Photobiological hydrogen production from the cyanobacterium Synechocystis sp. PCC 6803 encapsulated in sol-gel processed silica Public Deposited

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  • Photobiological hydrogen production from live cells of the cyanobacterium Synechocystis sp. PCC 6803 encapsulated in silica gel was investigated. Hydrogen is a dense energy carrier with potential to reduce dependence on fossil fuels, provided a renewable and sustainable means of production can be achieved. Under certain conditions, Synechocystis sp. PCC 6803 can produce molecular hydrogen through the activity of a reversible [Ni-Fe] hydrogenase using energy derived from photosynthesis. The current work improved hydrogen production through encapsulation of viable cells in silica gel and through manipulation of the phycobilisomes (PBS), the light harvesting antennae. First, various formulations of silicon alkoxide-derived gels were screened with a high throughput screening assay to verify the organism could be successfully encapsulated and hydrogen production was comparable to similar cells in liquid culture. It was found that wild-type (wt) Synechocystis sp. PCC 6803 and a mutant known as M55 are both amenable to encapsulation within gels derived from tetraethoxysilane (TEOS) precursors of low silica content, and observed hydrogen production was comparable to liquid cultures. Subsequent investigations explored the effects of encapsulation stress and possible effects of gel additives on the photosynthetic activity and efficiency of this organism. Synechocystis sp. PCC 6803 was found to be moderately robust against salt stress, somewhat sensitive to ethanol stress, and very sensitive to osmotic stresses and interference with excitation transfer created by polyethylene glycol and glycerol, respectively. Finally, three PBS mutants known as ΔcpcAB, ΔapcE, and ΔapcF were evaluated for glycogen accumulation, pigment content, hydrogen production, photosynthetic activity, and suitability for silica sol-gel encapsulation. The ΔcpcAB strain lacks PBS rods, the ΔapcE strain completely lacks a functional PBS, and the ΔapcF strain has a functional PBS with disrupted excitation transfer to the photosystems. Under 200 μEm⁻²s⁻¹ light, all strains accumulated similar levels of glycogen, and the ΔapcE and ΔapcF strains produced similar levels of hydrogen while the ΔcpcAB strain produced less hydrogen compared to wt cells, although its photosynthetic efficiency was actually improved. Under 400μEm⁻²s⁻¹ light, the ΔapcE and ΔapcF mutants both produced approximately 25% more hydrogen than wt cells from the same amount of glycogen, accumulating a 1.2% headspace hydrogen concentration. Hydrogen production from cells encapsulated in gels derived from aqueous precursors was approximately 3.5 times higher than similar cells in liquid culture, and more than double that from cells in alkoxide-derived gels. This investigation demonstrates the efficacy of silica sol-gel encapsulation as a tool to enhance photobiological hydrogen production from Synechocystis sp. PCC 6803 and that manipulation of photoantennae can lead to improved hydrogen production.
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