Abstract
Microbial electrolysis cells (MECs) represent an emerging technology that uses heterotrophic microbes to convert organic substrates into fuel products, such as hydrogen gas (H2). The recent development of biophotovoltaic cells (BPVs), which use autotrophic microbes to produce electricity with only light as a substrate, raises the possibility of exploiting similar systems to harness photosynthesis to drive the production of H2. In the current study we explore the capacity of the cyanobacterium Synechocystis sp. PCC 6803 to generate electrons by oxygenic photosynthesis and facilitate H2 production in a two-chamber bio-photoelectrolysis cell (BPE) system using the electron mediator potassium ferricyanide ([Fe(CN)6]3−). The performance of a wild-type and mutant strain lacking all three respiratory terminal oxidase activities (rto) was compared under low or high salt conditions. The rto mutant showed a decrease in maximum photosynthetic rates under low salt (60% lower Pmax than wild-type) but significantly increased rates under high salt, comparable to wild-type levels. Remarkably, rto demonstrated a 3-fold increase in (Fe[CN]6)3− reduction rates in the light under both low and high salt compared to the wild-type. Yields of H2 and efficiency parameters were similar between wild-type and rto, and highest under high salt conditions, resulting in a maximum rate of H2 production of 2.23 ± 0.22 ml H2 l−1 h−1 (0.68 ± 0.11 mmol H2 [mol Chl]−1 s−1). H2 production rates were dependent on the application of a bias-potential, but all voltages used were significantly less than that required for water electrolysis. These results clearly show that production of H2 using cyanobacteria is feasible without the need to inhibit photosynthetic O2 evolution. Optimising the balance between the rates of microbial-facilitated mediator reduction with H2 production may lead to long-term sustainable H2 yields.
Highlights
Hydrogen gas (H2) has many merits as a clean energy resource
In the bio-photoelectrolysis cell (BPE) system used here (Fig. 1), photosynthesis and respiration can be considered as the primary sources of electrons for H2 production
It is likely that (Fe[CN]6)3À is still reduced in the anodic chamber during the production of H2, the time taken for complete oxidation of (Fe[CN]6)4À depended on the biaspotential applied, with a maximum rate of 2.23 Æ 0.22 ml H2 lÀ1 hÀ1 for ca. 4 hours
Summary
Hydrogen gas (H2) has many merits as a clean energy resource. there are several chemical processes by which H2 can be produced,[1] microbial biotechnologies are attractive as they are renewable and o en relatively cheap to maintain. Far, sustained H2 production with algae (e.g. Chlamydomonas sp.) has been achieved only in the presence of a major decrease in activity of photosystem II – the O2-evolving complex of the photosynthetic apparatus. This was achieved using sulphur deprivation to decrease the O2 production rate below the rate of O2 use through mitochondrial respiration.[5,7,8] To produce H2 with lamentous cyanobacteria, such as Anabaena sp., nitrogen sources in the culture media must be depleted to allow H2-producing heterocysts to form.[4] Unicellular cyanobacteria require the removal of O2 from the growth medium for sustained H2 production.[4,9] Extensive work in this eld has been carried out to identify phenotypes/mutants with increased O2 tolerance and H2 yields.[6,9,10,11,12,13]
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