Redox-induced Mobilization of Rare Earth Elements in Musa Estuary Sediment, Southwest of Iran

In aquatic environments, metals tend to sink into sedimentary beds, but under different redox conditions they might be released to the water column. Oxic and anoxic microcosm experiments were conducted using an automatic microcosm apparatus to investigate how redox potential (Eh) affects the release of rare earth elements (REEs) in an estuary sediment sample. Results indicated that REEs mobility increases in both oxic and anoxic conditions although the average amount of REEs mobilization was higher in oxic conditions than anoxic conditions. Lu and Sm were the most mobile elements while Ce showed the lowest amount of release. REEs release is influenced by a mix of controlling factors including dissolution of Fe-Mn oxyhydroxides, low pH condition, and increase in dissolved organic carbon (DOC) under anoxic condition. An increase in Eh caused a decreasing pH from 8.08 to 6.80 and increasing DOC from 18.93 to 425 mg/l which resulted in the release of REEs. Enrichment factors (EFs) revealed that Musa estuary is threatened with Ce, Dy, La, Nd, Pr, and Sm. The high ecotoxicology effects of soluble REEs calling for potential environmental risks in Musa estuary.

An Unexpectedly Broad Thermal and Salinity-Tolerant Estuarine Methanogen Community

Moderately thermophilic (Tmax, ~55 °C) methanogens are identified after extended enrichments from temperate, tropical and low-temperature environments. However, thermophilic methanogens with higher growth temperatures (Topt ≥ 60 °C) are only reported from high-temperature environments. A microcosm-based approach was used to measure the rate of methane production and methanogen community structure over a range of temperatures and salinities in sediment from a temperate estuary. We report short-term incubations (<48 h) revealing methanogens with optimal activity reaching 70 °C in a temperate estuary sediment (in situ temperature 4–5 °C). While 30 °C enrichments amended with acetate, H2 or methanol selected for corresponding mesophilic trophic groups, at 60 °C, only hydrogenotrophs (genus Methanothermobacter) were observed. Since these methanogens are not known to be active under in situ temperatures, we conclude constant dispersal from high temperature habitats. The likely provenance of the thermophilic methanogens was studied by enrichments covering a range of temperatures and salinities. These enrichments indicated that the estuarine sediment hosted methanogens encompassing the global activity envelope of most cultured species. We suggest that estuaries are fascinating sink and source environments for microbial function study.

Baseline Survey on the Accumulation of Microdebris in the Intertidal Sediments of a Reference Estuarine System (Mira Estuary, Portugal)

Microdebris are nowadays a new emerging threat to marine ecosystems. Estuarine systems are considered sinks of contaminants, retaining these in their sediments. Nevertheless, baseline studies are essential, especially in reference estuarine systems such as the Mira Estuary, in order to establish reference conditions for other ecosystem assessment studies. Microdebris were extracted by density separation with saturated NaCl from sediment samples collected at the bare intertidal area of the lower sector of the Mira Estuary. Sediment vertical profiles showed a high abundance of microdebris, dominated by spherical and fibre-like microdebris, but also presenting metallic and uncategorized items. It was possible to observe that microdebris accumulation had its maximum concentration at a depth of 10 cm, indicating a recent increase in its usage and prevalence on this estuarine system. Comparing the item density from this work with previous studies on other aquatic ecosystems, it was found that the sediments analysed here present a very high concentration of microdebris. Although the Mira Estuary has been considered as a reference condition for the classical chemical contaminant descriptors considered in the Water Framework Directive, the data here presented indicate that this concept should be revisited adding microdebris contamination.