Spirulina, the dried biomass of Limnospira platensis and its congener L. maxima, is cultivated worldwide at industrial scale, exceeding 12,000 tons per year, and holds GRAS status from both the U.S. Food and Drug Administration and the European Food Safety Authority. Its market value rests on an exceptional nutritional profile, including protein content reaching up to 70 percent of dry weight, essential amino acids, vitamins B12 and E, bioactive lipids such as gamma-linolenic acid, and a suite of photosynthetic pigments. Among these, C-phycocyanin is the most valuable, prized as a natural colorant and functional ingredient across the food, cosmetic, and pharmaceutical sectors for its antioxidant, immunomodulatory, and anti-inflammatory properties. But extracting this pigment leaves behind a mountainside of residual material whose fate has remained largely unexamined, despite European policies that strongly encourage waste valorization and circular economy approaches in agriculture.

Researchers at Tor Vergata University of Rome, working with biomass supplied by the Italian producer Algaria Srl, set out to determine whether the residue left after phycocyanin extraction, which they call POST biomass, could serve as an effective biofertilizer. They compared it with untreated biomass collected before extraction, termed PRE biomass, and with a commercial NPK fertilizer, using chicory (Cichorium intybus) as the test plant. Chicory is a leafy vegetable of high nutritional and cultural importance in the Mediterranean diet, making it an ideal candidate for a fertilizer destined for sustainable horticulture. The team grew the cyanobacterium in a 300 square meter open raceway pond under natural sunlight, collected both biomass fractions, centrifuged and freeze-dried them, and then characterized their chemical composition.

The compositional analysis delivered the first encouraging surprise. Although phycocyanin extraction removed some of the protein, the POST biomass still contained 50.2 percent protein by dry weight, alongside 6.8 percent total nitrogen and 0.78 percent phosphorus, only moderately lower than the intact PRE biomass. In other words, the industrial extraction process strips out the blue pigment but leaves behind a nutrient-dense matrix. This matters because cyanobacterial biomass is known to carry phytohormones including auxins, cytokinins, gibberellins, and abscisic acid, plus amino acids, carbohydrates, and exopolymeric substances with documented biostimulant effects on plants.

Germination experiments revealed a nuanced early response. Seeds treated with POST biomass suspensions showed a transient delay in radicle emergence during the first days, an effect that strengthened with increasing concentration. Yet by day six, final germination had caught up with, or exceeded, the water control, with the 5 milligrams per milliliter treatment reaching a germination percentage of 96.6 percent against the control’s 91.6 percent. The researchers interpret this pattern of initial slowdown followed by equal or improved final germination as a priming-like response, in which bioactive compounds activate metabolic pathways associated with germination without harming the seed. By contrast, PRE biomass proved more inhibitory, significantly reducing germination at intermediate and high concentrations, suggesting the extraction process actually improves the bioavailability profile of the residual material.

The growth results were even more striking. Under controlled laboratory conditions, POST biomass increased shoot fresh weight in a dose-dependent fashion, with values of 0.76 grams at 7.5 milligrams per milliliter and 1.06 grams at 10 milligrams per milliliter, far exceeding the commercial fertilizer’s 0.39 grams. Leaf fresh weight followed the same trend, peaking at 0.94 grams versus 0.31 grams for the fertilizer control. Under greenhouse conditions, where temperature ranged from 19.7 to 32.5 degrees Celsius and humidity fluctuated naturally, the effects grew stronger rather than weaker. The highest POST treatment pushed shoot fresh weight to 4.0 grams, nearly triple the 1.47 grams achieved by the commercial product, and roots benefited significantly as well. The robustness of the response under variable environmental conditions is critical evidence that the effect can survive the messiness of real-world agriculture.

Photosynthetic pigment measurements helped explain where the extra biomass came from. POST-treated plants accumulated significantly more chlorophyll a and chlorophyll b than either control, reaching 981.9 micrograms per gram fresh weight of chlorophyll a in the laboratory and 578.3 micrograms per gram in the greenhouse, compared with 641.9 and 320.9 micrograms per gram respectively for fertilizer-treated plants. Carotenoid levels rose in parallel. The coordinated increase in chlorophyll b is particularly telling, since this pigment anchors the light-harvesting antenna complexes of photosystem II; its accumulation implies an expanded photosynthetic apparatus and greater capacity to capture light, which plausibly drove the observed gains in plant mass. PRE biomass produced weaker and less consistent pigment responses, with chlorophyll b often lagging behind the control and the chlorophyll a to chlorophyll b ratio rising, a signature of limited antenna development.

Just as important was what the researchers did not find. Phenolic compounds and flavonoids are classic markers of oxidative and environmental stress in plants, so their accumulation would have signaled that the biofertilizer was stressing the crop rather than helping it. In POST-treated greenhouse plants, total phenolic and flavonoid contents remained essentially unchanged relative to controls at most concentrations, confirming that the growth promotion was achieved without triggering adverse physiological responses. The residue acts as a genuine biostimulant, not a low-grade stressor masquerading as one. This biochemical evidence complements the growth and pigment data in portraying the leftover biomass as a safe and multifunctional agricultural input.

The study builds on the group’s earlier demonstration that spirulina processing waste can fertilize lettuce in aquaponic systems, and together the two works suggest the activity of L. platensis leftovers holds across different crops, substrates, and cultivation environments. The broader implications touch the economics of the entire spirulina sector. Commercial cyanobacterial biostimulants remain scarce largely because production costs exceed those of conventional fertilizers, and nearly half of all research on cyanobacteria in agriculture between 2006 and 2020 focused on L. platensis without translating into widespread products. By folding agricultural reuse into the existing phycocyanin biorefinery chain, producers could convert a disposal liability into a second revenue stream while reducing the environmental footprint of both the pigment industry and the farms that adopt the residue. The researchers conclude that integrating this upcycling step into spirulina biorefineries could substantially improve the economic and environmental sustainability of cyanobacterial production, turning process waste into functional bio-based products for sustainable agriculture. Remaining questions include optimizing dosing for different species and soils and validating performance at field scale, but the central message of this work is clear: the blue gold of the spirulina industry may be hiding a green treasure in what it throws away.

Subject of Research: Upcycling of industrial Limnospira platensis residual biomass as a biofertilizer for chicory cultivation

Article Title: From waste to resource: agricultural potential of Limnospira platensis (spirulina) leftovers in chicory cultivation

Article References: Savio, S., Di Cave, A., Ortenzi, F., Rugnini, L., Scuderi, F., Migliore, G., Canini, A., Braglia, R., & Congestri, R. (2026). From waste to resource: agricultural potential of Limnospira platensis (spirulina) leftovers in chicory cultivation. Plant Biosystems, 160(5), Article 265. https://doi.org/10.1007/s44473-026-00249-8

Image Credits: AI Generated

DOI: 10.1007/s44473-026-00249-8

Keywords: spirulina, Limnospira platensis, biofertilizer, phycocyanin, chicory, circular economy, biostimulant, plant biotechnology, cyanobacteria, sustainable agriculture, seed priming, industrial leftovers