Fig. 2. Hypoxic photosynthetic hydrogen production in surface-augmented algal cells.

a Schematic illustration of photosynthesis in PPy/CaCO₃-coated Chlorella cells. Hypoxic conditions are generated by depletion of oxygen levels due to Fe(III)/PPy-mediated oxidation of ascorbate (shown as molecular graphic) present in the external environment, giving rise to hydrogenase activation and hydrogen production (paired cyan circles). The decrease of pH arises from the oxidation of added ascorbate and is buffered by the CaCO₃ shell, which also facilitates prolonged hydrogenase activity. b–e Time-dependent measurements of dissolved oxygen concentration (b), hydrogen production (c), pH (d) and photosynthetic activity (F_v/F_m) (e) in an ascorbate-containing TAP culture medium in the presence of native, PPy-coated or PPy/CaCO₃-coated Chlorella cells. Data are presented as mean values ± SD, error bars indicate standard deviations (n = 3, biologically independent samples). Both the PPy- and PPy/CaCO₃-coated algal cells generate hypoxic conditions and photosynthetically produce hydrogen (b, c). The CaCO₃ exoskeleton acts as a pH buffer against acidification of the environment (d) and enhances hydrogen production (c). f Plots of average production rate for the natural instantaneous biomass-to-fuel yield, PPy-coated cells and PPy/CaCO₃-coated algal cells. The production rates are determined between time points of 2 and 7 d. Data are presented as mean values ± SD, error bars indicate standard deviations (n = 3, biologically independent samples). All Samples are cultivated in seal vials with sodium ascorbate-containing TAP culture medium and exposed to daylight with an intensity of 65 μE m^–2 s⁻¹. All relevant experiments are performed independently at least three times with similar results. Source data are provided as a Source Data file.