Initial electron-transfer in the reaction center from Rhodobacter sphaeroides.

The initial electron transfer steps in the photosynthetic reaction center of the purple bacterium Rhodobacter sphaeroides have been investigated by femtosecond time-resolved spectroscopy. The experimental data taken at various wavelengths demonstrate the existence of at least four intermediate states within the first nanosecond. The difference spectra of the intermediates and transient photodichroism data are fully consistent with a sequential four-step model of the primary electron transfer: Light absorption by the special pair P leads to the state P*. From the excited primary donor P*, the electron is transferred within 3.5 +/- 0.4 ps to the accessory bacteriochlorophyll B. State P+B- decays with a time constant of 0.9 +/- 0.3 ps passing the electron to the bacteriopheophytin H. Finally, the electron is transferred from H- to the quinone QA within 220 +/- 40 ps

The Role of Fire in the Coevolution of Soils and Temperate Forests

Climate drives the coevolution of vegetation and the soil that supports it. Wildfire dramatically affects many key eco‐hydro‐geomorphic processes, but its potential role in coevolution of soil‐forest systems has been largely overlooked. The steep landscapes of southeastern Australia provide an excellent natural laboratory to study the role of fire in the coevolution of soil and forests, as they are characterized by temperate forest types, fire frequencies, and soil depths that vary systematically with aridity. The aims of this study were (i) to test the hypothesis that in Southeastern Australia, fire‐related processes are critical to explain the variations in coevolved soil‐forest system states across an aridity gradient and (ii) to identify the key processes and (iii) feedbacks involved. To achieve these aims, we developed a numerical model that simulates the coevolution of soil‐forest systems which employ eco‐hydro‐geomorphic processes that are typical of the flammable forests of southeastern Australia. A stepwise model evaluation, using measurements and published data, confirms the robustness of the model to simulate eco‐hydro‐geomorphic processes across the aridity gradient. Simulations that included fire replicated patterns of observed soil depth and forest cover across an aridity gradient, supporting our hypothesis. The contribution of fire to coevolution increased in magnitude with aridity, mainly due to the higher fire frequency and lower post‐fire infiltration capacity, increasing the rates of fire‐related surface runoff and erosion. Our results show that critical feedbacks between soil depth, vegetation, and fire frequency dictate the trajectory and pace of the coevolution of flammable temperate forests and soils

X-ray crystal structure of the YM210W mutant reaction centre from Rhodobacter sphaeroides.

AbstractThe X-ray crystal structure of a reaction centre from Rhodobacter sphaeroides with a mutation of tyrosine M210 to tryptophan (YM210W) has been determined to a resolution of 2.5 Å. Structural conservation is very good throughout the body of the protein, with the tryptophan side chain adopting a position in the mutant complex closely resembling that of the tyrosine in the wild-type complex. The spectroscopic properties of the YM210W reaction centre are discussed with reference to the structural data, with particular focus on evidence that the introduction of the bulkier tryptophan in place of the native tyrosine may cause a small tilt of the macrocycle of the BL monomeric bacteriochlorophyll