Yantai Institute of Coastal Zone Research

Coastal eutrophication, driven by nutrients and trace metal enrichment, poses a significant threat to marine ecosystems worldwide. This study investigated the nutrient and trace metal dynamics driving eutrophication along the Cox's Bazar Coast of the Bay of Bengal (BoB), Bangladesh. Summer surface seawater samples were collected from 12 strategically located stations between 2021 and 2023 and analyzed for physicochemical parameters, dissolved nutrients, and trace metals using standard spectrophotometric and ICP-MS techniques. The results revealed marked spatial and temporal variations in nutrient concentrations (NO₃^-: 0.01-0.066 mg/L, NO₂^-: 0.004-0.035 mg/L, NH₄⁺: 0.014-0.096 mg/L, PO₄^3-: 0.273-1.243 mg/L, DSi: 0.011-0.073 mg/L), trace metals (Fe: 0.127-0.362 mg/L, Mn: 0.021-0.086 mg/L, Cu: 0.025-0.075 mg/L, Zn: 0.011-0.221 mg/L) and eutrophication (EI: 1.674-25.982), with nearshore stations exhibiting higher levels of concertation enrichment compared to offshore areas. Elevated nutrient levels, particularly nitrogen and phosphorus species, along with high concentrations of trace metals, were strongly correlated with anthropogenic inputs from industrial discharge, urban runoff, and aquaculture activities. PCA further identified two key components representing combined influences of trace metals with PO₄^3- and DSi, and a nitrogen-based nutrient factor. The integrated findings suggested that natural processes and intensified human activities drive eutrophication in the region, potentially promoting harmful algal blooms and threatening coastal ecosystem stability. This study underscored the need for effective management and pollution control strategies to mitigate nutrient overload and preserve the ecological integrity of the BoB.

Selenium, an essential micronutrient for human health, has garnered significant interest owing to its multifaceted biological activities including antioxidant, anti-inflammatory, and antitumor properties. In this study, 14 novel chitosan derivatives bearing selenourea groups were synthesized by incorporating selenium into chitosan via nucleophilic substitution reactions. Structural confirmation was achieved through FTIR, ¹H NMR, and ICP-MS analyses, confirming successful selenium incorporation. The derivatives demonstrated superior antioxidant performance, achieving >85 % DPPH radical scavenging at 0.8 mg/mL and exceptional superoxide anion radical inhibition (>95 % at 0.4 mg/mL), surpassing conventional urea-chitosan analogues (<75 % under identical conditions) from our previous study. Biocompatibility was assessed via MTT assay, revealing no significant cytotoxicity in L929 cells at concentrations ≤125 μg/mL. These selenourea-modified chitosan derivatives demonstrate potential as novel selenium-enriched dietary supplements and antioxidants for health foods and pharmaceuticals.

Dissolved oxygen (DO) critically regulates biogeochemical processes in mariculture sediments, yet its integrated effects on sulfur-metal interactions during sediment aging remain poorly understood. This study investigated the migration and transformation of reduced inorganic sulfur (RIS) and heavy metals in mariculture sediments under varying DO levels. Lower DO accelerated the dominance of acid-volatile sulfide (AVS) within the reduced inorganic sulfur pool (AVS/RIS = 56.11% under DO-1 vs. 41.18% under DO-3) and shortened the time required for AVS to become the dominant component by 16.67% (compared to DO-3). DO variations minimally affected overall metal mobility (coefficient of variation, CV < 12%), but Cu exhibited exceptional sensitivity. The influence of DO concentration was particularly significant on the oxidizable (F3) and residual (F4) fractions of Cu, and the F4 fraction of Pb. The F4 fraction of both Cu and Pb increased under low DO, likely due to reductive activation and lattice diffusion. The differing impacts of DO on RIS (significant) versus heavy metal speciation (relatively minor) resulted in significant variations in the correlations between sulfur species and heavy metal fractions across DO treatments. Sediment microbial richness and diversity declined with decreasing DO, significantly altering the relative abundance of bacterial phyla and the genus Desulfuromonas. DO primarily regulated sediment carbon, sulfur, and iron cycling, exerting its most pronounced impacts on sulfur transformations. Depleted DO intensifies the environmental hazards of inorganic sulfides through enhanced mobilization. Proactive control strategies are therefore imperative during hypoxic episodes to mitigate sulfide-driven ecological risks. This study provides new insights for managing sediment redox conditions in mariculture systems.