Photosynthetic biohybrid systems combine the best attributes of biological wholecell catalysts and semiconducting nanomaterials. Enzymatic machinery enveloped in its native cellular environment offers exquisite product selectivity and low substrate activation barriers while semiconducting nanomaterials harvest light energy stably and more efficiently than biomolecules. In this topical review, we illustrate the evolution and advances of photosynthetic biohybrid systems focusing on the conversion of CO 2 to value-added chemicals. We begin by considering the potential of this nascent field to meet global energy challenges while comparing it to alternate approaches. This is followed by a discussion of the advantageous coupling of electrotrophic organisms with light-active electrodes for solar-to-chemical conversion. We detail the dynamic investigation of photosensitized microorganisms creating direct light harvesting within unicellular organisms while describing complementary developments in cytoprotection and understanding of charge transfer mechanisms. Lastly, we focus on trends and improvements needed of photosynthetic biohybrid systems in order to address future challenges and enhance their widespread adoption for the production of solar fuels and chemicals.

Microbial electro-and photoelectrochemical CO2 fixation, in which CO2-reducing microorganisms are directly interfaced with a cathode material, represent promising approaches for sustainable fuel production. Although considerable efforts have been invested to optimize microorganism species and electrode materials, the microorganismcathode interface has not been systematically studied. Here, investigation of the interface allowed us to optimize the CO2-reducing rate of silicon nanowire/Sporomusa ovata system. Tuning the bulk electrolyte pH and increasing its buffering capacity supported the formation of a close-packed nanowire-bacteria cathode. Consequently, the resulting closepacked biohybrid achieved a CO2-reducing current density of ~0.65 mA cm-2. When coupled with a photovoltaic device, our system enabled solar-to-acetate production with ~3.6% efficiency over seven days.

Providing life-support materials to crewed space exploration missions is pivotal for mission success. However, as missions become more distant and extensive, obtaining these materials from in situ resource utilization is paramount. The combination of microorganisms with electrochemical technologies offers a platform for the production of critical chemicals and materials from CO2 and H2O, two compounds accessible on a target destination like Mars. One such potential commodity is poly(3-hydroxybutyrate) (PHB), a common biopolyester targeted for additive manufacturing of durable goods. Here, we present an integrated two-module process for the production of PHB from CO2. An autotrophic Sporomusa ovata (S. ovata) process converts CO2 to acetate which is then directly used as the primary carbon source for aerobic PHB production by Cupriavidus basilensis (C. basilensis). The S. ovata uses H2 as a reducing equivalent to be generated through electrocatalytic solar-driven H2O reduction. Conserving and recycling media components is critical, therefore we have designed and optimized our process to require no purification or filtering of the cell culture media between microbial production steps which could result in up to 98% weight savings. By inspecting cell population dynamics during culturing we determined that C. basilensis suitably proliferates in the presence of inactive S. ovata. During the bioprocess 10.4 mmol acetate L –1 day–1 were generated from CO2 by S. ovata in the optimized media. Subsequently, 12.54 mg PHB L–1 hour–1 were produced by C. basilensis in the unprocessed media with an overall carbon yield of 11.06% from acetate. In order to illustrate a pathway to increase overall productivity and enable scaling of our bench-top process, we developed a model indicating key process parameters to optimize.

The conversion of CO 2 to value-added products powered with solar energy is an ideal solution to establishing a closed carbon cycle. Combining microorganisms with light-harvesting nanomaterials into photosynthetic biohybrid systems (PBSs) presents an approach to reaching this solution. Metabolic pathways precisely evolved for CO 2 fixation selectively and reliably generate products. Nanomaterials harvest solar light and biocompatibly associate with microorganisms owing to similar lengths scales. Although this is a nascent field, a variety of approaches have been implemented encompassing different microorganisms and nanomaterials. To advance the field in an impactful manner, it is paramount to understand the molecular underpinnings of PBSs. In this perspective, we highlight studies inspecting charge uptake pathways and singularities in photosensitized cells. We discuss further analyses to more completely elucidate these constructs, and we focus on criteria to be met for designing photosensitizing nanomaterials. As a result, we advocate for the pairing of microorganisms with naturally occurring and highly biocompatible mineral-based semiconductor nanomaterials.

Solar-driven bioelectrosynthesis represents a promising approach for converting abundant resources into value-added chemicals with renewable energy. Microorganisms powered by electrochemical reducing equivalents assimilate CO 2 , H 2 O, and N 2 building blocks. However, products from autotrophic whole-cell biocatalysts are limited. Furthermore, biocatalysts tasked with N 2 reduction are constrained by simultaneous energy-intensive autotrophy. To overcome these challenges, we designed a biohybrid coculture for tandem and tunable CO 2 and N 2 fixation to value-added products, allowing the different species to distribute bioconversion steps and reduce the individual metabolic burden. This consortium involves acetogen Sporomusa ovata , which reduces CO 2 to acetate, and diazotrophic Rhodopseudomonas palustris , which uses the acetate both to fuel N 2 fixation and for the generation of a biopolyester. We demonstrate that the coculture platform provides a robust ecosystem for continuous CO 2 and N 2 fixation, and its outputs are directed by substrate gas composition. Moreover, we show the ability to support the coculture on a high–surface area silicon nanowire cathodic platform. The biohybrid coculture achieved peak faradaic efficiencies of 100, 19.1, and 6.3% for acetate, nitrogen in biomass, and ammonia, respectively, while maintaining product tunability. Finally, we established full solar to chemical conversion driven by a photovoltaic device, resulting in solar to chemical efficiencies of 1.78, 0.51, and 0.08% for acetate, nitrogenous biomass, and ammonia, correspondingly. Ultimately, our work demonstrates the ability to employ and electrochemically manipulate bacterial communities on demand to expand the suite of CO 2 and N 2 bioelectrosynthesis products.