Low oxygen levels contribute to improve photohydrogen production in mixotrophic non-stressed Chlamydomonas cultures

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.

H2 production pathways in nutrient-replete mixotrophic Chlamydomonas cultures under low light. Response to the commentary article “On the pathways feeding the H2 production process in nutrient-replete, hypoxic conditions,” by Alberto Scoma and Szilvia Z. Tóth

“…AQThese data are in good agreement with those published by Jurado-Oller et al [3], who reported H 2 production in sealed cultures after 24 h (≈7 ml l −1 ). Both studies reported that H 2 production lasted less than 24 h, and afterwards no more H 2 production was observed during the next 10 days.…”

“…Note that in the report of Jurado-Oller et al [3], control cultures incubated in dark showed a substantially lower H 2 production than cultures incubated in low light, indicating a minor role for the fermentative H 2 production pathway operating during low light conditions. As explained in more detail below, the contribution of the PSII-independent pathway (deduced from the DCMU treatments) could be underestimated when oxygen availability of the cultures is very limited.…”

“…It is possible that oxygen levels in the cultures can determine the degree of inhibition caused by DCMU [3]. The effect caused by DCMU could not only be linked to the inhibition of the PSII activity per se and to the concomitant loss of electrons that can enter in the photosynthetic chain, but also to the elimination of the main source of oxygen in sealed cultures.…”

“…Moreover, Jurado-Oller et al [3] also demonstrated that acetate uptake is greatly dependent on oxygen availability. Hence, the effect of DCMU can also affect greatly the acetate uptake rates depending on the different oxygenation regimes of the cultures.…”

Produção de hidrogênio por <i>Chlamydomonas </i>spp. e <i>Anabaena</i> spp.

“…Além da ausência do enxofre, a presença do ácido acético nos cultivos com Chlamydomonas são de extrema importância para a produção de hidrogênio, pois o seu consumo pela alga, permite melhor crescimento da cultura e auxilia a proporcionar ambiente anóxico (KOSOUROV et al, 2003;FOUCHARD et al, 2005;JURADO-OLLER et al, 2015). Existem pesquisas que relatam que os cultivos mixotróficos produzem mais hidrogênio do que os autotróficos KOSOUROV et al, 2007;MEUSER et al, 2009;BALLESTER et al, 2015).…”