Molecular Soil Biology 2026, Vol.17, No.1, 1-11 http://bioscipublisher.com/index.php/msb 3 microalgal growth, increase biomass, and enhance lipid accumulation, although the effects are modest (Gao, 2024). High concentrations of calcium ions also promote biofilm formation derived from algal organic matter in the microalgal strain SA-2 (Fan et al., 2025).Currently, most studies regarding microalgae and calcium focus on the role of microalgae in balancing environmental calcium ions, with calcium concentrations negatively correlated with the growth of calcified microalgae (Zhao et al., 2020). Research on the effects of calcium on microalgal growth and biomass accumulation remains limited.However, in certain plants—such as Brassica campestris—supplementing calcium under stress conditions promotes growth and photosynthesis. Foliar calcium application increases flavonoid content, enhances electron transport rates, alleviates photosynthetic inhibition, and improves photosynthetic efficiency. Calcium ions also mitigate excessive acidification of the thylakoid membrane, maintain membrane integrity, and enhance ATPase activity (Cheng, 2020).As calcium ions support thylakoid membrane structure and the oxygen-evolving complex in PSII, appropriate concentrations of calcium may similarly promote microalgal growth and biomass synthesis. Excessive accumulation of EDTA-Ca and EDTA-Fe can elevate calcium and iron concentrations in the environment, potentially causing heavy metal-like stress and impairing normal microalgal growth. However, studies indicate that the exogenous addition of EDTA-Fe and EDTA-Ca complexes can significantly regulate the photosynthetic metabolic system of the microalgae SA-2 under mixotrophic conditions. Iron sources not only enhance biomass production by improving light energy conversion efficiency but also exhibit dose-dependent effects on intracellular chlorophyll, carbohydrate, protein, and macromolecular synthesis. Particularly in lipid metabolic regulation, EDTA-Fe displays concentration-dependent modulation, with oleic acid showing specific enrichment in triacylglycerols when the concentration reaches a threshold level (Kona et al., 2017). 2 Results and Analysis This study aims to investigate in depth the variation patterns and influencing factors of microalgal biomass content under EDTA-Fe and EDTA-Ca treatments. The expected outcomes will provide a theoretical basis for the efficient cultivation of microalgae and the optimized utilization of biomass resources, as well as technical support for further research on biomass accumulation in microalgae. Experimental results showed that supplementing the nutrient phase of microalgae SA-2 with EDTA-Ca at concentrations below 0.3 mg/L and EDTA-Fe at concentrations below 0.2 mg/L could promote the growth of SA-2 to a certain extent and increase its biomass yield. The optimal concentration for EDTA-Ca to promote SA-2 growth was 0.3 mg/L, whereas that for EDTA-Fe was 0.2 mg/L. At their respective optimal concentrations, both EDTA-Ca and EDTA-Fe enhanced cell density, dry weight, cell viability, and the contents of proteins, lipids, pigments, and carbohydrates in SA-2. The addition of EDTA-Ca and EDTA-Fe did not alter the logarithmic growth phase of SA-2, which was consistent with that of the untreated control. The promotive effect of EDTA-Fe at its optimal concentration on the growth and biomass synthesis of SA-2 was greater than that of EDTA-Ca at its optimal concentration. This indicates that iron, compared with calcium, plays a more critical role in the growth, reproduction, and biomass formation of microalgae, and that microalgae have a higher demand for iron. Furthermore, the optimal concentration of EDTA-Fe for SA-2 growth was lower than that of EDTA-Ca, suggesting that microalgae are more sensitive to iron than to calcium and are more susceptible to stress caused by high concentrations of iron ions. 2.1 Effects of EDTA-Fe and EDTA-Ca treatments on microalgal cell density Microalgae SA-2 exhibited the highest cell density and best growth performance when the concentration of EDTA-Ca was 0.3 mg/L. Concentrations of EDTA-Ca above 0.3 mg/L showed an inhibitory effect on SA-2 cell density (Figure 1a). For EDTA-Fe, the optimal concentration for promoting SA-2 growth was 0.2 mg/L, whereas concentrations above 0.2 mg/L began to inhibit microalgal growth (Figure 1b). These results indicate that EDTA-Ca at concentrations below 0.3 mg/L and EDTA-Fe at concentrations below 0.2 mg/L promoted the growth of SA-2, while higher concentrations of EDTA-Ca (>0.3 mg/L) and EDTA-Fe (>0.2 mg/L) inhibited growth. The cell density of SA-2 increased exponentially between days 9 and 10 (Figure 1), indicating that day 10 corresponds to the logarithmic growth phase. The addition of EDTA-Ca and EDTA-Fe did not significantly affect the duration or timing of the logarithmic growth phase of SA-2.
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