A humble red seaweed harvested from the turbulent waters of the Gulf of Mannar in southern India may hold one of the most tantalizing leads in marine biomedicine. Researchers have extracted a sulfated galactan polysaccharide from Gracilaria spinulosa and shown that the compound can kill human colon cancer cells in the laboratory while leaving healthy colon epithelial cells largely untouched. The work, published in Discover Chemistry, adds a promising new candidate to a growing catalog of marine molecules whose biological activities rival anything dreamed up by synthetic chemists. What makes the finding particularly striking is the selectivity: at concentrations where the polysaccharide halved the viability of HT-29 colon adenocarcinoma cells, normal CCD-841 colon epithelial cells retained roughly 86.6 percent of their viability, well above the accepted cytotoxicity threshold of 75 percent.
The story begins on the Pamban Coast in December 2022, when the research team collected roughly three kilograms of fresh Gracilaria spinulosa, a red macroalga belonging to the order Gigartinales and the family Rhizophyllidaceae. This species thrives on reef rocks in high-current marine environments, an ecological niche that exposes it to constant physical stress and, perhaps, drives the production of the defensive molecules that make it chemically interesting. The specimens were taxonomically validated by the Centre for Advanced Studies in Botany at the University of Madras, then cleaned of epiphytes, rinsed thoroughly with distilled water, and stored at minus 20 degrees Celsius until analysis began. Only fresh, healthy specimens were chosen, and the dry weight of the biomass was determined at 28 percent of the fresh weight, a figure consistent with other red algal species such as Ceramium elongata and Laurencia mannarensis, which range between 22 and 38 percent.
Extracting the polysaccharide required a deliberate chemical strategy. The researchers first defatted and decolorized one hundred grams of pulverized algal biomass by stirring it in 85 percent ethanol for twelve hours at 37 degrees Celsius, removing proteins and pigments that could contaminate the final product. The dried residue was then subjected to acidic hot-water extraction, with the pH adjusted to 3.0 using concentrated hydrochloric acid and the mixture held at 85 degrees Celsius for two hours under continuous stirring. This procedure was repeated three times to maximize recovery. The combined supernatants were clarified by centrifugation, and the dissolved polysaccharides were precipitated with an equal volume of ice-cold acetone held overnight at 4 degrees Celsius. After further centrifugation, the crude polysaccharide pellet was dialyzed for twenty-four hours through tubing with a 14 kilodalton molecular weight cutoff, removing low-molecular-weight impurities, and finally freeze-dried. The yield was 7.2 percent of the dry seaweed, within the 2.0 to 7.7 percent range reported for other red seaweeds, though lower than the 22 to 56 percent yields achieved from species such as Gracilaria amansii, Gracilaria livida, Laurencia papillosa, Portieria hornemannii, and Acanthophora spicifera, reflecting how temperature, season, and species physiology shape extraction outcomes.
Chemical characterization revealed a composition dominated by carbohydrates. Total sugars accounted for 68.2 percent of the extract, sulfate esters contributed 20.6 percent, ash content was a modest 4.0 percent, and moisture measured 10 percent. Elemental analysis detected 28.57 percent carbon, 6.45 percent hydrogen, and 1.68 percent nitrogen, values closely mirroring earlier work on sulfated polysaccharides from Gracilaria debilis. When the researchers hydrolyzed the polysaccharide with trifluoroacetic acid to break it into its monosaccharide building blocks, galactose emerged as the overwhelming majority at 78.2 percent, followed by xylose at 10.6 percent, arabinose at 9.7 percent, and mannose at 2.3 percent. This profile confirms that the compound is a true sulfated galactan, with galactose forming the primary backbone of the polymer, and it aligns closely with the monosaccharide composition previously reported for the related red seaweed Acanthophora spicifera.
Structural confirmation came from two complementary imaging and spectroscopic techniques. Fourier-transform infrared spectroscopy of the potassium bromide pellets produced a spectrum with prominent peaks at 1246 and 829 wavenumbers. The first corresponds to the sulfur-oxygen double bond stretching vibration characteristic of sulfate ester groups, while the second signals carbon-oxygen-sulfur bonds indicative of secondary axial sulfation, features that distinguish this galactan from related carrageenan families. Scanning electron microscopy, performed at an acceleration voltage of 8.0 kilovolts on platinum-coated samples, showed irregular particles with heterogeneous surfaces at lower magnification and well-defined rectangular microstructures with small dimensions at higher magnification. Together, these data establish both the chemical identity and the physical architecture of the extract, essential groundwork for any future attempt to link structure to function.
The antioxidant results were solid rather than spectacular, and the researchers report them with appropriate candor. In the DPPH radical scavenging assay, activity climbed in a concentration-dependent manner from 17.81 percent at 100 micrograms per milliliter to 76.90 percent at 1000 micrograms per milliliter, a maximum scavenging performance comparable to values of 55.55 percent and 73.96 percent reported for sulfated polysaccharides from Acanthophora spicifera and Sargassum horneri. The half-maximal inhibitory concentration, or IC50, for DPPH scavenging was 464.12 micrograms per milliliter, higher than the 271.56 micrograms per milliliter achieved by ascorbic acid, meaning the seaweed compound required roughly 1.7 times more material to neutralize the same fraction of free radicals. Prior research suggests that phenolic content strongly influences these numbers, and the new study did not measure phenolics directly, so the moderate potency likely reflects the polysaccharide’s structural properties rather than any impurity.
The superoxide radical scavenging assay told a similar story. Activity rose from 9.68 percent at 100 micrograms per milliliter to 57.12 percent at 1000 micrograms per milliliter, with an IC50 of 841.54 micrograms per milliliter against the 385.11 micrograms per milliliter benchmark set by ascorbic acid. These figures sit within the range reported for sulfated galactans from tropical seaweeds, which have shown superoxide scavenging values ranging from 23.3 to 64.5 percent across different concentrations. Notably, some related red algal polysaccharides, including those from Gracilaria caudata, Sargassum filipendula, and Sargassum schroederi, showed no detectable superoxide scavenging whatsoever, making the measurable activity here a genuine point of differentiation. The researchers note that superoxide scavenging tends to correlate positively with sulfate content, and the 20.6 percent sulfate measured in this extract is consistent with that relationship.
The cytotoxicity findings are where the study earns its headline. Using the MTT assay, which measures metabolic activity as a proxy for cell viability, the team tested concentrations of 10 to 100 micrograms per milliliter against both HT-29 colon adenocarcinoma cells and CCD-841 normal colon epithelial cells, with doxorubicin as the positive control. The polysaccharide killed HT-29 cells in a concentration-dependent fashion, achieving an IC50 of 64.22 plus or minus 0.67 micrograms per milliliter. That figure is higher than the 29.35 plus or minus 1.15 micrograms per milliliter of doxorubicin, the clinical gold standard, but doxorubicin is famously toxic to healthy tissue as well, so a natural product with any selectivity is noteworthy. In stark contrast, CCD-841 cells retained approximately 86.6 percent viability even at the highest concentration tested, comfortably above the 75 percent threshold that conventionally separates non-cytotoxic from cytotoxic outcomes.
How this selectivity works at the molecular level remains an open question, and the authors are appropriately cautious about their interpretation. Previous studies have linked the anticancer activity of sulfated polysaccharides to their degree of sulfation, molecular weight, monosaccharide composition, and glycosidic linkages, and a recent paper suggested that sulfated polysaccharides from Gracilaria corticata can trigger reactive-oxygen-species-mediated apoptosis in colon cancer cells. The current work did not include desulfation experiments or formal structure-activity analyses, so the plausible link between the extract’s 20.6 percent sulfate content and its cytotoxic effect is inferred from the broader literature rather than demonstrated directly. Still, the dose-response curve is clear, the contrast with normal cells is pronounced, and the extraction chemistry is well characterized, giving the field a solid foundation for the next round of investigation.
The road ahead is clearly mapped by the study’s own limitations. The authors call for testing against additional cancer and non-cancerous cell lines, detailed structural characterization, mechanistic and in vivo studies, and formal structure-activity relationship work to confirm whether sulfate groups drive the observed biological effects. If those steps succeed, Gracilaria spinulosa could join the ranks of marine organisms whose polysaccharides, from fucoidans to carrageenans, are being developed as antioxidant supplements, anti-inflammatory agents, and anticancer drug leads. For now, the message is simpler and no less exciting: a seaweed that clings to reefs in one of the world’s richest marine environments is producing a sugar-based molecule that distinguishes cancerous colon cells from healthy ones under laboratory conditions. In the ongoing search for gentler and more selective cancer therapies, that is a distinction worth chasing.
Subject of Research: Sulfated galactan polysaccharides from the red seaweed Gracilaria spinulosa and their antioxidant and selective cytotoxic effects against colon cancer cells in vitro.
Article Title: In vitro antioxidant activity and cytotoxic effects of sulfated galactan polysaccharides from Gracilaria spinulosa
Article References: Kannaiah, S., Sekar, N., Devi, W. S., Arunprakash, N., Raju, P., Mani, A., Selvarajan, R., & Abia, A. L. K. (2026). In vitro antioxidant activity and cytotoxic effects of sulfated galactan polysaccharides from Gracilaria spinulosa. Discover Chemistry, 3(1), Article 450. https://doi.org/10.1007/s44371-026-00913-0
Image Credits: AI Generated
DOI: 10.1007/s44371-026-00913-0
Keywords: Gracilaria spinulosa, sulfated polysaccharides, red seaweed, antioxidant, colon cancer, HT-29, cytotoxicity, Gulf of Mannar, marine natural products, DPPH, galactose, vitro
News Source: Nathaniel Bowman. (October 14, 2026). Red Seaweed Compound Kills Colon Cancer Cells While Sparing Healthy Ones. Scienmag.
