The research team set out to compare how the two strains respond to heavy metal stress at the physiological and biochemical level. Arthronema africanum is a filamentous, non-heterocystous cyanoprokaryote with unique morphological and physiological traits. It thrives in extreme desert habitats, adapts readily to harsh conditions, and is a promising producer of phycobiliproteins, light-harvesting pigments valued in biotechnology. Despite its potential, the strain remains poorly explored, with only a handful of studies examining its antitumor and antioxidant properties. Coelastrella sp. BGV, by contrast, is a fast-growing green microalga newly isolated in Bulgaria that has already attracted attention for its high bioactive and antitumor capacity. The researchers grew both strains in intensive batch cultures under continuous illumination at 28 to 30 degrees Celsius, supplied with sterile carbon dioxide, and then exposed them for ten days to copper, cadmium, and lead ions added to the nutrient medium at concentrations of 50 and 100 micromolar.

The results revealed a striking difference in sensitivity between the two organisms. Cadmium and lead significantly inhibited growth and protein biosynthesis in both strains, but the cyanoprokaryote suffered far more than the green alga. At 100 micromolar copper and 100 micromolar cadmium, the dry biomass of Arthronema africanum dropped by 55 to 57 percent compared with untreated controls. Coelastrella sp. BGV also endured the toxic effects, but less severely: the same high concentrations of cadmium and lead reduced its dry biomass by only 30 to 33 percent by the tenth day. Interestingly, low concentrations of copper actually stimulated growth in the early stages of treatment in both strains, a hint that copper, an essential trace element, behaves differently from the nonessential and highly toxic cadmium and lead.

Protein content told a similar story. In Arthronema africanum, the lowest protein levels were measured after cadmium and lead treatment, while copper caused the least inhibition — a decrease of just 8 to 22 percent. In Coelastrella sp. BGV, both cadmium concentrations produced a maximal reduction of protein biosynthesis of roughly 36 percent, whereas 50 micromolar copper decreased protein content by only about 5 percent. The researchers note that the protein content of both strains was the least affected parameter compared with pigments and oxidative stress markers, indicating that the photosynthetic apparatus bore the brunt of the metal toxicity.

Pigment analysis exposed the photosynthetic damage in vivid detail. Chlorophyll a biosynthesis in Arthronema africanum plummeted by 62 to 64 percent under 100 micromolar cadmium and lead. The green alga reacted with an extreme reduction of both chlorophyll a and chlorophyll b after cadmium exposure, with lead producing a similar though less pronounced effect — chlorophyll a fell by about 60 percent and chlorophyll b by 50 percent. Copper ions caused the smallest changes in chlorophyll content in both organisms. The decline in chlorophyll may result from distortion of pigment structure or direct inhibition of biosynthesis, mechanisms consistent with reports on other metal-stressed algae.

Yet amid this photosynthetic damage, the researchers observed a remarkable biochemical counterattack. Carotenoid content rose sharply, especially in Coelastrella sp. BGV, where 100 micromolar cadmium and lead triggered increases of 93 and 82 percent respectively. Carotenoids are well known antioxidants that quench reactive oxygen species, and their surge indicates a protective reaction against the oxidative stress provoked by cadmium and particularly lead. In Arthronema africanum, the picture was more complicated: cadmium actually decreased carotenoid content by about 40 percent, while copper enhanced carotenoid levels by 24 percent at the higher concentration. This contrast with findings in the cyanoprokaryote Synechocystis sp., where cadmium reduced carotenoid content, underscores how differently related organisms can deploy the same biochemical tools.

To quantify the cellular damage, the team measured malondialdehyde, a standard marker for lipid peroxidation — the oxidative destruction of membrane molecules caused by heavy-metal-induced generation of reactive oxygen species. All three metals sharply increased malondialdehyde in Arthronema africanum, with cadmium and lead producing the largest effects. Coelastrella sp. BGV showed a similar trend but to a lesser extent, with malondialdehyde levels roughly three times higher in response to cadmium and lead. The stress response of the cyanoprokaryote was clearly more intense, mirroring its more severely inhibited growth.

The enzymatic antioxidant defenses followed suit, though with telling species-specific differences. Superoxide dismutase activity in Arthronema africanum more than doubled under both cadmium concentrations, while copper and lead produced less pronounced changes. In Coelastrella sp. BGV, the pattern was reversed: the highest superoxide dismutase values were registered under lead treatment, with an 80 to 90 percent enhancement, as well as at 100 micromolar copper. Catalase activity, which breaks down hydrogen peroxide, was similarly elevated in both organisms — roughly two times above control at 100 micromolar cadmium and lead — while copper had a weaker effect. The researchers observed that catalase changes did not strictly correspond to those of superoxide dismutase and malondialdehyde, suggesting that the elimination of reactive oxygen species likely involves additional protective mechanisms beyond these two enzymes, mechanisms that remain to be studied in detail. Notably, the enzymatic antioxidant protection of Arthronema africanum appears to have been insufficient to fully overcome metal toxicity, as manifested by its severely inhibited growth.

Perhaps the most practically significant finding concerned metal uptake. Both strains accumulated large amounts of the metals added to the medium, with a clear preference order: lead was absorbed most readily, followed by cadmium and then copper. Coelastrella sp. BGV showed a particularly high tendency to accumulate metals in its biomass, with a much higher uptake capacity than the cyanoprokaryote. When exposed to 100 micromolar lead, the green alga accumulated over 12,000 milligrams of lead per kilogram of dry weight — roughly twice the amount taken up by Arthronema africanum under the same conditions. For cadmium, Coelastrella sp. BGV accumulated more than 5,000 milligrams per kilogram of dry weight. Crucially, this voracious absorption did not affect the green alga’s growth as strongly as it did the cyanoprokaryote’s, meaning the strain can keep growing and sequestering metals even under heavy exposure.

The researchers conclude that both strains, and especially Coelastrella sp. BGV, could find real application in the treatment of water bodies contaminated with copper, lead, and cadmium. The study also deepens understanding of how heavy metal stress operates in microalgae at the mechanistic level — how a single metal ion can interact specifically with a particular strain, how pigment biosynthesis and antioxidant defenses respond in concert, and how uptake capacity and tolerance can be uncoupled. After further research, the team believes Coelastrella sp. BGV could be suggested as a suitable species for non-toxic and effective heavy metal removal, positioning this unassuming Bulgarian green alga as a potential workhorse of the emerging phytoremediation industry.

Subject of Research: Comparative stress response and heavy metal uptake of the microalgal strains Arthronema africanum and Coelastrella sp. BGV

Article Title: Heavy metal stress response of microalgal strains Arthronema africanum and Coelastrella sp. BGV

Article References: Karcheva, Z., Georgieva, Z., Tomov, A., Petrova, D., Zhiponova, M., Vasileva, I., & Chaneva, G. (2022). Heavy metal stress response of microalgal strains Arthronema africanum and Coelastrella sp. BGV. BioRisk, 17, 83-94. https://doi.org/10.3897/biorisk.17.77483

Image Credits: AI Generated

DOI: 10.3897/biorisk.17.77483

Keywords: microalgae, heavy metals, phytoremediation, oxidative stress, antioxidant enzymes, cadmium, lead, copper, bioremediation, water treatment, Heavy, metal