Engineered biocatalytic architecture for enhanced light utilisation in algal H2 production†

Abstract

Thin-layer photosynthetic biocatalysts (PBCs) offer an innovative and promising approach to the solar-powered generation of renewable chemicals and fuels. Thin-layer PBCs incorporate photosynthetic microbes, engineered for the production of targeted chemicals, into specifically tailored bio-based polymeric matrices. This unique integration forms a biocatalytic architecture that allows controlled distribution of light, nutrients, and substrates to the entrapped cells, optimising their performance. The research outlined in this study offers a systematic engineering approach to developing a biocatalytic architecture with improved light utilisation and enhanced photosynthetic conversion of captured light energy to molecular hydrogen (H₂), an important energy carrier and fuel. This was achieved by entrapping wild-type green alga Chlamydomonas reinhardtii and its mutants with truncated light-harvesting chlorophyll antenna (Tla) complexes within thin-layer (up to 330 μm-thick) polymeric matrices under sulphur-deprived conditions. Our step-by-step engineering strategy involved: (i) synchronising culture growth to select cells with the highest photosynthetic capacity for entrapment, (ii) implementing a photosynthetic antenna gradient in the matrix by placing Tla cells atop the wild-type algae for better light distribution, (iii) replacing the conventional alginate formulation with TEMPO-oxidised cellulose nanofibers for improved matrix stability and porosity, and (iv) employing a semi-wet production approach to simplify the removal of produced H₂ from the matrix with entrapped cells, thus preventing H₂ recycling. The engineered PBCs achieved a fourfold increase in H₂ photoproduction yield compared to conventional alginate films under the same irradiance (0.65 vs. 0.16 mol m⁻² under 25 μmol photons m⁻² s⁻¹, respectively) and maintained H₂ photoproduction activity for over 16 days. This resulted in a remarkable 4% light energy to hydrogen energy conversion efficiency at peak production activity and over 2% throughout the entire production period. These significant advancements highlight the potential of engineered thin-layer PBCs for efficient H₂ production. The technology could be adapted for biomanufacturing various renewable chemicals and fuels.