Revealing the Dynamics of Thylakoid Membranes in Living Cyanobacterial Cells

Abstract

Cyanobacteria are photosynthetic prokaryotes that make major contributions to the production of the oxygen in the Earth atmosphere. The photosynthetic machinery in cyanobacterial cells is housed in flattened membrane structures called thylakoids. The structural organization of cyanobacterial cells and the arrangement of the thylakoid membranes in response to environmental conditions have been widely investigated. However, there is limited knowledge about the internal dynamics of these membranes in terms of their flexibility and motion during the photosynthetic process. We present a direct observation of thylakoid membrane undulatory motion in vivo and show a connection between membrane mobility and photosynthetic activity. High-resolution inelastic neutron scattering experiments on the cyanobacterium Synechocystis sp. PCC 6803 assessed the flexibility of cyanobacterial thylakoid membrane sheets and the dependence of the membranes on illumination conditions. We observed softer thylakoid membranes in the dark that have three-to four fold excess mobility compared to membranes under high light conditions. Our analysis indicates that electron transfer between photosynthetic reaction centers and the associated electrochemical proton gradient across the thylakoid membrane result in a significant driving force for excess membrane dynamics. These observations provide a deeper understanding of the relationship between photosynthesis and cellular architecture.

Figure 1. S(q, t)/S(q, 0) of cyanobacteria at 20 °C for light and dark states.

All scattering functions start at unity and have been shifted for better visualization. Solid lines represent the stretched exponential function fitting with stretching exponent of 2/3; 0.035 Å⁻¹, 0.05 Å⁻¹ and 0.095 Å⁻¹ minimum value of momentum transfer were measured according to the correlation peaks observed in the SANS experiment. Five different groupings of the time channels were applied, leading to 15 q values. Only four q values for each state are shown here in relation to Figs 2 and 3.

Figure 2. q³ dependence of the decay rate Γ for cyanobacteria during light and dark states.

The vertical dashed lines show the position of the corresponding observed distances within membrane pairs, with q3 at the peak of Γ function. With increasing distance observed (from q1 to q2), the influence of the neighboring membrane becomes apparent in the increase of the relaxation rate up to a maximum where the membranes repel each other. This leads to a decrease of the relaxation rate to the state where the influence of the neighboring membrane is no longer visible (q4). Small Angle Neutron Scattering profiles are super-imposed to observe the correlation of dynamics features with the structure.

Figure 3. Thylakoid membrane sheet-like structure dynamics.

A small region of two adjacent closely appressed membranes that cannot accommodate phycobilisomes is presented undulating freely in the cytoplasm, and the corresponding interthylakoidal distances. From q1 to q4 NSE samples a larger distance (57 Å–175 Å) within the interthylakoidal space and different relaxation behavior is observed. The excess of protons (H⁺) in the thylakoid lumen results in restricted membrane dynamics observed during light. At dark, H⁺ pressure is alleviated by chemiosmosis and the membranes undulate freely resulting in higher relaxation rates and excess dynamics. Purple feature is the ATP-Synthase responsible for moving down the electrochemical gradient. The center-to-center distance of a membrane pair corresponds to SANS correlation peaks for closely appressed membranes in Fig. 2 (100 Å–180 Å).