Coastal wetland sediments are vital to the global carbon cycle as they represent large sinks of blue carbon – carbon from atmospheric and oceanic sources – which are threatened by ecosystem loss. The forms of sequestered carbon and the sequestration capability are affected by many bio- and geochemical factors that change unpredictably along coastal locales. In the present study, we investigated three unique coastal sites – a coastal mangrove and two sabkhas with contrasting geology and tidal influence in the Qatar peninsula – for their carbon capture ability to determine how biogeochemical indices affect their blue carbon sequestration potential. We applied a suite of biological and geochemical tools, collecting the sediment cores of approximately 40 cm depth; analysed sediment porewater; performed depth-profiling of the organic matter, sedimentary minerals, microbial community and analysis of sediment surface for pH, oxygen (O2); redox potential and hydrogen sulfide (H2S) by microsensors. High-resolution transmission electron microscopy with energy-dispersive X-ray spectroscopy (TEM-EDXS) and scanning transmission X-ray microscopy (STXM) revealed templating effects that promoted Mg-carbonate nucleation in coastal hypersaline environments. Microsensing reveals the intricacy of the oxic/anoxic transition at the sediment surface. Microbial DNA sequencing at various sediment depths shows the occurrence of microbial genera, whose functions explain the geochemical trends and carbon sequestration pathways observed at each site. Notably, we found that carbon sequestration in the mangrove and carbonate-sand sabkha was correlated with organic matter degradation and inorganic carbon content, while in the siliciclastic sabkha it was solely influenced by sediment density and depth.

Selenium is a widespread contaminant released by industrial activities such as coal combustion. In selenium-contaminated groundwater, native microbial communities commonly have the capability of reducing the toxic oxyanions selenate and selenite to insoluble elemental selenium. The impact of local hydrogeography on microbial selenium reduction was tested by constructing laboratory microcosms using biofilm and groundwater collected from four monitoring wells screened in three distinct stratigraphic units near fly ash disposal ponds in southeastern Montana. Glycerol, methanol and molasses were tested as carbon amendments. Nitrate and selenium concentrations were monitored, and microbial communities were sequenced to examine differences among sites and carbon amendments. All site and carbon combinations resulted in nitrate removal, though molasses had the highest removal rate. Selenium removal was significantly impacted by stratigraphic unit, with microcosms from alluvial wells removing more total selenium than those from coal and interburden wells. Microbial community composition was correlated with site, carbon amendment, and nitrate and selenium removal. Furthermore, two genera from the order Clostridiales, Desulfosporosinus and Gracilibacter, emerged as potential indicator organisms for selenium reduction in this environment. The site, carbon amendment, and microbial community were all found to potentially impact remediation efficacy.