The review, led by Khayali Das and colleagues at Suresh Gyan Vihar University in Jaipur, India, takes aim at a familiar problem: conventional bloom control methods simply do not scale sustainably. Copper-based algaecides and hydrogen peroxide deliver rapid suppression, but they can cause secondary pollution, trigger toxin release when cells lyse, and harm non-target organisms. Biological strategies using algicidal bacteria, viruses or grazers are gentler, yet their effectiveness fluctuates with environmental conditions and species specificity. Physical removal, meanwhile, remains costly and impractical for large water bodies. What is needed, the authors contend, is an approach that harnesses an abundant, free energy source, sunlight, while minimizing chemical inputs and collateral ecological damage.

The core mechanism behind visible-light photocatalysis is elegant in its simplicity. A semiconductor material contains a valence band and a conduction band separated by a characteristic band-gap energy. When a photon with sufficient energy strikes the material, an electron is promoted across the gap, leaving behind a positively charged hole. These electron-hole pairs migrate to the catalyst surface, where electrons reduce dissolved oxygen to superoxide radicals while holes oxidize water or hydroxide ions to generate hydroxyl radicals. Together with singlet oxygen and hydrogen peroxide, these reactive oxygen species form a potent oxidative arsenal capable of destroying organic contaminants and, crucially, inactivating living algal cells.

The catch has always been that the workhorse photocatalyst, titanium dioxide, possesses a wide band gap of roughly 3.2 electron volts, meaning it can only be activated by ultraviolet light, which accounts for barely five percent of the solar spectrum reaching Earth. The review details how materials scientists have attacked this limitation through a suite of engineering strategies. Elemental doping with metals and non-metals reshapes the electronic structure to absorb lower-energy visible photons. Defect engineering introduces oxygen vacancies and Ti3+ species that create localized electronic states within the band gap, improving light absorption and charge migration. Nanostructure design maximizes surface area and active sites. Perhaps most importantly, heterojunction construction couples two semiconductors with aligned bands to accelerate directional charge transfer and suppress the recombination of electron-hole pairs, the single greatest efficiency killer in photocatalysis.

Heterojunction architecture has itself evolved through distinct generations. Conventional Type-II heterojunctions separate charges effectively but sacrifice redox power, since electrons and holes end up on bands with weaker oxidation and reduction potentials. Z-scheme systems, inspired by natural photosynthesis, recombine the low-energy carriers while preserving the highly energetic ones at the surface for redox chemistry. The more recent S-scheme design adds an internal electric field that drives simultaneous charge separation and retention of strong redox capacity. Composite systems built on these principles, such as graphitic carbon nitride coupled with bismuth vanadate, silver phosphate or tungsten trioxide, have demonstrated markedly improved algal inactivation compared with single-component materials, along with better resistance to photocorrosion.

Among the materials highlighted, graphitic carbon nitride stands out as a metal-free platform with a band gap near 2.7 electron volts, excellent physicochemical stability, low toxicity and simple synthesis from inexpensive precursors like melamine and urea. Its weaknesses, a small specific surface area, modest electrical conductivity and rapid charge recombination, have been addressed through doping with sulfur, phosphorus, boron and transition metals, the deliberate introduction of nitrogen and carbon vacancies, and morphological tuning into porous sheets, nanotubes and hollow spheres. Bismuth vanadate, with its narrow 2.4 electron volt gap and strong visible-light absorption, and magnetic zinc ferrite, which allows catalyst recovery with a simple magnet, round out a growing toolkit. Emerging classes including MXenes, metal-organic frameworks, covalent organic frameworks, quantum dots, plasmonic nanoparticles and single-atom catalysts promise tunable electronic structures and abundant active sites, though most remain at early developmental stages for bloom applications.

The review devotes particular attention to how reactive oxygen species actually kill algae, a cascade that unfolds in stages. Hydroxyl radicals, with an oxidation potential near 2.8 volts, rapidly attack membrane phospholipids, peroxidizing them and increasing membrane permeability. Vital electrolytes such as potassium, calcium and magnesium leak out, while microscopy reveals cell shrinkage, membrane rupture and eventual lysis. Oxidative species then penetrate the damaged envelope and degrade chlorophyll a, carotenoids and phycobiliproteins, shutting down Photosystem II, halting electron transport and cutting off ATP synthesis and carbon fixation. Algal cells mount antioxidant defenses through enzymes like superoxide dismutase and catalase, but when ROS production overwhelms these systems, protein oxidation, metabolic collapse and a self-amplifying cycle of injury follow. Persistent stress ultimately reaches the genome, causing base oxidation, strand breaks and DNA-protein cross-links that trigger necrosis or programmed cell death.

Notably, photocatalysis offers a dual benefit that conventional algaecides lack: it can destroy the toxins released when bloom cells rupture. Reactive oxygen species oxidize microcystin-LR through ring-opening and peptide bond cleavage, converting it into progressively less toxic intermediates that are ultimately mineralized to carbon dioxide, water and inorganic ions. This simultaneous inactivation of algae and degradation of cyanotoxins positions photocatalysis as a holistic water treatment strategy rather than a mere suppression tool, addressing one of the most serious drawbacks of chemical bloom control.

Yet the review is candid about the gulf between laboratory performance and field reality. Natural waters are chemically complex: pH fluctuations alter catalyst surface charge and radical redox potentials, bicarbonate and carbonate ions scavenge hydroxyl radicals, chloride can generate secondary reactive chlorine species, and natural organic matter competes for catalyst surfaces, absorbs light and quenches ROS before they reach target cells. Turbidity limits light penetration, confining photocatalysis to surface layers and motivating innovations such as floating photocatalytic films and platforms. Algal biology matters too: mucilaginous sheaths shield some species like Microcystis aeruginosa, filamentous cyanobacteria tolerate oxidative stress better than unicellular forms, and bloom densities exceeding one million cells per milliliter vastly exceed typical laboratory concentrations, intensifying competition for reactive species. Nanoparticle aggregation, photocorrosion, catalyst recovery and potential ecotoxicity of released nanomaterials add further hurdles.

The authors close with a roadmap for bridging these gaps. Future photocatalysts must combine high activity with thermal and photochemical stability, reusability and resistance to photocorrosion, and should be designed as immobilized, magnetic or floating systems that can be recovered after use. Standardized testing protocols reporting light intensity, catalyst loading, initial algal density and ROS production rates are needed to make studies comparable. Pilot-scale solar reactors, continuous-flow and modular hybrid systems coupling photocatalysis with membranes, wetlands or biological treatment must be validated in real water matrices, accompanied by techno-economic analysis and long-term ecological risk assessment. Computational tools, including artificial intelligence, machine learning and density functional theory, are expected to accelerate catalyst discovery and illuminate interfacial charge-transfer mechanisms. If those pieces come together, the review concludes, sunlight-driven photocatalysis could transform harmful algal bloom management from a reactive, chemically intensive struggle into a sustainable, solar-powered solution for the world’s increasingly troubled waters.

Subject of Research: Visible-light photocatalysis for the mitigation of harmful algal blooms

Article Title: Advances in visible light photocatalysis for harmful algal bloom mitigation from mechanistic understanding to sustainable application

Article References: Das, K., Kumar, N., Yakubu, E., Sharma, G., Sharma, R. K., & Aachhera, S. (2026). Advances in visible light photocatalysis for harmful algal bloom mitigation from mechanistic understanding to sustainable application. Discover Chemistry, 3(1), Article 504. https://doi.org/10.1007/s44371-026-00962-5

Image Credits: AI Generated

DOI: 10.1007/s44371-026-00962-5

Keywords: harmful algal blooms, visible-light photocatalysis, reactive oxygen species, semiconductor photocatalysts, heterojunctions, graphitic carbon nitride, algal inactivation, cyanotoxin degradation, water treatment, solar energy, advanced oxidation processes, environmental remediation