Influence of Chemically Disrupted Photosynthesis on Cyanobacterial Thylakoid Dynamics in Synechocystis sp. PCC 6803

Abstract

The photosynthetic machinery of the cyanobacterium Synechocystis sp. PCC 6803 resides in flattened membrane sheets called thylakoids, situated in the peripheral part of the cellular cytoplasm. Under photosynthetic conditions these thylakoid membranes undergo various dynamical processes that could be coupled to their energetic functions. Using Neutron Spin Echo Spectroscopy (NSE), we have investigated the undulation dynamics of Synechocystis sp. PCC 6803 thylakoids under normal photosynthetic conditions and under chemical treatment with DCMU (3-(3,4-dichlorophenyl)-1,1-dimethylurea), an herbicide that disrupts photosynthetic electron transfer. Our measurements show that DCMU treatment has a similar effect as dark conditions, with differences in the undulation modes of the untreated cells compared to the chemically inhibited cells. We found that the disrupted membranes are 1.5-fold more rigid than the native membranes during the dark cycle, while in light they relax approximately 1.7-fold faster than native and they are 1.87-fold more flexible. The strength of the herbicide disruption effect is characterized further by the damping frequency of the relaxation mode and the decay rate of the local shape fluctuations. In the dark, local thicknesses and shape fluctuations relax twice as fast in native membranes, at 17% smaller mode amplitude, while in light the decay rate of local fluctuations is 1.2-fold faster in inhibited membranes than in native membranes, at 56% higher amplitude. The disrupted electron transfer chain and the decreased proton motive force within the lumenal space partially explain the variations observed in the mechanical properties of the Synechocystis membranes, and further support the hypothesis that the photosynthetic process is tied to thylakoid rigidity in this type of cyanobacterial cell.

Figure 1

S(q, t)/S(q, 0) of native and inhibited cyanobacterial cells. All scattering functions start at unity and are shifted for better visualization. Solid lines represent fitting by a stretched exponential model^, according to Eq. (1); Only one q value per time-of-flight grouping for each sample is shown here (note: the full set of data is available). Panel “a” displays the evolution of membranes in dark and Panel “b” shows the evolution of membranes in light conditions. Error bars represent statistical error (1σ), in some cases smaller than the size of the symbol used.

Figure 2

q³ dependence of the decay rate Γ for native and inhibited cyanobacterial cells. Panel “a” displays the membrane relaxation rates during the dark cycle and Panel “b” shows the relaxation rates during light conditions. Solid lines in the same color are the Lorentz fit according to Eq. (2). Distances are marked according to q probed by NSE, SANS profiles are superimposed, and the region where 1/q ≪ interthylakoidal distance is labeled Zilman-Granek region. Error bars represent statistical error (1σ), in some cases smaller than the size of the symbol used.

Figure 3

Summary diagram of the observed mechanical properties of native WT and inhibited WT-DCMU membranes. The green circles represent Synechocystis 6803 cyanobacterial cells with one pair of thylakoid membranes, under different illumination conditions. The distance d represents the interthylakoidal distance = the local length scale where thickness and shape fluctuations are sampled. The red arrows show the direction of increase flexibility.

Figure 4

q-dependence of the q³-normalized decay rate Γ/q³ for DCMU inhibited cyanobacterial cells. A plot of membrane relaxation rate during dark and light conditions is superimposed by Small Angle Scattering data (small symbols). Solid lines in same color are the Lorentz fit according to Eq. (2). Peak position, q₀, is marked, corresponding to length scale of probed local shape fluctuations. Error bars represent statistical error (1σ), in some cases smaller than the size of the symbol used.