Biological hydrogen production offers a promising pathway for sustainable energy generation, leveraging natural processes to convert biomass or sunlight into hydrogen gas. Two primary methods dominate this space: photobiological hydrogen production, which uses photosynthetic microorganisms, and dark fermentation, which relies on anaerobic bacteria to break down organic matter. A detailed lifecycle assessment of these systems reveals critical differences in energy inputs, carbon footprint, water usage, and scalability, each with distinct environmental trade-offs.

Energy Inputs
Photobiological hydrogen production relies on sunlight as its primary energy source, making it inherently renewable. However, the process requires additional energy inputs for cultivation systems, nutrient supply, and gas separation. Photobioreactors, whether open ponds or closed systems, demand energy for mixing, temperature control, and maintaining optimal light penetration. The energy return on investment (EROI) for photobiological systems is highly variable, depending on the efficiency of the photosynthetic organisms and the design of the cultivation infrastructure.

Dark fermentation, in contrast, derives energy from the biochemical breakdown of organic substrates, such as agricultural waste or wastewater. The energy inputs here are tied to feedstock preparation, reactor operation, and downstream purification. Pretreatment of lignocellulosic biomass, for instance, may require mechanical, chemical, or enzymatic steps, each contributing to the overall energy burden. While dark fermentation avoids direct reliance on sunlight, its EROI is influenced by the availability and processing energy of the feedstock.

Carbon Footprint
The carbon footprint of photobiological hydrogen production is largely determined by the energy sources used for auxiliary processes. If renewable energy powers the cultivation and separation systems, the net emissions can be minimal. However, the carbon intensity increases if fossil fuels are used for these inputs. Additionally, some photobiological systems produce oxygen as a byproduct, which can indirectly influence atmospheric chemistry.

Dark fermentation generates CO2 as an inherent byproduct of microbial metabolism. The carbon footprint depends on the feedstock origin; waste biomass can be considered carbon-neutral if it avoids landfill emissions, whereas purpose-grown crops may introduce land-use change emissions. Methane, a potent greenhouse gas, can also form in mixed microbial communities, requiring careful system management to prevent leakage.

Water Usage
Water is a critical resource in both systems but plays different roles. Photobiological production consumes significant quantities of water for microbial growth and evaporation losses in open systems. Closed photobioreactors reduce evaporation but still require water for nutrient delivery and cooling. In regions with water scarcity, this demand can be a limiting factor.

Dark fermentation uses water as a reaction medium and for feedstock preparation. The water footprint varies with feedstock type; high-moisture waste streams may reduce additional water needs, while dry biomass requires hydration. Wastewater can sometimes serve as both feedstock and medium, offering synergistic benefits for water recycling.

Scalability Constraints
Photobiological systems face scalability challenges due to land and light requirements. Large-scale open ponds compete with agriculture for arable land, while closed photobioreactors have high capital costs. Light distribution becomes a bottleneck as system size increases, with self-shading reducing overall efficiency. Seasonal and diurnal light variations further complicate consistent production.

Dark fermentation scales more easily in terms of physical footprint, as reactors can be stacked or operated continuously. However, feedstock supply chains must be robust to avoid logistical bottlenecks. Transportation of bulky biomass increases costs and emissions, while decentralized systems may struggle with feedstock consistency.

Environmental Trade-offs
Photobiological production excels in minimizing direct carbon emissions but struggles with land and water use efficiency. The cultivation of photosynthetic organisms may also compete with food production or natural ecosystems if not carefully managed. Nutrient runoff from open systems can lead to eutrophication, requiring mitigation strategies.

Dark fermentation offers higher volumetric production rates and better compatibility with waste streams, reducing waste disposal impacts. However, its reliance on organic feedstocks introduces variability in environmental performance. Pretreatment chemicals, if not recovered, can pose pollution risks, and the process may require additional steps to upgrade biogas or purify hydrogen.

Comparative Summary
The choice between photobiological and dark fermentation pathways hinges on regional resources and priorities. Photobiological systems align well with sunny, water-rich areas and can achieve low carbon footprints when integrated with renewables. Dark fermentation suits regions with abundant organic waste, offering faster deployment and waste valorization benefits. Both pathways require advances in organism efficiency, system design, and integration with circular economy principles to realize their full potential.

Ultimately, biological hydrogen production is not a one-size-fits-all solution. Hybrid systems, combining photobiological and fermentation steps, may emerge as a way to balance the strengths and weaknesses of each approach. Lifecycle assessments must continue to evolve, incorporating dynamic factors like climate variability and technological advancements, to guide sustainable deployment.