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Photon echo studies of photosynthetic light harvesting.

ABSTRACT: The broad linewidths in absorption spectra of photosynthetic complexes obscure information related to their structure and function. Photon echo techniques represent a powerful class of time-resolved electronic spectroscopy that allow researchers to probe the interactions normally hidden under broad linewidths with sufficient time resolution to follow the fastest energy transfer events in light harvesting. Here, we outline the technical approach and applications of two types of photon echo experiments: the photon echo peak shift and two-dimensional (2D) Fourier transform photon echo spectroscopy. We review several extensions of these techniques to photosynthetic complexes. Photon echo peak shift spectroscopy can be used to determine the strength of coupling between a pigment and its surrounding environment including neighboring pigments and to quantify timescales of energy transfer. Two-dimensional spectroscopy yields a frequency-resolved map of absorption and emission processes, allowing coupling interactions and energy transfer pathways to be viewed directly. Furthermore, 2D spectroscopy reveals structural information such as the relative orientations of coupled transitions. Both classes of experiments can be used to probe the quantum mechanical nature of photosynthetic light-harvesting: peak shift experiments allow quantification of correlated energetic fluctuations between pigments, while 2D techniques measure quantum beating directly, both of which indicate the extent of quantum coherence over multiple pigment sites in the protein complex. The mechanistic and structural information obtained by these techniques reveals valuable insights into the design principles of photosynthetic light-harvesting complexes, and a multitude of variations on the methods outlined here.

SUBMITTER: Read EL

PROVIDER: S-EPMC2744827 | biostudies-other | 2009 Aug-Sep

REPOSITORIES: biostudies-other

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Project description:BACKGROUND: Photosynthetic light-harvesting proteins are the mechanism by which energy enters the marine ecosystem. The dominant prokaryotic photoautotrophs are the cyanobacterial genera Prochlorococcus and Synechococcus that are defined by two distinct light-harvesting systems, chlorophyll-bound protein complexes or phycobilin-bound protein complexes, respectively. Here, we use the Global Ocean Sampling (GOS) Project as a unique and powerful tool to analyze the environmental diversity of photosynthetic light-harvesting genes in relation to available metadata including geographical location and physical and chemical environmental parameters. METHODS: All light-harvesting gene fragments and their metadata were obtained from the GOS database, aligned using ClustalX and classified phylogenetically. Each sequence has a name indicative of its geographic location; subsequent biogeographical analysis was performed by correlating light-harvesting gene budgets for each GOS station with surface chlorophyll concentration. CONCLUSION/SIGNIFICANCE: Using the GOS data, we have mapped the biogeography of light-harvesting genes in marine cyanobacteria on ocean-basin scales and show that an environmental gradient exists in which chlorophyll concentration is correlated to diversity of light-harvesting systems. Three functionally distinct types of light-harvesting genes are defined: (1) the phycobilisome (PBS) genes of Synechococcus; (2) the pcb genes of Prochlorococcus; and (3) the iron-stress-induced (isiA) genes present in some marine Synechococcus. At low chlorophyll concentrations, where nutrients are limited, the Pcb-type light-harvesting system shows greater genetic diversity; whereas at high chlorophyll concentrations, where nutrients are abundant, the PBS-type light-harvesting system shows higher genetic diversity. We interpret this as an environmental selection of specific photosynthetic strategy. Importantly, the unique light-harvesting system isiA is found in the iron-limited, high-nutrient low-chlorophyll region of the equatorial Pacific. This observation demonstrates the ecological importance of isiA genes in enabling marine Synechococcus to acclimate to iron limitation and suggests that the presence of this gene can be a natural biomarker for iron limitation in oceanic environments.

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Photon echo studies of photosynthetic light harvesting.

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