Study of the Effect of Autotrophic and Mixotrophic Culture Media on Heavy Metal Uptake by <i>Fischerella</i> sp. and <i>Desmonostoc alborizicum</i>

Heavy metals, even at low concentrations, can have detrimental effects on human health and the environment. The use of biological methods for heavy metal removal is considered a suitable alternative to conventional physicochemical methods due to lower costs and environmental compatibility. Among these methods, exopolysaccharides produced by cyanobacteria play a significant role in the adsorption of metal ions. This study aimed to evaluate the capability of two native Iranian cyanobacteria, Fischerella sp. and Desmonostoc alborizicum, in removing the heavy metals chromium, nickel, and copper. The cyanobacterial strains Fischerella sp. and Desmonostoc alborizicum were cultured in mixotrophic media containing glucose, maltose, lactose, and sucrose (10 g/L), as well as in a control medium (carbohydrate-free). Dry cell weight (6–48 hours), exopolysaccharide content (at 48 hours), and heavy metal removal (copper, chromium, and nickel, 10–90 minutes) were measured. Carbohydrate and protein contents were determined using the phenol-sulfuric acid method and the Lowry method, respectively. Additionally, volatile compounds produced by Fischerella sp. in the presence of copper were analysed using gas chromatography-mass spectrometry (GC-MS). The results demonstrated that Fischerella sp. exhibited superior performance in maltose-enriched culture media compared to Desmonostoc alborizicum. This strain led to a 1.3-fold increase in exopolysaccharide production, a 1.02-fold increase in copper removal, and higher dry cell weight relative to Desmonostoc alborizicum. The removal efficiency of chromium and nickel ranked second and third, respectively, after copper in maltose-enriched media. Quantitative comparison of the compounds identified by GC-MS in Fischerella sp. under control conditions and in the presence of maltose and copper revealed an increase in the levels of 3-methylbutane, 2-methylbutane, methyl acetate, 2-butanone, acetic acid, 3-methylbutyl acetate, 1-butanol, 3-methyl-1-butanol, 3-hydroxy-2-butanone, 2-phenylethanol, and 3-octen-2-ol. This study demonstrated that Fischerella sp. exhibited high efficiency in heavy metal removal, exopolysaccharide, and protein production in maltose-enriched media. Therefore, this strain can be considered an eco-friendly candidate for reducing environmental pollution.