A Two-Stage Photobiological Bio-Hydrogen Reactor for Cyanobacterial Bloom Mitigation and Toxin Neutralization in Brackish Ecosystems

Harmful cyanobacterial blooms (Nodularia spumigena) in brackish aquatic environments such as the Archipelago Sea and the Gulf of Bothnia present severe ecological and toxicological risks due to the production of potent hepatotoxins (microcystins). Conventional mechanical harvesting and thermal processing approaches entail high energy consumption and create substantial risks of secondary cell lysis and toxin release into the water column. This paper introduces a closed-loop, two-stage biotechnological reactor architecture designed to redirect cyanobacterial metabolism toward enzymatic hydrogen (H2) production under anaerobic stress, effectively neutralizing toxin synthesis prior to biomass disposal. The process segregates metabolic phases: Phase A utilizes a light-transmitting quartz/FEP circuit for photosynthetic glycogen accumulation coupled with continuous vacuum membrane degassing of photolytic oxygen (O2), preventing the inhibition of downstream nitrogenase and hydrogenase enzymes. Phase B transitions the concentrated biomass into a sulfur- and nitrogen-depleted dark anaerobic environment, down-regulating RuBisCO activity, halting microcystin synthesis, and inducing biological hydrogen gas evolution. We evaluate two operational scales: mobile, shallow-draft catamarans for targeted remediation of stagnant coastal incubators, and stationary barrier filtration modules integrated into offshore wind turbine monopiles. Energy self-sufficiency is maintained via an internal Power-to-Gas-to-Power loop utilizing proton-exchange membrane fuel cells. This decentralized bio-electrochemical strategy transforms toxic organic waste into clean energy and safe organic fertilizer without relying on extensive external power grids.