Our project goal is to maximize the photosynthetic productivity for a culture of Rhodobacter sphaeroides under both high and low light intensities in a bioreactor by synthetically regulating the size of the light-harvesting antenna. We chose to undertake such a project in R. sphaeroides due to its well-characterized photosynthetic and genetic system.
The antenna system functions to expand the spectrum of light available for photosynthesis by absorbing different wavelengths than that of the reaction center. Essentially, it is like the dish around the main receiver of any antenna. Marine bacteria, such as R. sphaeroides, evolved very large antenna complexes to absorb light in a natural environment where there is great competition for photons. As a result, the photosynthetic machinery is saturated at a rather low light intensity in a synthetic non-competitive environment, such as a bioreactor. This causes up to 95% of incidental photons to be dissipated as heat or fluorescence by the bacteria at the exterior of a bioreactor through a process called Non-Photochemical Quenching (NPQ) (Mussgnug et al., 2007). In essence, these photons are being wasted as NPQ reduces light penetration into a bioreactor and starves cells on the interior for photons. One method that has been shown to improve photosynthetic efficiency is the reduction of light-harvesting antenna sizes (Polle et al., 2002, Mussgnug et al., 2007). Though, current approaches to this end rely on genetic knockouts and as such are difficult to precisely control from the perspective of metabolic engineering and synthetic biology. Our intention is to create a more dynamic system to vary antenna size that is dependent on incidental light intensity and that can be readily optimized using synthetic biology principles. This system should result in the bacteria at the exterior of the bioreactor expressing fewer light harvesting antenna proteins than the cells at the interior, reducing NPQ while maintaining a high absorbance of incidental photons.
We will focus on altering the quantity of the Light Harvesting Complex 2 (LH2) by regulating the pucB/A genes that code for the two subunits of the complex. LH2 absorbs photons maximally at the wavelength of 842 nm and funnels its energy to LH1 and the reaction center. The ratio of LH2 complexes to LH1 naturally ranges from 3.0 to 6.7 under varying light conditions (Scheuring et al., 2005). We propose that if this ratio of LH2 to LH1 is altered to range from 0-7 or more in response to incidental light intensity, then the photosynthetic efficiency and productivity for a culture of R. sphaeroides may be maximized.
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