Molecular Soil Biology 2026, Vol.17, No.1, 1-11 http://bioscipublisher.com/index.php/msb 2 the prolonged environmental presence of EDTA can chelate toxic heavy metal ions already deposited in sediments, allowing these ions to re-enter aquatic systems and migrate freely, leading to environmental pollution (Claudia and Jaime, 2003) and disrupting the balance of essential nutrients required for algal growth. For example, metal ions such as iron, zinc, and magnesium are essential micronutrients in the photosynthetic and metabolic processes of algae. Excessive EDTA reduces the availability of these ions, thereby inhibiting normal algal growth and reproduction, slowing growth rates, and reducing cell division. EDTA also affects essential metabolic processes, including photosynthesis and respiration. It may interfere with chlorophyll synthesis and function, reduce photosynthetic efficiency, and limit the ability of algae to acquire energy, ultimately impairing growth and development. For instance, when the EDTA concentration exceeds 13.5 μmol/L, it significantly inhibits the growth of Microcystis aeruginosa (Chu et al., 2007). At low concentrations, however, EDTA can promote microalgal growth and biomass formation. Some studies have shown that low concentrations of EDTA can significantly alter metal toxicity to algae through chelation (Fawaz et al., 2018). When metals form stable complexes with EDTA, their bioavailability is markedly reduced, primarily through decreased concentrations of free metal ions in water, which are the most biologically accessible forms. This chelating mechanism diminishes the biological toxicity of metals, leading to a substantial reduction in toxicity indicators. Such phenomena have been confirmed in multiple toxicological studies involving metal–EDTA complex systems, verifying that the formation of metal–EDTA complexes is the primary mechanism for toxicity reduction (Geis et al., 2000). Some studies have found that high concentrations of EDTA strongly inhibit the growth of Microcystis aeruginosa but have little effect on Scenedesmus quadricauda (Zeng et al., 2009). Other reports indicate that EDTA supplementation can enhance lipid accumulation in microalgae (Ren et al., 2014). For instance, in Nannochloropsis oculata, both biomass and lipid accumulation increase progressively with rising EDTA concentrations (Xiao et al., 2013). Iron is one of the most important trace mineral elements in living organisms and is an essential component of intracellular redox reactions. It plays critical roles in cellular respiration, photosynthesis, and catalytic reactions involving metalloproteins. As an indispensable micronutrient for the growth and development of photosynthetic organisms, iron’s metabolic functions are mainly reflected in the molecular regulation of enzyme cofactors. Its transition metal properties provide a central role in electron transport chains and redox-catalyzed reactions, participating in metabolism through diverse structural forms, including iron–sulfur clusters, heme, di-iron centers, and mononuclear iron. In higher plants and microalgae, the iron–sulfur cluster biosynthesis systems—evolved from endosymbiotic bacteria—are located in the mitochondria and chloroplasts. Their precise assembly mechanisms support the continuous cofactor demand required for photosynthesis, respiration, and other energy metabolism processes (Balk and Schaedler, 2014). Iron deficiency disrupts electron transport and reduces energy conversion efficiency. Moreover, iron is an essential cofactor for enzymes such as RuBisCO and catalase, participating in carbon fixation and reactive oxygen species detoxification. Iron is also a key element in chlorophyll synthesis, the photosynthetic electron transport chain (e.g., cytochromes, ferredoxin), and enzymatic activities (e.g., catalase). Thus, it plays crucial roles in microalgal growth, metabolism, and biomass formation. Supplementing iron during the nutrient phase positively influences the photosynthetic mechanisms of the microalgae strain SA-2. Under mixotrophic conditions, iron significantly affects biomass, chlorophyll, carbohydrate, protein, and lipid synthesis in microalgae (Xia et al., 2010). Furthermore, relevant studies show that iron plays an essential role in microalgal growth and lipid accumulation. At certain concentrations, iron ions influence biomass, lipid composition, and metabolite synthesis, with high iron concentrations significantly affecting oleic acid accumulation (Zhang et al., 2014). Calcium is recognized as the second essential nutrient element in plants and is an important component of cell membranes. It affects the middle lamella of cell walls and plays a vital role in cell division, growth, and death (Lei et al., 2012). Calcium ions also significantly influence carbohydrate formation and transformation. Some studies indicate that calcium ions promote lipid synthesis in microalgae, and their oxides can catalyze microalgal oil synthesis for biodiesel production (Chen et al., 2016). At low concentrations, calcium ions promote
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