Direct electricity production from Microalgae Choricystis sp. and investigation of the boron to enhance the electrogenic activity

In this work, a new bio-electrochemical fuel cell was constructed and the effect of boron was investigated to obtain improved electrogenic activity of a green algae (Choricystis sp.). In a specially designed bio-electrochemical fuel cell, electrode combinations of indium tin oxide (ITO) and a carbon layer are used. Open circuit potential (OCP) measurements of algal cells placed bio-electrochemical fuel cells were performed with a cyclic on-off illumination of a LED light source. The results revealed that 60 μM is the efficient boron concentration for the algal growth and pigmentation. Higher doses of boron limited the algal growth. However, the algal growth reduced slightly at higher doses, pointing out a possible boron export mechanism in green algae. OCP analysis showed that hydrogen was electro-catalytically reduced at the cathode site and an 18 mV voltage was obtained from boron-deficient (0 μM) Choricystis sp. samples. A significant enhancement in voltage output up to 33 mV was achieved from 60 μM of boron-treated algal samples. The maximum power density was calculated from the boron-treated Choricystis sp. at pseudo-steady state as 42.2 mW m⁻² at a current density of 154 mA m⁻².     

Continuous photosynthetic biohydrogen production from acetate-rich wastewater: Influence of light intensity

Photo-biohydrogen by microalgae is attractive sustainable energy caused by the utilization of solar energy and water. However, due to oxygen (O₂) sensitive hydrogenase (HydA) activity, effective control of O₂ and light intensity is critical for achieving sustainable photosynthetic hydrogen (H₂) production. Here we demonstrate continuous algal H₂ production using acetate-enriched fermenter effluent, achieving the complete O₂ cessation without sulfur depletion. Average H₂ production of 108 ± 4 μmol L^?1 for Chlamydomonas reinhardtii and 88 ± 7 μmol L^?1 for Chlorella sorokiniana at 100 μmol m^?2 s^?1 were observed for 15 days, respectively. The highest light energy to H₂ energy conversion efficiency (LHCE) of 1.61% for C. reinhardtii and 1.06% for C. Sorokiniana was obtained under low light intensity (50 μmol m^?2 s^?1) but the LHCE decreased with the increase of light intensity followed by photoinhibition, which led to a decrease of HydA activity and H₂ production. Low H₂ production was observed at 50 μmol m^?2 s^?1 under the highest LHCE, in which microalgae exhibited photoinhibition biomass growth kinetics to produce chlorophyll a (Chl a) for electron generation. These results demonstrate that light is a feasible strategy for producing electron for H₂ production under anoxygenic photosynthesis.     

Preparation of a microalgal photoanode for hydrogen production by photo-bioelectrochemical water-splitting

In this study, a microalga Tetraselmis subcordiformis (synonym: Platymonas subcordiformis)-based photoanode was prepared by a novel method developed in our lab. The optimal photocurrent density of microalgae photoanode, 37 μA/cm², was achieved under illumination of 145 μmol s⁻¹ m⁻² at anode potential of 0.5 V vs Ag|AgCl|sat. KCl, immobilized cell density of 2.08 × 10⁶/cm² and BQ concentration of 300 μmol/L. The results of measurements showed that oxygen evolution peak, hydrogen evolution peak and photocurrent response were all synchronous to light impulse in a three-electrode system. It revealed that there occurred a process of photo-bioelectrochemical water-splitting. Hydrogen can be produced by the method. The investigation for whole photo-bioelectrochemical process also indicated that the electrons for hydrogen evolution had two sources, microalgal metabolic process in dark condition and photosynthetic water oxidation. The photo-hydrogen evolution was twice more than hydrogen evolution in dark condition.     

LED light-induced enhanced photocatalytic hydrogen evolution on Au@TiO2 core–shell modified by nitrogen doping and reduced graphene oxide

This study focused on the large band gap of TiO₂ for its use as a photocatalyst under light emitting diode (LED) light irradiation. The photocatalytic activities of core–shell structured Au@TiO₂ nanoparticles (NPs), nitrogen doped Au@TiO₂ NPs, and Au@TiO₂/rGO nanocomposites (NCs) were investigated under various light intensities and sacrificial reagents. All the materials showed better photocatalytic activity under white LED light irradiation than under blue LED light. The N-doped core–shell structured Au@TiO₂ NPs (Au@N–TiO₂) and Au@TiO₂/rGO NCs showed enhanced photocatalytic activity with an average H₂ evolution rate of 9205 μmol h^?1g^?1 and 9815 μmol h^?1g^?1, respectively. All these materials showed an increasing rate of hydrogen evolution with increasing light intensity and catalyst loading. In addition, methanol was more suitable as a sacrificial reagent than lactic acid. The rate of hydrogen evolution increased with increasing methanol concentration up to 25% in DI water and decreased at higher concentrations. Overall, Au@TiO₂ core–shell-based nanocomposites can be used as an improved photocatalyst in photocatalytic hydrogen production.     

A truncated antenna mutant of Chlamydomonas reinhardtii can produce more hydrogen than the parental strain

Photoproduction of H₂ gas was examined in the Chlamydomonas reinhardtii tla1 strain, CC-4169, containing a truncated light-harvesting antenna, along with its parental CC-425 strain. Although enhanced photosynthetic performance of truncated antenna algae has been demonstrated previously (Polle et al. Planta 2003; 217:49-59), improved H₂ photoproduction has yet to be reported. Preliminary experiments showed that sulfur-deprived, suspension cultures of the tla1 mutant could not establish anaerobiosis in a photobioreactor, and thus, could not photoproduce H₂ gas under conditions typical for the sulfur-deprived wild-type cells (Kosourov et al. Biotech Bioeng 2002; 78:731-40). However, they did produce H₂ gas when deprived of sulfur and phosphorus after immobilization within thin (∼300 μm) alginate films. These films were monitored for long-term H₂ photoproduction activity under light intensities ranging from 19 to 350 μE m⁻² s⁻¹ PAR. Both the tla1 mutant and the CC-425 parental strain produced H₂ gas for over 250 h under all light conditions tested. Relative to the parental strain, the CC-4169 mutant had lower maximum specific rates of H₂ production at low and medium light intensities (19 and 184 μE m⁻² s⁻¹), but it exhibited a 4-times higher maximum specific rate at 285 μE m⁻² s⁻¹ and an 8.5-times higher rate at 350 μE m⁻² s⁻¹ when immobilized at approximately the same cell density as the parental strain. As a result, the CC-4169 strain accumulated almost 4-times more H₂ than CC-425 at 285 μE m⁻² s⁻¹ and over 6-times more at 350 μE m⁻² s⁻¹ during 250-h experiments. These results are the first demonstration that truncating light-harvesting antennae in algal cells can increase the efficiency of H₂ photoproduction in mass culture at high light intensity.     

Hydrogen production from aqueous triethanolamine solution using Eosin Y-sensitized ZnO photocatalyst doped with platinum

Photocatalytic system for hydrogen production comprising ZnO as a photocatalyst, Eosin Y photo-sensitizer, triethanolamine electron donor and platinum co-catalyst is prepared and systematically tested under visible and simulated solar light irradiation. The results of laboratory experiments show that all studied parameters, such as a solution pH, Pt/ZnO dose, triethanolamine concentration and light intensity, notably influence the rate of photocatalytic hydrogen generation. The maximum hydrogen generation rate of 6.50 μmol min⁻¹ is achieved at pH 7.0 and 2 g L⁻¹ ZnO loaded with 0.75 wt% platinum, and 0.757 M triethanolamine concentration. The rate of hydrogen production as a function of triethanolamine concentration follows Langmuir-Hinshelwood kinetic model with reaction rate constant 4.50 μmol min⁻¹ and adsorption constant 14.84 M⁻¹ in solar light and 5.26 μmol min⁻¹ and 6.67 M⁻¹ in visible light, respectively. The reaction mechanism of hydrogen generation in the tested system is proposed and discussed.     

Isochrysis sp. IOAC724S,a newly isolated,lipid-enriched,marine microalga for lipid production,and optimized cultivation conditions

An oleaginous, unicellular, marine microalga termed IOAC724S was isolated from the South China Sea. Morphology and genetic analyses indicated it belongs to the genus Isochrysis. Gas chromatography (GC) results showed that more than 10 types of fatty acids existed in Isochrysis sp. IOAC724S and that 90% of them were suitable for lipid production. The culture conditions suitable for cell growth were progressively optimized through photosynthetic and respiratory analyses. The optimal culture conditions were: photon flux 200–500 μmol m⁻² s⁻¹, temperature 35 °C during daytime and 24 °C at night, pH value between 7 and 8, NaNO₃ 160 g m⁻³ and NaH₂PO₄·2H₂O 80 g m⁻³ for starting culture. When microalgal cultures were exposed to these optimal conditions, the specific growth rate reached to 0.26 d⁻¹ on average and 1.0 d⁻¹ in MAX. Lipid production was optimized through nutrient starvation processes, including nitrate or phosphate deprivation and simultaneous nitrate and phosphate deprivation. The highest lipid mass fraction of dry cell weight (about 55.6%) was obtained after the stationary phase algal culture was transferred into phosphate-free medium for 3 days. GC data demonstrated that the enhancement of lipid accumulation in algal cells maintained under nutrient starvation came mainly from an increase of C16:0 and C18:1 fatty acids; however, the lipids with a chain length appropriate for fuel use (C14 to C18) were unchanged at 90% mass fraction of the dry cell weight. Based on these good characteristics, Isochrysis sp. IOAC724S appeared to be a strong candidate for lipid production.     

“Effect of light intensity and the light: dark cycles on the long term hydrogen production of Chlamydomonas reinhardtii by batch cultures”

The photomixotrophic hydrogen production was investigated in sulfur deprived Chlamydomonas reinhardtii cultures. The cultures were exposed to continuous illumination of various light intensities in 27-day batches. Light intensity of 70 × 2 ??E m⁻² s⁻¹ was selected for hydrogen production. Subsequent experiments involving 27-day long light:dark cycles were conducted at the selected light intensity. The cycles consisted of hour divisions (h:h; 18:6, 14:10, 12:12) or minute divisions (min:min; 45:15, 35:25, 30:30). The results showed an adverse effect of the light:dark cycles on hydrogen production. All experiments, irrespective of the type of illumination indicated that cultures needed a lag phase for production and the highest hydrogen production was obtained during first 7-10 days of production reaching a peak in the first 5 days.     

Photocatalytic hydrogen production on the hetero-junction CuO/ZnO

The co-precipitation is successfully used for the synthesis of the hetero-junction CuO/ZnO. Thermal analysis, ATR spectroscopy and diffuse reflectance were used to assess the photoactivity of the hetero-system for the hydrogen formation upon visible light. As expected, the X-ray diffraction shows mixed phases of CuO (tenorite) and ZnO (Wurtzite). The specific surface area is around ~7 m² g⁻¹ with a crystallite size lying between 20 and 49 nm. The diffuse reflectance indicates an indirect transition at 3.13 eV for ZnO and a direct transition at 1.60 eV for the sensitizer CuO. The capacitance-potential (C⁻² - E) characteristic of CuO plotted in Na₂SO₄ electrolyte exhibits p-type comportment with a flat band potential of −0.315 V_RHE and a holes concentration of 8.7 × 10¹⁸cm⁻³. The Electrochemical Impedance Spectroscopy exhibits a semicircle characteristic of the bulk material with an impedance of 1725 Ω cm² which decreases down to 453 Ω cm² under irradiation, supporting the semiconducting character of CuO. ZnO mediates the electrons transport thanks to its conduction band, formed from Zn²⁺: 4s orbital (−0.92 V_RHE); it is positioned cathodically with respect to the H₂O/H₂ level (~-0.74 V_RHE), producing a H₂ evolution under visible light illumination. The performance peaks at pH ~7 on 5% CuO/ZnO for a catalyst dose of 0.25 mg of catalyst/mL of solution in the presence of SO₃²⁻ as holes scavenger. A liberation rate of 340 μmol h⁻¹ (g of catalyst)⁻¹ is obtained with a quantum efficiency of 0.38% under a photons flux of 2.09 × 10¹⁹ s⁻¹.     

Photocatalytic hydrogen evolution assisted by aqueous (waste)biomass under simulated solar light: Oxidized g-C3N4 vs. P25 titanium dioxide

Oxidized graphitic carbon nitride (o-g-C₃N₄) and Evonik AEROXIDE^® P25 TiO₂ were compared for lab-scale photocatalytic H₂ evolution from aqueous sacrificial biomass-derivatives, under simulated solar light. Experiments in aqueous starch using Pt or Cu–Ni as the co-catalysts indicated that H₂ production is affected by co-catalyst type and loading, with the greatest hydrogen evolution rates (HER) up to 453 and 806 μmol g⁻¹ h⁻¹ using TiO₂ coupled with 3 wt% Cu–Ni or 0.5 wt% Pt, respectively. Despite the lower surface area, o-g-C₃N₄ gave HERs up to 168 and 593 μmol g⁻¹ h⁻¹ coupled with 3 wt% Cu–Ni or 3 wt% Pt. From mono- and di-saccharide solutions, H₂ evolution was in the range 504–1170 μmol g⁻¹ h⁻¹ for Pt/TiO₂ and 339–912 μmol g⁻¹ h⁻¹ for Cu–Ni/TiO₂, respectively; o-g-C₃N₄ was efficient as well, providing HERs of 90–610 μmol g⁻¹ h⁻¹. The semiconductors were tested in sugar-rich wastewaters obtaining HERs up to 286 μmol g⁻¹ h⁻¹. Although HERs were lower compared to Pt/TiO₂, a cheap, eco-friendly and non-nanometric catalyst such as o-g-C₃N₄, coupled to non-noble metals, provided a more sustainable H₂ evolution.     

Enhancement of biomass production in Scenedesmus bijuga high-density culture using weakly absorbed green light

To increase microalgae biomass production and support high density cultures in photobioreactors artificial illumination systems have been designed to increase photosynthetic activity. Supplemental lighting systems are commonly composed by a combination of chlorophyll (a + b) strongly absorbed wavelengths, while weakly absorbed wavelengths are not present. At this work we compared the photosynthetic activity and biomass production induced by chlorophyll (a + b) strongly versus weakly absorbed wavelengths in Scenedesmus bijuga microalgae cultures at different biomass densities. Photosynthetic activity and biomass production induced by 4 different wavelengths using LEDs (blue – peak at λ470 nm; green – peak at λ530 nm; red – peak at λ655 nm; and white-4100 K) were measured and analyzed on high-density cultures of S. bijuga. As culture density increased the chlorophyll (a + b) weakly absorbed green light penetrated deeper into the samples inducing higher oxygen evolution at culture concentration of 1.45 g L⁻¹ compared to the chlorophyll (a + b) strongly absorbed red light. High-density culture (2.19 g L⁻¹) cultivated under green light showed higher biomass production rate (30 mg L⁻¹ d⁻¹) with a 8.43% biomass growth in a 6-day period compared to the same quantum flux of red light that induced 4.35% biomass growth on the same period. The integration of green LEDs into photobioreactors lighting apparatus could improve the existing systems composed predominantly by red and blue LEDs increasing biomass productivity of high-density cultures at latter stages of microalgae cultivation.     

Construction of Cu7.2S4/g-C3N4 photocatalyst for efficient NIR photocatalysis of H2 production

Graphite-like carbon nitride (g-C₃N₄) has been regarded as a promising photocatalyst for solar-to-chemical conversion. Nevertheless, the narrow absorption of light extremely limited its photocatalytic performance under near-infrared (NIR) irradiation. Herein, the Cu₇.₂S₄ with outstanding NIR absorption was successfully introduced to g-C₃N₄ nanosheets through a simple in-situ growth procedure. As expected, the constructed Cu_7.2S₄/g-C₃N₄ (CSCN) photocatalysts exhibit superior H₂ production activity of 82 μmol g⁻¹ h⁻¹ under NIR light irradiation (λ > 800 nm), which outperforms currently reported g–C₃N₄–based NIR-driven H₂ production systems. Especially, the optimal sample CSCN-5 displays a robust activity of 66 μmol g⁻¹ h⁻¹ at λ = 850 nm monochromatic light irradiation. The excellent photocatalytic performance is linked to the extended optical absorption as well as the efficient separation efficiency of photoinduced carriers, which are evidenced by the UV-visible absorption spectroscopy and photoelectrochemical test. This work provides an effective approach for constructing a Cu_7.2S₄/g-C₃N₄ photocatalytic system for the transformation of NIR solar energy into hydrogen.     

Green synthesized carbon quantum dots as TiO2 sensitizers for photocatalytic hydrogen evolution

Carbon quantum dots (CQDs) have attracted growing interest due to their superior luminescent properties, which make them excellent photosensitizers for TiO₂. This study presents the green-synthesis of CQDs from edible mushroom Agaricus bisporus through microwave irradiation. In the study as-synthesized CQDs were used as a sensitizer for TiO₂ in photocatalytic hydrogen evolution in aqueous triethanolamine (sacrificial reagent) under visible-light irradiation. Photocatalytic hydrogen production activity of CQD-sensitized TiO₂ was found to be 472 μmol g⁻¹ h⁻¹ (without loading any noble metal co-catalyst) and 1458 μmol g⁻¹ h⁻¹ (with loading Pt co-catalyst). The study revealed that the CQDs from mushroom A. bisporus can be used as an efficient sensitizer for TiO₂ in photocatalytic hydrogen production.     

(In,Cu) Co-doped ZnS nanoparticles for photoelectrochemical hydrogen production

In and Cu co-doped ZnS nanoparticles were successfully synthesized in DI water and ethanol solvent by a sonochemical approach using citric acid as surfactants in aqueous medium. FESEM micrographs show that In and Cu co-doped ZnS crystallites have a rough surface nanostructure and the as-synthesized photocatalysts were tested for the photocatalytic hydrogen evolution from water splitting via the irradiation of simulated sunlight. Among In and Cu co-doped ZnS products, 4In4CuZnS photocatalyst can achieve the maximum hydrogen production rate (752.7 μmol h⁻¹ g⁻¹) in 360 min under simulated sunlight illumination. Meanwhile, we separated the hydrogen and oxygen cells using an ion exchange membrane. Both electrodes (working electrode and Pt electrode) are dipped into each cell containing an aqueous solution containing 0.1 M Na₂S at pH 3 to convert water into hydrogen and oxygen under solar irradiation. As expected, the photoelectrochemical water splitting cells could significantly improve the photocatalytic activity, where the 4In4CuZnS nanoparticles shows the photoelectrochemical performance with photocurrent density of 12.2 mA cm⁻² at 1.1 V and hydrogen evolution rate of 1189.4 μmol h⁻¹ g⁻¹.     

Renewable algal photo H2 production without S control using acetate enriched fermenter effluents

Renewable energy production using microorganisms is one of the challenging issues for environmental sustainability. Algal hydrogen (H₂) production has often been achieved by sulfur (S) and chloride ion (Cl^?) deprivation in a growth medium; however, it may not be realistic to control S or Cl^? concentrations in natural sources (e.g., wastewater). In this study, two different green algal species, Chlamydomonas reinhardtii and Chlorella sorokiniana were selected and their photosynthetic activities were compared with different acetate/Cl^? ratios both in batch and continuous modes. At 150 of acetate/Cl^? ratio, the H₂ production rates were 0.25–0.33 μmol L^?1 min^?1 for C. sorokiniana and 0.20–0.38 μmol L^?1 min^?1 for C. reinhardtii, respectively. The hydrogenase (HydA) reactivation and photosystem II (PSII) inhibitor test revealed that biohydrogen production by algae is due to photosynthetic activity. It was found that maintaining acetate/Cl^? ratios greater than 60–100 leads to continuous O₂ depletion and thus renewable H₂ production for both algal species. Molecular dynamics (MD) simulations of hydrogen bonding between Y_z and His₁₉₀ in PSII supported O₂ inhibition using acetate. Using fermenter effluents, C. sorokiniana and C. reinhardtii showed a successful continuous H₂ production of ~80 μmol L^?1 and ~95 μmol L^?1, respectively, for 15 days.     

Two-stage photoautotrophic cultivation to improve carbohydrate production in Chlamydomonas reinhardtii

A two-stage strategy was developed to improve the microalgal carbohydrate accumulation for advanced biofuel production. In the first stage, Chlamydomonas reinhardtii CC125 was cultivated in photo-bioreactors by repeated fed batch operation to improve the biomass production. Optimal culture conditions achieved in the batch operation were applied to the repeated fed batch operation. Biomass productivity reached 0.47 g L⁻¹ d⁻¹ with 70% medium replacement ratio and 5% CO₂ under continuous light of 135 μmol m⁻² s⁻¹. In the second stage, reducing CO₂ content to 0.04% led to a high carbohydrate content of 71%, showing more than 9 times improvement compared to that in the biomass from the first stage culture. These results suggest that photoautotrophic two-stage cultivation is an effective approach to accumulate microalgal carbohydrate as a feedstock for biofuel production.     

Facile fabrication of nickel/porous g-C3N4 by using carbon dot as template for enhanced photocatalytic hydrogen production

Ni/porous g-C₃N₄ was prepared by high temperature thermal polymerization process using carbon dots as soft template and photodeposition. With nickel nanoparticles supported as co-catalyst, the hydrogen evolution reaction (HER) activity of the photocatalyst has been significantly enhanced under visible light, which is up to 1273.58 μmol g⁻¹ h⁻¹, superior to pristine g-C₃N₄ (4.12 μmol g⁻¹ h⁻¹). This is attributed to the inhibited recombination of photogenerated electron-hole pairs and the much better electron transport efficiency. The formed porous structure of carbon nitride could facilitate light utilization and together with nickel nanoparticles, better charge separation can be realized which are proved by the photoluminescence, time-resolved photoluminescence spectra, transient photocurrent measurements and electrochemical impendence spectroscopy. This work provides a useful route to obtain less expensive and efficient photocatalyst containing no noble metals for hydrogen production.     

Application of experimental design methodology for optimization of biofuel production from microalgae

Optimization of biofuel productivity, in terms of lipid content, polysaccharide content, and calorific value, from microalgae was performed by varying four variables (temperature, light intensity, nitrogen content, and CO₂ addition) using a 2⁴ full factorial design. A statistical analysis showing the influence of each variable and their interactions was conducted. The selected variables all influence biofuel productivity, but their importance varies according to the sequence: CO₂ addition > temperature > nitrogen content > light intensity. Interactive effects of temperature with light intensity and nitrogen with CO₂ addition for lipid and polysaccharide productivities were identified, respectively. For calorific value, interactive effects of CO₂ addition with light intensity and nitrogen content were observed. The highest biofuel productivity was obtained at the following conditions: temperature (>25 °C), light intensity (>60 μmol photons m⁻² s⁻¹), nitrogen content (<50 mg L⁻¹), and CO₂ addition (>18 mL L⁻¹ d⁻¹). 10 days was found to be the most favorable cultivation time for lipid production under the investigated conditions.     

Photoautotrophic hydrogen production by nitrogen-deprived Chlamydomonas reinhardtii cultures

In this study we have demonstrated the possibility of phototrophic hydrogen production in C. reinhardtii under N-deprived conditions. When tested under air + CO₂, and Ar + CO₂ N-deprived C. reinhardtii demonstrated decrease in PSII activity mainly due to over reduction of PQ, in addition no ascorbate accumulation was observed in cells. Under air + CO₂ atmosphere cells accumulated excessive amounts of starch. When incubated under Ar + CO₂ atmosphere cells accumulated starch as nitrogen replete cultures and no hydrogen production was observed. Hydrogen production (86 ml H₂ per one l of culture) occurred under Ar + CO₂ atmosphere when particular two-step illumination protocol was implicated. In oxygen producing and early oxygen consuming stage cells were illuminated under light intensity 169 μE m^?2 s^?1. When light was switched to 30 μE m^?2 s^?1, cultures quickly respired all oxygen and transient to anaerobic conditions with subsequent hydrogen production 2 h later. Actual quantum yield of C. reinhardtii cultures was measured in photobioreactor and maximal quantum efficiency of PSII of dark adapted cells together with JIP test were studied.     

Optimization of photosynthetic hydrogen yield from platinized photosystem I complexes using response surface methodology

Light-dependent hydrogen production by platinized Photosystem I isolated from the cyanobacterium Thermosynechococcus elongatus BP-1 was optimized using response surface methodology (RSM). The process parameters studied included temperature, light intensity and wavelength, and platinum salt concentration. Application of RSM generated a model that agrees well with the data for H₂ yield (R² = 0.99 and p < 0.001). Significant effects on the total H₂ yield were seen when the platinum salt concentration and temperature were varied during platinization. However, light intensity during platinization had a minimal effect on the total H₂ yield within the region studied. The values of the parameters used during the platinization that optimized the production of H₂ were light intensity of 240 μE m⁻² s⁻¹, platinum salt concentration of 636 μM and temperature of 31 °C. A subsequent validation experiment at the predicted conditions for optimal process yield gave the maximum H₂ yield measured in the study, which was 8.02 μmol H₂ per mg chlorophyll.