Fermentative and Photochemical Production of Hydrogen in Algae

Abstract

The investigation reported in this paper rendered mainly two results which are of importance for the analysis of photosynthesis in chlorophyllous plants. Firstly: Unicellular algae (Scenedesmus and similar species), which under anaerobic conditions are capable of reducing carbon dioxide with molecular hydrogen in the light, will liberate hydrogen slowly in the dark if air is replaced by nitrogen in the surrbunding gas phase. This faculty for a hydrogen fermentation in Scenedesraus agrees with our knowledge of the dark metabolism of bacteria which utilize hydrogen. These reactions with hydrogen have often been found to be reversible. Secondly: Illumination of the fermenting algae enhances the liberation of hydrogen, particularly if the substrates of the' photochemical reduction process, carbon dioxide and hydrogen, are both absent. The release of hydrogen by the algae under the influence of light occurs at a rate about ten times that of the hydrogen formation in the dark. The rate, however, is already limited at low intensities by factors other than the light intensity. Experiments with specific inhibitors, like dinitrophenol, allow us to differentiate between the dark and the photochemical liberation of hydrogen.Upon return of the photosynthesizing cell to aerobic conditions both phenomena disappear as in the case of photoreduction.Besides hydrogen and carbon dioxide, the anaerobic metabolism of Scenedesmus and of similar green algae includes many other substances, notably organic acids formed by fermentation of internal or artificially provided carbohydrates. As far as the nature of these fermentation products is concerned, the present investigation was extended only to the analysis of added glucose and to the identification and determination of lactic acid. MethodsA description of the methods used in the present study has been reported elsewhere (Jack Rubin (27)), so that a few summarizing remarks will suffice.Pure cultures of Scenedesmus species D1, S. species D3, and S. obl~tuus were grown at 20°C. in inorganic saline through which a slow stream of 4 per cent CO2 in air was passed. The culture flasks, each containing 200 co. of medium, were inoculated from an agar slant and illuminated for 3 to 4 days with incandescent lamps yielding about 4,000 lux at the bottom of the flasks. The gas exchange of the algae was determined by Warburg's manometric method 219

Photoautotrophic hydrogen production by eukaryotic microalgae under aerobic conditions

“…No further hydrogen production was observed at very high partial pressures of oxygen for both C. vulgaris YSL01 and YSL16. An analogous observation was previously reported, in which hydrogen was consumed by photoheterotrophic eukaryotes under the dark/light anaerobic conditions 2,4 .…”

“…1). The biomassnormalized hydrogen production (ml of H 2 per g of biomass) was similar for elevating headspace CO 2 (Fig. 1c), but it significantly decreased from 1.8±0.01 and 1.1±0.03 to 0.6±0.02 and 0.3 ± 0.01 ml g À 1 (Po0.0001), with the elevation of O 2 levels from 5 to 15% at 10% CO 2 for YSL01 and YSL16, respectively (Fig.…”

“…We used the one way ANOVA test within the GraphPad Prism5 software to generate all P-values. Hydrogen production was even observed in the control atmospheric CO 2 and O 2 condition. Cumulative hydrogen production was relatively higher for C. vulgaris YSL01 compared with YSL16 (Fig.…”

“…M icroalgae are microorganisms that use solar energy to combine water with carbon dioxide to create biomass 1,2 . The growth kinetics of photoautotrophic algae depend on the extent of available carbon sources (for example, bicarbonate), which are mainly generated from the dissolution of atmospheric carbon dioxide [3][4][5][6] .…”

“…Green microalgae, such as Chlamydomonas reinhardtii, C. noctigama, C. moewusii and Chlorella fusca, synthesize [FeFe]-H 2 ase that is highly active in hydrogen evolution 15 . However, owing to the oxygen sensitivity of [FeFe]-H 2 ases, most studies on photosynthetic hydrogen production have largely relied on experimental manipulations, including intermittent illumination and/or anaerobic conditions 2,16 (for example, dissolved oxygen was completely removed from the culture media). A review of previous studies revealed that there are no reports on the occurrence of photoautotrophic H 2 production in the presence of CO 2 and O 2 .…”