Abstract
BackgroundCurrently, hydrogen fuel is derived mainly from fossil fuels, but there is an increasing interest in clean and sustainable technologies for hydrogen production. In this context, the ability of some photosynthetic microorganisms, particularly cyanobacteria and microalgae, to produce hydrogen is a promising alternative for renewable, clean-energy production. Among a diverse array of photosynthetic microorganisms able to produce hydrogen, the green algae Chlamydomonas reinhardtii is the model organism widely used to study hydrogen production. Despite the well-known fact that acetate-containing medium enhances hydrogen production in this algae, little is known about the precise role of acetate during this process.ResultsWe have examined several physiological aspects related to acetate assimilation in the context of hydrogen production metabolism. Measurements of oxygen and CO2 levels, acetate uptake, and cell growth were performed under different light conditions, and oxygenic regimes. We show that oxygen and light intensity levels control acetate assimilation and modulate hydrogen production. We also demonstrate that the determination of the contribution of the PSII-dependent hydrogen production pathway in mixotrophic cultures, using the photosynthetic inhibitor DCMU, can lead to dissimilar results when used under various oxygenic regimes. The level of inhibition of DCMU in hydrogen production under low light seems to be linked to the acetate uptake rates. Moreover, we highlight the importance of releasing the hydrogen partial pressure to avoid an inherent inhibitory factor on the hydrogen production.ConclusionLow levels of oxygen allow for low acetate uptake rates, and paradoxically, lead to efficient and sustained production of hydrogen. Our data suggest that acetate plays an important role in the hydrogen production process, during non-stressed conditions, other than establishing anaerobiosis, and independent of starch accumulation. Potential metabolic pathways involved in hydrogen production in mixotrophic cultures are discussed. Mixotrophic nutrient-replete cultures under low light are shown to be an alternative for the simultaneous production of hydrogen and biomass.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-015-0341-9) contains supplementary material, which is available to authorized users.
Highlights
Hydrogen fuel is derived mainly from fossil fuels, but there is an increasing interest in clean and sustainable technologies for hydrogen production
Acetate‐containing media and low light are non‐stressful conditions that elicits H2 production Hermetically sealed vessels containing 100 ml of low cell density (10 μg chl./ml; ~3 million cells/ml) Chlamydomonas cells cultured in Tris–Acetate–Phosphate (TAP) were placed under four different light conditions (12, 22, 50 and 100 μmol photons m−2 s−1; hereafter, 12 photosynthetically active radiation (PAR), 22 PAR, 50 PAR, and 100 PAR) and dark
Most of the studies about H2 production on Chlamydomonas rely in the use of nutrient starved cultures, which reduces the viability of large-scale productions
Summary
Hydrogen fuel is derived mainly from fossil fuels, but there is an increasing interest in clean and sustainable technologies for hydrogen production. Three pathways for H2 production can be described in Chlamydomonas depending on the electron source and the electron transport pathway to HYDA1 With two of these pathways, the electrons come from photosynthetic electron transport, and they are termed photosystem II (PSII)-dependent and PSII-independent pathways. Degradation of starch is proposed as the main source for NADPH [15, 16] Both electrons routes connect to FDX1 prior to final electron donation to HYDA1. Fermentative H2 production is linked to the activity of the pyruvate:ferredoxin oxidoreductase (PFR), which can donate electrons to HYDA1 via FDX1 [20]
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