Biohydrogen production by Anabaena sp. PCC 7120 wild-type and mutants under different conditions: Light, nickel, propane, carbon dioxide and nitrogen

Abstract

Abstract Increasing awareness of environmental problems caused by the current use of fossil fuel-based energy, has led to the search for alternatives. Hydrogen is a good alternative and the cyanobacterium Anabaena sp. PCC 7120 is naturally able to produce molecular hydrogen, photosynthetically from water and light. However, this H 2 is rapidly consumed by the uptake hydrogenase. This study evaluated the hydrogen production of Anabaena sp. PCC 7120 wild-type and mutants: hupL − (deficient in the uptake hydrogenase), hoxH − (deficient in the bidirectional hydrogenase) and hupL − /hoxH − (deficient in both hydrogenases) on several experimental conditions, such as gas atmosphere (argon and propane with or without N 2 and/or CO 2 addition), light intensity (54 and 152 μEm −2 s −1 ), light regime (continuous and light/dark cycles 16 h/8 h) and nickel concentrations in the culture medium. In every assay, the hupL − and hupL − /hoxH − mutants stood out over wild-type cells and the hoxH − mutant. Nevertheless, the hupL − mutant showed the best hydrogen production except in an argon atmosphere under 16 h light/8 h dark cycles at 54 μEm −2 s −1 in the light period, with 1 μM of NiCl 2 supplementation in the culture medium, and under a propane atmosphere. In all strains, higher light intensity leads to higher hydrogen production and if there is a daily 1% of CO 2 addition in the gas atmosphere, hydrogen production could increase 5.8 times, related to the great increase in heterocysts differentiation (5 times more, approximately), whereas nickel supplementation in the culture medium was not shown to increase hydrogen production. The daily incorporation of 1% of CO 2 plus 1% of N 2 did not affect positively hydrogen production rate.