The appeal of microalgae in wastewater treatment lies in their basic biology. These photosynthetic microorganisms feed on exactly the compounds that make sewage an environmental hazard: nitrogen and phosphorus compounds that otherwise drive eutrophication, the oxygen-depleting overgrowth of aquatic ecosystems. Conventional treatment plants remove these nutrients through energy-intensive physicochemical and biological processes that also generate secondary sludge. Microalgae, by contrast, assimilate ammonium and phosphate directly into their cells, converting pollutants into biomass that can then be harvested for feed, fertilizers, nutraceuticals, or biofuels. The catch has always been economics: large-scale algal cultivation demands enormous quantities of freshwater and synthetic nutrients, precisely the inputs that wastewater could replace.

The Brazilian team, led by researchers at the Federal University of Pelotas, tackled this challenge by working with a strain isolated directly from local wastewater, on the logic that microorganisms adapted to local conditions and effluent variability should outperform laboratory strains in real-world systems. Molecular identification confirmed the isolate as Tetradesmus obliquus: sequencing of the 18S ribosomal RNA gene revealed 99.69 percent identity with reference sequences, and phylogenetic analysis placed the strain, designated CICLO_00125, within a well-supported Tetradesmus obliquus clade. Microscopy showed the characteristic four-cell coenobia and ellipsoidal cells typical of the species, a genus already recognized for rapid growth, high nutrient uptake, and metabolic plasticity in eutrophic environments.

The experimental design paired two wastewater matrices with three inoculation densities. Raw domestic wastewater, collected before any treatment, was compared with primary-treated wastewater that had undergone mesh filtration, chemical flocculation with aluminum sulfate, and sedimentation. Each effluent was inoculated with the alga at 10, 20, or 30 percent of the culture volume, and the six combinations were cultivated for 14 days in aerated flasks under continuous LED illumination, alongside uninoculated controls. This setup allowed the researchers to disentangle two variables that are rarely evaluated together: how much the effluent has been pretreated, and how many algal cells are added at the start.

The two effluents turned out to be strikingly different cultivation environments. Raw wastewater carried higher concentrations of ammonium, phosphate, iron, and manganese, along with high turbidity that limits light penetration. Primary treatment removed nearly all the suspended particles and trace metals, dropping optical density from 0.34 to 0.07 and pushing iron and manganese below detection, but it also stripped away some nutrients, reducing phosphate from 7.22 to 4.03 milligrams per liter. The clarified effluent offered better light for photosynthesis but a leaner nutritional menu, a trade-off that shaped everything that followed.

The standout bioremediation result came from the lowest inoculum density in treated wastewater. There, the alga removed 62.46 percent of ammonium and an impressive 98.56 percent of phosphate over 14 days, while simultaneously achieving the highest specific growth rate of the entire study, 0.16 per day, and the highest biomass productivity, 42.54 milligrams per liter per day. Counterintuitively, adding more cells did not improve removal. At the two higher inoculum densities in treated wastewater, phosphate removal collapsed to 42.80 percent and a mere 7.59 percent, even though final biomass concentrations were comparable. The researchers attribute this to self-shading: denser cultures reduce the light reaching each individual cell, and light drives the energy-hungry active transport of phosphate and its storage as polyphosphate, the so-called luxury uptake mechanism. High pH values above 10 in those dense cultures confirmed that bulk photosynthesis continued, pointing the finger at per-cell irradiance rather than culture-wide metabolic failure.

Nutrient removal in the raw wastewater followed a different logic. Turbidity constrained algal photosynthesis, but the effluent’s rich organic load fueled resident bacteria, which likely contributed substantially to ammonium transformation through nitrification and other microbial processes. The highest volumetric ammonium removal rate of the study, 3.40 milligrams per liter per day, occurred in raw wastewater at the highest inoculum density, under conditions least favorable to algal photosynthesis, a hint that the microalgae-bacteria consortium, not the alga alone, was doing much of the work. The authors are careful to note that bacterial contributions were not directly quantified, since nitrogen fluxes and community composition were not measured, but the pattern is consistent with a growing literature on metabolically coupled algal-bacterial systems, in which algae supply oxygen for bacterial nitrification while bacteria release carbon dioxide that feeds algal growth.

Metal removal added another dimension to the bioremediation story. Iron concentrations fell by up to 99 percent in inoculated treatments, and in the treated wastewater both iron and manganese dropped below detection limits, with manganese removal efficiencies of 80 to 100 percent. Because removal did not scale with inoculum density, the researchers conclude that a rapid, saturable passive biosorption onto cell-surface and exopolysaccharide functional groups dominates the process, followed by slower intracellular uptake, with abiotic co-precipitation of metal hydroxides at elevated pH contributing as well. The consistently lower residual metal concentrations in inoculated cultures compared with controls mark the alga’s contribution clearly.

Perhaps the most commercially intriguing finding is that the two wastewaters yielded biochemically distinct biomass. Biomass grown in raw wastewater accumulated significantly more carbohydrates, up to 32.68 percent, and more chlorophyll, reaching 14.46 milligrams per gram, likely a photoacclimation response to the dim, turbid environment. Biomass from treated wastewater, especially at the lowest inoculum, contained up to 45.83 percent protein, a level comparable to conventional protein crops, and a lipid fraction in which polyunsaturated fatty acids made up 72.70 to 78.90 percent of the total, dominated by alpha-linolenic and linoleic acids, the omega-3 and omega-6 essential fatty acids prized in nutrition. The researchers link this PUFA enrichment to the thylakoid membrane lipids that proliferate in fast-growing, light-replete cells. Putative long-chain PUFAs, including signals matching docosahexaenoic acid, appeared at levels higher than typically reported for this genus, though the authors caution that these identifications are preliminary and based on relative peak areas rather than absolute yields.

The study stops short of declaring the technology ready for prime time. Harvesting dilute algal cultures remains an energy bottleneck, effluent composition varies seasonally, and non-sterile cultivation carries contamination risks, while the fate of trace contaminants in wastewater-grown biomass would need application-specific assessment before any use in feed or food. Still, the demonstration that a single, locally adapted algal strain can clean real sewage, remove trace metals, and produce biomass whose composition can be steered between carbohydrate-rich and protein-and-PUFA-rich profiles simply by choosing the effluent and the inoculum density, is a meaningful step toward wastewater treatment plants that do not just dispose of waste but harvest it. Pilot-scale trials with techno-economic and life-cycle analysis are the logical next test for this circular bioeconomy vision.

Subject of Research: Bioremediation of domestic wastewater by a native Tetradesmus obliquus strain for production of protein- and PUFA-rich microalgal biomass

Article Title: Integration of domestic wastewater and native Tetradesmus obliquus for bioremediation and production of biomass rich in protein and polyunsaturated fatty acids

Article References: do Amaral, D. S., Werner, L., Pagano, A. D., Leitzke, A. F., Hakbart Boneman, D., La Rosa Novo, D., Nunes, L. S., Bueno, D. T., Laporta, L. V., Crizel, R. L., Filho, A. V., Campos, V. F., & de Pereira, C. M. P. (2026). Integration of domestic wastewater and native Tetradesmus obliquus for bioremediation and production of biomass rich in protein and polyunsaturated fatty acids. Environmental Science and Pollution Research, 33(30), 15417-15442. https://doi.org/10.1007/s11356-026-38213-w

Image Credits: AI Generated

DOI: 10.1007/s11356-026-38213-w

Keywords: Tetradesmus obliquus, microalgae, wastewater treatment, bioremediation, polyunsaturated fatty acids, protein, biomass productivity, inoculum density, circular bioeconomy, nutrient removal, eutrophication, biosorption

The study, conducted by scientists at the State University of Ponta Grossa and the Federal University of Santa Catarina and published in the journal Blue Biotechnology, tested two commercially available mineral fertilizers enriched with hydrolyzed extracts of the red seaweed Kappaphycus alvarezii and the brown seaweed Ascophyllum nodosum. One product, designated FR and intended for seed treatment, contained 3.0 percent total nitrogen, 10.0 percent P2O5, cobalt, molybdenum, boron, zinc and nickel alongside the algal extracts. The second, designated FG and used as a foliar spray, contained 2.0 percent nitrogen, 2.0 percent P2O5, boron, zinc, sulfur, manganese, magnesium and nickel with the same seaweed extracts.

Alongside these commercial formulations, the researchers prepared aqueous extracts from a blend of three microalgae species grown in their own laboratory: the cyanobacterium Arthrospira platensis, the green microalga Tetradesmus obliquus, and the marine diatom Chaetoceros muelleri. Cultures were grown to the stationary phase in tailored media, harvested by centrifugation, frozen at minus 80 degrees Celsius, lyophilized, and then macerated in deionized water with the pH adjusted to 5.5. The team used 2.5 milligrams of dry biomass from each species per treatment batch, delivering the blend either as a seed coating or as a foliar spray in 150 liters of solution per hectare.

The field experiment took place during the 2023 winter growing season at the Capão da Onça School Farm in Ponta Grossa, Paraná, at roughly 1,002 meters above sea level on a sandy Dystrophic Haplic Cambisol managed under no-till for fifteen years. The researchers employed a randomized block design with four replications in a 3 by 3 factorial arrangement: three seed treatments crossed with three foliar treatments produced 36 experimental units. The barley cultivar Imperatriz was sown at a target density of 2.5 million plants per hectare with base fertilization of 200 kilograms per hectare of 10-20-20 N-P-K and a topdressing of 120 kilograms per hectare of nitrogen at tillering. Foliar applications occurred at 24, 45, 65 and 87 days after sowing, and the 140-day crop cycle was assessed for plant stand, height, stem diameter, tillers, grains per ear, ear length, 100-grain weight and final yield.

The headline result was striking. Seed treatment with the commercial FR product combined with foliar application of the commercial FG product increased barley yield by 49.73 percent, equivalent to 784.24 kilograms per hectare, while the same seed treatment paired with the blended microalgae spray raised yield by 50.53 percent, or 788.84 kilograms per hectare, both compared with untreated controls. Even FR seed treatment alone, when followed by either foliar option, delivered yield gains of roughly 32 percent, translating to approximately 583 to 584 extra kilograms per hectare, or about 9.7 sixty-kilogram sacks.

The physiological story behind these numbers is intricate. FR seed treatment increased the number of tillers per plant by 19.37 percent and plant height by 8.47 percent, likely because the seaweed extracts supply auxin-like, gibberellin-like and cytokinin-like compounds that drive cell division and elongation, while the product’s 10 percent phosphorus content fuels ATP-dependent growth. However, the same treatment shortened ear length by 10.80 percent and reduced 100-grain weight by 5.43 percent. The authors suggest the surge in tillering, and the resulting yield, may partly reflect a compensatory response: unusually heavy early rainfall cut the initial plant stand by 43.13 percent below the intended population, and the sparser crop responded with more tillers and taller plants.

Intriguingly, foliar application alone produced no significant effect on any growth or yield parameter. The researchers attribute this to the formidable physicochemical barriers that dissolved molecules must cross, from the leaf cuticle through the apoplast, before reaching the protoplast of leaf cells. Molecules in the extracts, they note, may simply fail to reach their targets in sufficient concentrations, which helps explain why earlier studies using other growth stimulants reported positive foliar responses in spring barley. The yield benefits in this trial emerged only when the treatments were stacked: the foliar products acted synergistically with FR seed treatment rather than additively, mitigating the seed treatment’s negative effects on ear traits.

The economics look promising on paper. In March 2025 the combined cost of FR and FG was about 14.56 US dollars per hectare, while the extra grain harvested with both products was worth roughly 165 dollars at prevailing prices for a sixty-kilogram sack, a gross gain of about 150 dollars per hectare before labor, fuel and machinery costs. The authors caution, however, that a rigorous economic analysis accounting for all operational expenses remains necessary, and they stress that the FR dose used was based on the manufacturer’s wheat recommendation, since no barley-specific label exists.

The researchers are careful to flag the limitations. Total rainfall over the 140-day cycle hit 1,064.2 millimeters, far above the 450-to-600-millimeter optimum for barley and likely suppressing both disease-free growth and treatment responses, particularly for foliar products. The 100-grain weight averaged 3.79 grams, well below the cultivar’s expected 4.54 grams, and soil conditions were suboptimal. The team therefore calls for multi-year, multi-site trials to confirm the results, alongside deeper investigation into the physiological mechanisms by which macroalgal and microalgal biocompounds—polysaccharides, polyphenols, amino acids, sterols, vitamins and phytohormone mimics—act on germination, tillering and grain filling.

Even with those caveats, the study stands as one of the first field demonstrations that marine macroalgae and freshwater-to-marine microalgae can work together as a coherent biofertilizer system in a temperate cereal crop. With barley demand steady and growers under pressure to cut synthetic inputs, the message from Paraná is that the ocean’s flora, harnessed as seed coatings and leaf sprays, may offer a genuinely scalable route to larger harvests—potentially half again as much grain from the same land, without a single extra kilogram of conventional nitrogen or phosphate.

Subject of Research: Field evaluation of seaweed-extract biofertilizers and blended microalgae extracts for enhancing barley crop growth and yield

Article Title: Seed treatment with seaweed extract biofertilizers and foliar application of blended microalgae extracts enhanced barley (Hordeum vulgare) crop yield

Article References: de Moraes, V. C., Ruivo, L. B., Lopes, R. G., Barboza, L. E., Matiello, R. R., Owatari, M. S., Derner, R. B., & Ohse, S. (2026). Seed treatment with seaweed extract biofertilizers and foliar application of blended microalgae extracts enhanced barley (Hordeum vulgare) crop yield. Blue Biotechnology, 3(1), Article 3. https://doi.org/10.1186/s44315-026-00055-z

Image Credits: AI Generated

DOI: 10.1186/s44315-026-00055-z

Keywords: barley, Hordeum vulgare, biofertilizers, biostimulants, seaweed extract, Kappaphycus alvarezii, Ascophyllum nodosum, microalgae, Arthrospira platensis, Tetradesmus obliquus, Chaetoceros muelleri, sustainable agriculture