Sustainable hydrogen photoproduction by phosphorus-deprived marine green microalgae Chlorella sp

Abstract

Previously it has been shown that green microalga Chlamydomonas reinhardtii is capable of prolonged H2 photoproduction when deprived of sulfur. In addition to sulfur deprivation (-S), sustained H2 photoproduction in C. reinhardtii cultures can be achieved under phosphorus-deprived (-P) conditions. Similar to sulfur deprivation, phosphorus deprivation limits O2 evolving activity in algal cells and causes other metabolic changes that are favorable for H2 photoproduction. Although significant advances in H2 photoproduction have recently been realized in fresh water microalgae, relatively few studies have focused on H2 production in marine green microalgae. In the present study phosphorus deprivation was applied for hydrogen production in marine green microalgae Chlorella sp., where sulfur deprivation is impossible due to a high concentration of sulfates in the sea water. Since resources of fresh water on earth are limited, the possibility of hydrogen production in seawater is more attractive. In order to achieve H2 photoproduction in P-deprived marine green microalgae Chlorella sp., the dilution approach was applied. Cultures diluted to about 0.5-1.8 mg Chl·L-1 in the beginning of P-deprivation were able to establish anaerobiosis, after the initial growth period, where cells utilize intracellular phosphorus, with subsequent transition to H2 photoproduction stage. It appears that marine microalgae during P-deprivation passed the same stages of adaptation as fresh water microalgae. The presence of inorganic carbon was essential for starch accumulation and subsequent hydrogen production by microalgae. The H2 accumulation was up to 40 mL H2 gas per 1iter of the culture, which is comparable to that obtained in P-deprived C. reinhardtii culture.

Figure 1

The effect of the initial phosphates concentration on the accumulation of the biomass (measured as the final Chl concentration). Each point represents the average of three independent repetitions. The curve was drawn as nonlinear regression, modified Hyperbola III.

Figure 2

The effect of the initial culture density (measured as the total Chl concentration at the beginning of phosphorus deprivation) on accumulation of the biomass (measured as the final Chl concentration) and the total yield of H₂ gas produced by the culture. The phosphorus deprivation effect was achieved by using the dilution method. Each point represents the average of three independent repetitions. Curve that represents Chlorophyll was drown as nonlinear regression, modified Hyperbola III. For Hydrogen graph was used simple error bars curve.

Figure 3

Incubation of Chlorella sp. in TA-P/NaCl medium without CO₂ (A,C), and with CO₂ additions (B,D). At the start of incubation the gas phase was supplemented with CO₂ to 10% (v/v). Parameters that were examined: (A,B) O₂ concentration in a gas phase of vials (%) and volume of the H₂ gas produced (mL·L⁻¹ of suspension); (C,D) intracellular starch and acetate in the medium (mM·L⁻¹), total (a + b) chlorophyll (mg·L⁻¹). Figure represents data of typical experiments from three independent repetitions with the same trend. For all graphs on Figure 3 simple error bars curves were used.

Figure 4

Incubation of Chlorella sp. in TA-P/SW medium without CO₂ (A,C), and with CO₂ additions (B,D). At the start of incubation the gas phase was supplemented with CO₂ to 10% (v/v). Parameters that were examined: (A,B) O₂ concentration in a gas phase of vials (%) and volume of the H₂ gas produced (mL·L⁻¹ of suspension); (C,D) intracellular starch and acetate in the medium (mM·L⁻¹), total (a + b) chlorophyll (mg·L⁻¹). Figure represents data of typical experiments from three independent repetitions with the same trend. For all graphs on Figure 4 simple error bars curves were used.