Photobiological hydrogen production, dark fermentation, and microbial electrolysis represent three distinct biological pathways for generating hydrogen, each with unique mechanisms, energy requirements, yields, and environmental implications. While all three leverage microorganisms to produce hydrogen, their operational frameworks and outcomes differ significantly.

Photobiological hydrogen production utilizes photosynthetic microorganisms, such as algae and cyanobacteria, to convert sunlight and water into hydrogen through light-dependent processes. These systems typically rely on direct or indirect biophotolysis, where sunlight splits water into oxygen and hydrogen using specialized enzymes like hydrogenases or nitrogenases. The process is inherently renewable, as it depends on solar energy and water, with no need for external organic substrates.

Dark fermentation, by contrast, employs anaerobic bacteria to break down organic matter—such as agricultural waste, sewage, or industrial effluents—into hydrogen, carbon dioxide, and other byproducts. This process does not require light, making it operable in continuous conditions regardless of daylight availability. However, it depends on a steady supply of organic feedstock, which can introduce complexities in sourcing and pretreatment.

Microbial electrolysis cells (MECs) represent a hybrid approach, combining microbial metabolism with electrochemical assistance. In MECs, exoelectrogenic bacteria oxidize organic matter, releasing electrons and protons. An external voltage is applied to drive hydrogen evolution at the cathode. While this method can achieve higher hydrogen yields than dark fermentation alone, it requires electrical input, which affects overall energy efficiency.

Energy Inputs
Photobiological systems are uniquely advantageous in their reliance on sunlight, a renewable and abundant energy source. The primary energy input is solar radiation, with no need for additional electricity or heat, assuming optimal photosynthetic efficiency. However, challenges such as low light conversion efficiency and the oxygen sensitivity of hydrogen-producing enzymes can limit output.

Dark fermentation requires no light but depends on the chemical energy stored in organic substrates. The energy input is indirect, tied to the cultivation, transport, and preprocessing of biomass. While the process itself is anaerobic and can run continuously, the upstream energy costs associated with feedstock logistics can be substantial.

Microbial electrolysis demands both organic substrates and electrical energy. The applied voltage typically ranges between 0.2 and 0.8 V, which is lower than traditional water electrolysis but still constitutes a significant energy input. The overall efficiency hinges on the source of electricity; if derived from renewables, the environmental impact is reduced, but grid electricity diminishes sustainability.

Hydrogen Yields
Yield comparisons reveal stark differences among the three methods. Photobiological systems generally exhibit lower volumetric production rates due to the inherent limitations of photosynthetic efficiency and enzyme inactivation. Reported yields range between 0.1 and 4 mol H2 per mol of substrate (water or organic acid), with practical systems often achieving the lower end of this spectrum.

Dark fermentation outperforms photobiological methods in terms of volumetric productivity, with yields typically between 1 and 3 mol H2 per mol of glucose. However, the theoretical maximum of 4 mol H2 per mol glucose is rarely achieved due to metabolic bottlenecks and competing pathways that produce alternative byproducts like acetate or butyrate.

Microbial electrolysis can achieve higher yields by extracting additional hydrogen from fermentation byproducts. With an applied voltage, MECs can push yields toward 8–12 mol H2 per mol glucose, surpassing dark fermentation. The trade-off is the added energy cost of the electrochemical step, which must be factored into net efficiency calculations.

Environmental Impacts
Photobiological hydrogen production stands out for its minimal environmental footprint when operated under controlled conditions. It consumes only water and sunlight, emitting no greenhouse gases during operation. Challenges include land use for photobioreactors and potential water demand in arid regions, but these are offset by the absence of feedstock-related emissions.

Dark fermentation’s environmental impact is closely tied to feedstock sourcing. If waste biomass is used, the process can contribute to waste reduction and circular economy goals. However, if crops are cultivated specifically for hydrogen production, issues like land-use change, water consumption, and fertilizer runoff arise. The process also generates CO2, though this is often biogenic and thus carbon-neutral over short cycles.

Microbial electrolysis shares similar feedstock concerns with dark fermentation but adds the energy burden of electricity. If the electricity is sourced from fossil fuels, the carbon footprint increases significantly. Renewable electricity improves sustainability but does not eliminate the need for organic substrates, which remain a limiting factor.

Unique Advantages of Photosynthesis-Driven Systems
Photobiological hydrogen production offers several distinct benefits. First, it directly harnesses solar energy, avoiding the inefficiencies of intermediate energy conversions seen in other methods. Second, it operates at ambient temperatures and pressures, reducing energy-intensive processing steps. Third, it can be integrated with wastewater treatment, where algae or cyanobacteria simultaneously clean water and produce hydrogen, enhancing resource efficiency.

Another advantage is scalability in decentralized settings. Photobioreactors can be deployed in rural or off-grid areas with ample sunlight, providing localized hydrogen production without extensive infrastructure. Additionally, genetic engineering and synthetic biology hold promise for enhancing photosynthetic efficiency and hydrogenase stability, potentially unlocking higher yields in the future.

In contrast, dark fermentation and microbial electrolysis are constrained by their reliance on organic feedstocks and, in the latter case, external electricity. While they offer higher immediate yields, their sustainability is contingent on waste-derived substrates and renewable energy integration.

Conclusion
Photobiological hydrogen production, dark fermentation, and microbial electrolysis each occupy a niche in the biohydrogen landscape. Photobiological systems excel in sustainability and low operational inputs but face yield limitations. Dark fermentation offers higher productivity but depends on feedstock availability and preprocessing. Microbial electrolysis bridges the gap with enhanced yields but introduces energy trade-offs. For a future renewable hydrogen economy, photosynthesis-driven systems hold unique promise due to their direct solar utilization and minimal environmental impact, provided technological advancements can address current inefficiencies.