Iron is a limiting factor that controls the phytoplankton biomass of the ocean and plays an important role in the lipid production of microalgae. Elucidating the effects of different iron valences on microalgae is helpful for their commercial production. We investigated the growth, photosynthesis, and fatty acid profile of the model diatom Phaeodactylum tricornutum cultured with depleted Fe, Fe²⁺, Fe²⁺/Fe³⁺, and Fe³⁺. Samples were taken every 24 h for 8 days, and their cell density, photosynthetic pigment content, chlorophyll fluorescence, total fatty acid content, and fatty acid composition were analyzed. The cell densities of the Fe²⁺ and Fe²⁺/Fe³⁺ groups were significantly higher than those of the control and Fe³⁺ groups (p < 0.05). They were 1.26 times and 1.23 times higher than those in the Fe-depleted group. The contents of chlorophyll a and c in the Fe²⁺ group were significantly higher than those in the Fe-depleted group (p < 0.05). The chlorophyll fluorescence results show that Fe²⁺ enhanced the photosynthesis of P. tricornutum to a greater extent than Fe³⁺. On the eighth day of harvest, Fv/Fm and Y(II) in the Fe²⁺ group were 0.672 and 0.476, respectively, being 1.10 and 1.19 times greater than those in the Fe³⁺ group and 1.15 and 1.33 times greater than those in the Fe-depleted group, respectively. Compared with the control group, the levels of saturated fatty acids of the Fe²⁺/Fe³⁺ and Fe³⁺ groups were significantly higher (p < 0.05) at 21.36 ± 1.24% and 21.20 ± 0.13%, respectively. The levels of polyunsaturated fatty acids of the Fe²⁺/Fe³⁺ group were significantly lower (p < 0.05) at 29.82 ± 2.75%. Our results show that P. tricornutum exhibited physiological plasticity, including changes in photosynthetic activities and shifts in fatty acid composition, in response to different iron valences and that Fe²⁺ was more beneficial to the biomass production of this species than Fe³⁺. These findings are applicable to the production of biomass and polyunsaturated fatty acids.

To study metal enrichment and identify the influencing factors, 106 surface soils were collected in the Yellow River delta, including ten vegetation types. Concentrations of 10 different metals, including As, Cd, Cr, Ni, Cu, Pb, Zn, Mn, Al, Fe, and physicochemical properties pH, salinity, particle size were detected. The pH value was 7.35~9.17, and the salinity was in the range of 0.01~2.00%. The average value of silt was 76.18%, which was the main particle size for 99% of the samples. The mean concentrations of As, Cd, Cr, and Ni were higher than the background value of Shandong Province or the background value of yellow soil in China. A higher concentration of Fe occurred in Phragmites australis (mean concentration 2.50%) and paddy field. The concentrations of Cd, Cr, Ni, Cu, Pb and Zn were lower in the Suaeda salsa soil. The Nemerow pollution index indicated that 79% of all samples showed low-level metal pollution, and 7% of all samples showed moderate-level metal pollution. In the areas with higher salinity, the concentrations of As were higher, while the concentrations of Cd and Ni were lower. The correlation analysis showed that with increasing pH, the concentrations of As and Zn decreased significantly. The metal concentrations had a significant positive correlation with clay, except for As.