Fig. 4. Role of extracellular electrons in surface-conductive algal cell-mediated hypoxic photosynthesis.

a Chlorophyll fluorescence kinetic curves for PPy/CaCO₃-coated Chlorella cells with or without the addition of EY and TEOA. b Corresponding spider plots of chlorophyll fluorescence parameters from a. c–f Histograms of chlorophyll fluorescence characteristics (c), NADP⁺, NADPH and ATP contents (d), Rubisco activity (e) and carbohydrate, lipid and protein contents (f) for PPy/CaCO₃-coated Chlorella cells with or without EY and TEOA. Data are presented as mean values ± SD, error bars indicate standard deviations (n = 12 (c, F_v/F_m, qN, qP), n = 4 (c, ETR), n = 3 (d, e, f), biologically independent samples). g, h Time-dependent measurements of hydrogen production for DCMU- (g) or DBMIB-treated (h) PPy/CaCO₃-coated Chlorella cells with or without addition of EY and TEOA (solid lines). The corresponding un-inhibited hydrogen production data are shown by the dotted lines. Data are presented as mean values ± SD, error bars indicate standard deviations (n = 3, biologically independent samples). i Proposed mechanism of enhanced hydrogen production associated with the extracellular electron-modulated hypoxic photosynthesis of PPy/CaCO₃-coated Chlorella algal cells under the addition of extracellular electrons. The extracellular electrons derived from EY and TEOA participate in the photosynthesis pathway (PQ(H₂)→b₆/f→PC→PSI→Fd→hydrogenase), and both photolysis of water and endogenous organic matter contribute to photosynthetic hydrogen generation under hypoxic conditions. All samples are cultivated in sealed 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.