The vast potential of the ocean as a resource for renewable energy has led to increasing interest in marine biomass as a feedstock for hydrogen production. Among the various marine resources, seaweed and other forms of marine biomass present a promising avenue due to their rapid growth rates, high carbohydrate content, and ability to thrive in offshore environments without competing for arable land. Offshore hydrogen production using seaweed or marine biomass can be achieved through gasification or fermentation, leveraging the natural productivity of marine ecosystems while minimizing land-use conflicts.

Cultivation techniques for seaweed and marine biomass have evolved significantly, with innovations in offshore farming systems that maximize yield while minimizing ecological disruption. Macroalgae such as kelp, sargassum, and ulva are commonly cultivated using longline systems, submerged grids, or floating platforms. These systems allow for scalable production in open-ocean environments, where nutrient availability and sunlight penetration support high growth rates. Some cultivation methods incorporate depth-cycling mechanisms to optimize exposure to light and nutrients, enhancing biomass productivity. Additionally, integrated multi-trophic aquaculture systems combine seaweed cultivation with fish or shellfish farming, improving resource efficiency by recycling waste nutrients into algal growth.

Once harvested, marine biomass can be converted into hydrogen through biological or thermochemical pathways. Dark fermentation is a prominent biological method, where anaerobic bacteria break down organic matter in the absence of light, producing hydrogen along with byproducts such as volatile fatty acids. Certain strains of bacteria, such as Clostridium and Enterobacter, have demonstrated efficiency in fermenting seaweed-derived sugars into hydrogen. Pretreatment methods, including enzymatic hydrolysis or thermal processing, are often necessary to break down complex polysaccharides in seaweed into fermentable sugars.

Gasification represents another viable route for hydrogen production from marine biomass. This thermochemical process involves heating biomass at high temperatures in a controlled environment with limited oxygen, producing syngas—a mixture of hydrogen, carbon monoxide, and methane. The syngas can then undergo further processing, such as water-gas shift reactions, to increase hydrogen yield. Offshore gasification presents unique engineering challenges, including the need for robust bioreactors capable of withstanding harsh marine conditions. Innovations in floating gasification platforms and modular reactor designs aim to address these challenges, enabling continuous operation in open-ocean settings.

Bioreactor design is a critical factor in optimizing offshore hydrogen production from marine biomass. Traditional land-based bioreactors are not always suitable for marine deployment due to corrosion risks, wave motion, and biofouling. Recent advancements include the development of submersible bioreactors made from corrosion-resistant materials such as titanium or specialized polymers. These reactors are designed to operate under varying pressure conditions and incorporate antifouling coatings to prevent microbial growth on surfaces. Another innovation involves wave-energy-integrated bioreactors that use ocean motion to enhance mixing and gas exchange, improving process efficiency.

Environmental trade-offs must be carefully considered when scaling up offshore hydrogen production from marine biomass. Large-scale seaweed cultivation could influence local marine ecosystems by altering nutrient cycles and light availability. Excessive biomass growth may lead to eutrophication if decomposing algae deplete oxygen levels in surrounding waters. However, controlled cultivation and harvesting can mitigate these risks by preventing biomass accumulation and recycling nutrients back into the system. Life cycle assessments indicate that seaweed-based hydrogen production has a lower carbon footprint compared to fossil-derived hydrogen, provided that energy inputs for cultivation, harvesting, and processing are minimized.

The integration of offshore hydrogen production with other marine renewable energy systems, such as wind or wave energy, could further enhance sustainability. Hybrid platforms combining seaweed cultivation with offshore wind turbines have been proposed, where excess electricity powers electrolysis or biomass processing. Such systems could optimize space utilization and energy efficiency while reducing infrastructure costs.

Despite the promise of marine biomass for hydrogen production, technical and economic hurdles remain. Harvesting and transporting bulky seaweed biomass to processing facilities require efficient logistics, and the energy balance of offshore operations must be favorable to ensure net-positive hydrogen output. Research is ongoing to improve strain selection for higher carbohydrate content, optimize pretreatment methods, and develop cost-effective offshore bioreactors.

The potential of seaweed and marine biomass as a renewable hydrogen feedstock aligns with global efforts to decarbonize energy systems. By leveraging the productivity of marine ecosystems and advancing offshore conversion technologies, this approach could contribute to a sustainable hydrogen economy while minimizing land-use conflicts. Continued innovation in cultivation techniques, bioreactor design, and environmental management will be essential to realizing this potential at scale.