Energy-dependent quenching adjusts the excitation diffusion length to regulate photosynthetic light harvesting

An important determinant of crop yields is the regulation of photosystem II (PSII) light harvesting by energy-dependent quenching (qE). However, the molecular details of excitation quenching have not been quantitatively connected to the PSII yield, which only emerges on the 100 nm scale of the grana membrane and determines flux to downstream metabolism. Here, we incorporate excitation dissipation by qE into a pigment-scale model of excitation transfer and trapping for a 200 nm x 200 nm patch of the grana membrane. We demonstrate that single molecule measurements of qE are consistent with a weak-quenching regime. Consequently, excitation transport can be rigorously coarse-grained to a 2D random walk with an excitation diffusion length determined by the extent of quenching. A diffusion-corrected lake model substantially improves the PSII yield determined from variable chlorophyll fluorescence measurements and offers an improved model of PSII for photosynthetic metabolism.Comment: 19 pages, 4 figures, 3 supplementary figure

Models and measurements of energy-dependent quenching.

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

Stress-induced Metabolic Exchanges Between Complementary Bacterial Types Underly a Dynamic Mechanism of Inter-species Stress Resistance

Metabolic cross-feeding plays vital roles in promoting ecological diversity. While some microbes depend on exchanges of essential nutrients for growth, the forces driving the extensive cross-feeding needed to support the coexistence of free-living microbes are poorly understood. Here we characterize bacterial physiology under self-acidification and establish that extensive excretion of key metabolites following growth arrest provides a collaborative, inter-species mechanism of stress resistance. This collaboration occurs not only between species isolated from the same community, but also between unrelated species with complementary (glycolytic vs. gluconeogenic) modes of metabolism. Cultures of such communities progress through distinct phases of growth-dilution cycles, comprising of exponential growth, acidification-triggered growth arrest, collaborative deacidification, and growth recovery, with each phase involving different combinations of physiological states of individual species. Our findings challenge the steady-state view of ecosystems commonly portrayed in ecological models, offering an alternative dynamical view based on growth advantages of complementary species in different phases

Models and measurements of energy-dependent quenching

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

Light Harvesting and Its Regulation in Photosynthetic Grana Membranes

Photosystem II (PSII) initiates photosynthesis in plants by absorbing and converting light energy from the sun into chemical energy, a process called light harvesting. PSII is composed of proteins bound to pigment cofactors that can be grouped into antenna proteins, which absorb light and transfer excitation energy to other pigment-protein complexes, and reaction centers, which can convert excitation energy into chemical energy via a charge separation reaction. In plants, the proteins associated with PSII are located in the grana membrane, which is densely packed with photosystem II and major light harvesting complexes (LHCII). PSII reversibly binds with LHCII to form PSII supercomplexes. PSII supercomplexes and LHCIIs form a large, variably fluid array of pigment-protein complexes that gives rise to an energy transfer network that operates like a "smart" solar cell. In dim sunlight, the grana membrane harvests light with 90% efficiency. In response to light of fluctuating intensity and wavelength, the antenna proteins of PSII can regulate light harvesting. Understanding the design principles of light harvesting in grana membranes in different light conditions would be useful as a blueprint for designing robust solar cells. This thesis presents measurements and models for understanding light harvesting in variable light conditions. We have developed the fluorescence lifetime snapshot technique to monitor changes in the energy transfer network of the grana membrane of green algae and plant leaves in response to changes in incident light. Using this technique, we suggest that there are two mechanisms for green algae to acclimate to changes in light intensity. To fully leverage the snapshot data, a structure-based model of energy transfer for the grana membrane is required. We constructed a detailed model of energy transfer and trapping in PSII supercomplexes and show how to use this model to construct an energy transfer model for the grana membrane. Together, the snapshot technique and membrane model will aid in the elucidation of the principles of light harvesting and its regulation in grana membranes

Light Harvesting and Its Regulation in Photosynthetic Grana Membranes

Photosystem II (PSII) initiates photosynthesis in plants by absorbing and converting light energy from the sun into chemical energy, a process called light harvesting. PSII is composed of proteins bound to pigment cofactors that can be grouped into antenna proteins, which absorb light and transfer excitation energy to other pigment-protein complexes, and reaction centers, which can convert excitation energy into chemical energy via a charge separation reaction. In plants, the proteins associated with PSII are located in the grana membrane, which is densely packed with photosystem II and major light harvesting complexes (LHCII). PSII reversibly binds with LHCII to form PSII supercomplexes. PSII supercomplexes and LHCIIs form a large, variably fluid array of pigment-protein complexes that gives rise to an energy transfer network that operates like a "smart" solar cell. In dim sunlight, the grana membrane harvests light with 90% efficiency. In response to light of fluctuating intensity and wavelength, the antenna proteins of PSII can regulate light harvesting. Understanding the design principles of light harvesting in grana membranes in different light conditions would be useful as a blueprint for designing robust solar cells. This thesis presents measurements and models for understanding light harvesting in variable light conditions. We have developed the fluorescence lifetime snapshot technique to monitor changes in the energy transfer network of the grana membrane of green algae and plant leaves in response to changes in incident light. Using this technique, we suggest that there are two mechanisms for green algae to acclimate to changes in light intensity. To fully leverage the snapshot data, a structure-based model of energy transfer for the grana membrane is required. We constructed a detailed model of energy transfer and trapping in PSII supercomplexes and show how to use this model to construct an energy transfer model for the grana membrane. Together, the snapshot technique and membrane model will aid in the elucidation of the principles of light harvesting and its regulation in grana membranes

Technical Note: MRI-guided breast biopsy - our preliminary experience

The diagnostic potential of breast MRI can be fully utilized only when it is possible to biopsy lesions detected on MRI, especially when they are not visible on mammography or USG. We would like to describe our experience with MRI-guided wire localization and biopsy