Abstract
Hydrogen is a promising future energy source. Although the ability of green algae to produce hydrogen has long been recognized (since 1939) and several biotechnological applications have been attempted, the greatest obstacle, being the O2-sensitivity of the hydrogenase enzyme, has not yet been overcome. In the present contribution, 75 years after the first report on algal hydrogen production, taking advantage of a natural mechanism of oxygen balance, we demonstrate high hydrogen yields by lichens. Lichens have been selected as the ideal organisms in nature for hydrogen production, since they consist of a mycobiont and a photobiont in symbiosis. It has been hypothesized that the mycobiont’s and photobiont’s consumption of oxygen (increase of COX and AOX proteins of mitochondrial respiratory pathways and PTOX protein of chrolorespiration) establishes the required anoxic conditions for the activation of the phycobiont’s hydrogenase in a closed system. Our results clearly supported the above hypothesis, showing that lichens have the ability to activate appropriate bioenergetic pathways depending on the specific incubation conditions. Under light conditions, they successfully use the PSII-dependent and the PSII-independent pathways (decrease of D1 protein and parallel increase of PSaA protein) to transfer electrons to hydrogenase, while under dark conditions, lichens use the PFOR enzyme and the dark fermentative pathway to supply electrons to hydrogenase. These advantages of lichen symbiosis in combination with their ability to survive in extreme environments (while in a dry state) constitute them as unique and valuable hydrogen producing natural factories and pave the way for future biotechnological applications.
Highlights
Lichens are the symbiotic phenotype of nutritionally specialized fungi that acquire, in an ecologically obligate, mutualistic symbiosis, fixed carbon from a population of minute green algal and/or cyanobacterial cells [1,2]
The oxygen-sensitivity of hydrogenase is the main obstacle in photosynthetic hydrogen production by green algae
Lichens are symbiotic organisms consisting of a mycobiont that consumes oxygen and a photobiont that in the case of green algae has the ability to produce hydrogen in a hermitically closed system
Summary
Lichens are the symbiotic phenotype of nutritionally specialized fungi (the mycobiont) that acquire, in an ecologically obligate, mutualistic symbiosis, fixed carbon from a population of minute green algal and/or cyanobacterial cells (the photobiont) [1,2]. Concerning photobionts, about 85% of lichen-forming fungi associate with green algae, about 10% with cyanobacteria and about 4% simultaneously with both [3]. The majority of lichen-forming fungi form crustose, often quite inconspicuous thalli on or within the substratum, and only near 25% of lichen mycobionts form shrubby, leaf- or band-shaped, erect or pendulous thalli, usually known as macrolichens [5]. This symbiotic phenotype proved to be so successful that lichens dominate close to 10% of the earth's terrestrial ecosystems, which encompass areas, such as tundra, where higher plants are at their physiological limits
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