six out of fifteen mixtures of two SOA types to result in two phase particles. We demonstrate that the number of phases depends on the difference in the average O / C ratio between the two SOA types (Δ(O / C)). Using a threshold Δ(O / C) of 0.47, we can predict the phase behavior of over 90 % of our mixtures, with one- and two-phase particles predicted for Δ(O / C) < 0.47 and Δ(O / C) ≥ 0.47, respectively. This Δ(O / C) threshold further allows to predict if mixtures of fresh and aged SOA form one- or two-phase particles in the atmos-phere. In addition, we show that phase separated SOA particles form when mixtures of volatile organic compounds emitted from real trees are oxidized.

The earliest steps in bacterial photosynthesis require that an antenna system efficiently capture incident photons and shuttle the excitation energy to the ''special pair'' bacteriochlorophylls within the membrane-bound reaction center where charge separation occurs. Previous work has shown coherent energy transfer-a wavelike transfer process-among peripheral chromophores, bacteriopheophytins and accessory bacteriochlorophylls, at cryogenic temperatures. Whether or not this coherent transfer extends to the special pair, however, has remained elusive at any temperature. Here we report direct evidence that the special pair is coherently coupled to the accessory bacteriochlophylls and that this coherence dephases only upon transfer to the special pair-the maximal amount of coherence physically possible. We employ Gradient Assisted Photon Echo Spectroscopy to simultaneously excite the bacteriopheophytins, accessory bacteriochlorophylls and the special pair in the reaction center from Rhodobacter sphaeroides. These results suggest the bacteria exploits coherent energy transfer at room temperature.