English: (i) Algal cell-based spheroids: a Large numbers of Chlorella algal cells (green spheres) are spontaneously captured within w/w dextran-in-PEG micro-droplets by emulsification in the presence of denatured BSA micro-particles (yellow rectangles). b Cell-containing emulsion droplets are hyperosmotically compressed (red arrows) by transfer to a concentrated PEG solution to produce robust multicellular spheroids comprising a closely packed aggregate of algal cells immobilized in a dextran/BSA hydrogel matrix. c Cell-mediated depletion of oxygen over time in the hydrogel matrix (yellow triangular network) generates hypoxic (interior, cyan) and aerobic (surface, green) micro-niches due to light shading of the algal cells in the core domain by the outer shell of Chlorella cells. Depending on the size of the spheroids, photosynthetic oxygen generation is decreased in the core such that respiration becomes dominant over photosynthesis resulting in the net depletion of cellular storage compounds, hypoxic conditions, hydrogenase activity and hydrogen production in daylight under air. Corresponding reactions: Shell domain: H₂O → ½O₂ + 2H⁺ + 2e⁻ (PSII). Core domain: H₂O → ½O₂ + 2H⁺ + 2e⁻ (PSII) and 2H⁺ + 2e → H₂ (hydrogenase). (ii) Algal/bacterial hybrid spheroids: (d) Preparation of the droplets using mixtures of Chlorella and PEGylated E. coli cells (blue rods) results in a spatially segregated arrangement of photosynthetic algal cells enclosed by a thin oxygen-depleting layer of respiratory bacterial cells. e Hyperosmotic compression results in consolidation and immobilization of the two cellular micro-niches. f In daylight under air, the binary community acts synergistically to enhance the levels of hydrogen production produced by hypoxic photosynthesis in the core of the multicellular hybrid micro-reactor. Corresponding reactions: Shell domain: H₂O → ½O₂ + 2H⁺ + 2e⁻ (PSII) and O₂ → CO₂ (respiration from E. coli). Core domain: H₂O → ½O₂ + 2H⁺ + 2e⁻ (PSII) and 2H⁺ + 2e⁻ → H₂ (hydrogenase).