Extreme Microalga Turns Shrimp and Sludge Waste Into Protein-Rich Biomass

Aquaculture has become one of the fastest growing food production sectors on the planet, and with that growth comes an uncomfortable byproduct: an unrelenting stream of organic waste. By 2022, global aquaculture production had surpassed 223.2 million metric tons, and every ton of cultivated aquatic animal biomass generates residues such as leftover feed, feces, and dead organisms. When these residues are discharged untreated, they fuel eutrophication, degrade water quality, and release greenhouse gases. A new study published in Waste and Biomass Valorization now shows that this ecological liability could be converted into a nutritional asset, using one of the most resilient microorganisms known to science. Researchers led by Corina Kleps and Daniel Pleissner cultivated the thermo-acidophilic red microalga Galdieria sulphuraria on hydrolysates prepared from shrimp processing waste and aquaculture sludge, producing biomass that contained up to 62 percent protein along with measurable quantities of the commercially valuable blue pigment phycocyanin.

The choice of organism is central to the strategy. Galdieria sulphuraria, a unicellular red alga of the class Cyanidiophyceae, thrives in conditions that would kill nearly all competing microbes: acidic environments with pH values between 1 and 5 and temperatures up to 56 degrees Celsius. In this study, the alga was grown at pH 1.8 and 45 degrees Celsius in complete darkness, drawing energy from organic substrates rather than photosynthesis. This heterotrophic mode of growth offers clear advantages for industrial waste valorization. Because the extreme culture conditions suppress bacterial and fungal contaminants, the process can run under non-sterile conditions, dramatically reducing costs compared with conventional fermentation. At the same time, the organism tolerates the complex and variable chemistry of real industrial residues, a persistent stumbling block for cleaner cultivation systems.

The raw materials came from a German shrimp processing facility and included wastewater, sludge, and minced shrimp heads and shells. Compositional analysis by near-infrared spectroscopy revealed that shrimp residues were remarkably protein dense, containing 57 percent protein and 11 percent lipids by weight, while the sludge contained 28 to 32 percent protein, roughly 13 percent carbohydrates, and 2 to 8 percent fat. The wastewater, with only 0.85 percent solids and negligible free amino nitrogen and phosphate, served not as a nutrient source but as a dilution medium. To unlock the nutrients locked inside the solid residues, the team tested a series of hydrolysis strategies: enzymatic treatment with the acidic protease Protease S-02, enzymatic treatment with Glucoamylase AN, a combination of both enzymes, and a chemical pretreatment with 1 percent sulfuric acid at 90 degrees Celsius followed by enzymatic digestion.

The hydrolysis results carried an important and somewhat sobering message. Free amino nitrogen recovered from sludge hydrolysates remained low, at only 1 to 2 milligrams per gram of sludge, regardless of the treatment applied. Shrimp residues performed far better, yielding 10 to 15 times more free amino nitrogen when digested with protease, the enzyme cocktail, or acid-assisted enzymatic hydrolysis over 48 hours. Strikingly, control experiments without any enzymes showed that a large share of the recoverable nutrients, around 15 milligrams of free amino nitrogen per gram of shrimp residue and 100 to 150 milligrams of phosphate per gram of both residues, could be released simply by solubilizing and mixing the materials. The yields were also notably below the 50 to 90 percent recovery figures often reported for proteolytic hydrolysis of fish and meat processing wastes, a discrepancy the authors attribute to limited protein accessibility, suboptimal mixing and solids concentrations, or a mismatch between the chosen protease and its substrate.

With hydrolysates in hand, the researchers turned to cultivation trials. In shaken flask cultures held for seven days in the dark, Galdieria sulphuraria grew exponentially in sludge hydrolysate concentrations of 25 to 50 percent by volume, reaching a maximum growth rate of 1.36 per day at the 50 percent level supplemented with 2.5 grams per liter of glucose. This rate matches values previously reported for heterotrophic growth of the species. However, the windows of tolerance were narrow. At 75 percent sludge hydrolysate, growth was inhibited, and shrimp hydrolysate supported growth only at concentrations of 25 percent or less. The culprit appears to be organic acids: experiments showed that acetate concentrations of 0.3 grams per liter and above blocked growth, while lower levels were metabolized. The mechanism is well understood in acidophilic microbiology. Below the pKa of acetic acid, the molecule becomes protonated, diffuses across the cell membrane, and releases its proton inside the cell, acidifying the cytoplasm and disrupting metabolism.

To push biomass concentrations higher, the team moved to fed-batch cultivation in a 5-liter bioreactor. Cultures were started with a mixture of 15 percent sludge hydrolysate and 10 percent shrimp hydrolysate plus 10 grams per liter of glucose, and on days three and four were fed an additional 300 to 400 milliliters of a richer solution containing 70 percent sludge hydrolysate, 30 percent shrimp hydrolysate, and 22.5 grams per liter of glucose, delivered by peristaltic pump. The initial batch phase produced consistent growth rates of 1.19 to 1.25 per day, corresponding to doubling times between 13.3 and 14.0 hours, and biomass reached 6 to 8 grams per liter. In the best-performing run, continued growth after feeding pushed the biomass concentration to nearly 10 grams per liter, with the produced cells containing 47 to 62 percent protein and 7 to 8.6 percent lipids.

The pigment phycocyanin, a natural blue colorant with established markets in food and nutraceutical industries, accumulated to as much as 8 milligrams per gram of dry biomass during the initial exponential growth phase, falling to around 4 milligrams per gram in stationary phase in one culture. Yet another culture produced only 1 to 2 milligrams per gram despite similar nutrient conditions. This variability pointed the researchers toward a deeper question: whether growth depends not just on how much nitrogen is available, but on which nitrogen compounds are present. Because the nitrogen species in residue hydrolysates are difficult to identify, the team ran a systematic screening, supplying the alga with 50 millimolar solutions of individual amino acids and comparing growth against ammonium sulfate, the standard nitrogen source.

The screen revealed pronounced substrate specificity. Ammonium sulfate supported a growth rate of 0.95 per day, and only two amino acids came close: alanine at 0.87 per day and proline at 0.80 per day, marking them as highly suitable nitrogen donors. Glutamic acid, aspartic acid, leucine, arginine, glycine, and methionine supported measurable but weaker growth between 0.59 and 0.73 per day, while tryptophan, lysine, phenylalanine, isoleucine, threonine, serine, histidine, and valine performed poorly, with rates between 0.16 and 0.48 per day. Lysine and cysteine permitted no or only weak, delayed growth. The practical implication is significant: residues must be selected not only for their total nitrogen content but for the amino acid composition of that nitrogen, since the alga cannot universally exploit amino acids. The low phycocyanin yield in one fed-batch culture may reflect exactly this gap, where free amino nitrogen was abundant but the right amino acids were missing.

Taken together, the study maps both the promise and the constraints of a decentralized, circular bioeconomy in which aquaculture farms recycle their own residues into protein-rich algal feed or pigment feedstocks. The authors identify three dominant control points: the limited and variable release of nitrogen from residues, growth inhibition at high hydrolysate concentrations driven by organic acids such as acetate, and carbon limitation caused by glucose depletion even under fed-batch operation. Because much of the phosphate and some free amino nitrogen are released by mixing alone, pretreatment must be tailored to each substrate and enzyme combination to remain cost-effective. If those parameters can be mastered, the extreme lifestyle of Galdieria sulphuraria, its tolerance of heat, acidity, and contamination, could transform the waste streams of the world’s fastest growing food sector into a reliable source of protein and natural colorants, closing a nutrient loop that has until now leaked into rivers and coastal waters.

Beyond protein and phycocyanin, the biomass produced in such a process carries additional commercial value. Galdieria sulphuraria is known to accumulate highly branched glycogen, a form of storage carbohydrate of interest for nutritional applications, and its protein fraction is reported to offer a favorable amino acid profile suitable for animal feed and human nutrition. Because the alga was cultivated heterotrophically in the dark, productivity is not constrained by light availability or photobioreactor geometry, allowing the high cell densities typical of stirred-tank fermentation to be approached.

The fed-batch strategy used here also illustrates a broader principle of residue-based bioprocessing. Rather than exposing the culture to a single hydrolysate at full strength, the researchers diluted inhibitory components during start-up and then supplied nutrients progressively, keeping the alga in exponential growth while limiting the accumulation of organic acids. This kind of staged feeding mirrors established practice in industrial fermentation, where substrate toxicity is managed through controlled dosing rather than batch addition.

Decentralization is another notable aspect of the concept. Since the residues originate at shrimp processing facilities and aquaculture farms, a compact cultivation unit operating at acidic pH and elevated temperature could, in principle, run on-site without sterilization equipment, converting waste into feed or pigment precursors where it is generated. The remaining challenges are largely quantitative: stabilizing hydrolysate composition across batches, matching nitrogen quality to the alga’s substrate specificity, and balancing carbon dosing against acetate inhibition. Addressing these control points will determine whether laboratory yields of roughly ten grams per liter of protein-rich biomass can be translated into an economically viable, closed-loop component of the aquaculture industry.

Subject of Research: Heterotrophic cultivation of Galdieria sulphuraria on aquaculture residue hydrolysates for protein- and phycocyanin-rich biomass production.

Article Title: Valorization of Aquaculture Residues by Heterotrophic Cultivation of the Extremophilic Microalga Galdieria Sulphuraria for Protein- and Phycocyanin-Rich Biomass Production

Article References: Kleps, C., Händel, N., Schönfelder, S., Baum, L., Ogurek, M., & Pleissner, D. (2026). Valorization of Aquaculture Residues by Heterotrophic Cultivation of the Extremophilic Microalga Galdieria Sulphuraria for Protein- and Phycocyanin-Rich Biomass Production. Waste and Biomass Valorization. https://doi.org/10.1007/s12649-026-03778-7

Image Credits: AI Generated

DOI: 10.1007/s12649-026-03778-7

Keywords: Galdieria sulphuraria, aquaculture residues, phycocyanin, microalgae, waste valorization, circular bioeconomy, hydrolysis, heterotrophic cultivation, shrimp waste, protein-rich biomass, bioprocess engineering, Valorization