Abstract
Benthic ecosystems have come under intense pressure, due to eutrophication-driven oxygen decline and industrial metal contamination. One of the most toxic metals is Cadmium (Cd), which is lethal to many aquatic organisms already at low concentrations. Denitrification by facultative anaerobic microorganisms is an essential process to transform, but also to remove, excess nitrate in eutrophied systems. Cd has been shown to decrease denitrification and sequester free sulfide, which is available when oxygen is scarce and generally inhibits complete denitrification (i.e. N2O to N2). In polluted sediments, an interaction between oxygen and Cd may influence denitrification and this relationship has not been studied. For example, in the Baltic Sea some sediments are double exposed to both Cd and hypoxia. In this study, we examined how the double exposure of Cd and fluctuations in oxygen affects denitrification in Baltic Sea sediment. Results show that oxygen largely regulated N2O and N2 production after 21 days of exposure to Cd (ranging from 0 to 500 μg/L, 5 different treatments, measured by the isotope pairing technique (IPT)). In the high Cd treatment (500 μg/L) the variation in N2 production increased compared to the other treatments. Increases in N2 production are suggested to be an effect of 1) enhanced nitrification that increases NO3- availability thus stimulating denitrification, and 2) Cd successfully sequestrating sulfide (yielding CdS), which allows for full denitrification to N2. The in situ field sediment contained initially high Cd concentrations in the pore water (∼10 μg/L) and microbial communities might already have been adapted to metal stress, making the effect of low Cd levels negligible. Here we show that high levels of cadmium pollution might increase N2 production and influence nitrogen cycling in marine sediments.
Highlights
Benthic habitats are highly diverse ecosystems important for the ecosystem functions of nutrient cycling between the pelagic waters and the sediment (Griffiths et al, 2017)
This might have been due to an effect of cadmium sulfide (CdS) formation sequestering free sulfide which would allow for full denitrification to N2 (Bonaglia et al, 2016; Sørensen et al, 1980), and nitrification in the sediment surface (Stenstrom and Poduska, 1980) that has been shown to increase NO3− availability and denitrification in coastal sediments (Seitzinger, 1988)
Coastal sediment was incubated with different Cd concentrations for 21 days, and results showed that bottom water oxygen largely regulated the N2O and N2 production in the sediment
Summary
Benthic habitats are highly diverse ecosystems important for the ecosystem functions of nutrient cycling between the pelagic waters and the sediment (Griffiths et al, 2017). A significant part of biogeochemical transformation is conducted in sediments by bacteria and archaea (Burdige, 2006). These microbial communities are essential in, e.g., the reduction and oxidation of nitrogen and carbon compounds (Burdige, 2006). Denitrification is one of the most important microbially driven biogeochemical processes in transforming and removing NO3− (Galloway et al, 2004). The enzymes nitrite reductase (nir i.e. NO2− to NO) and Aquatic Toxicology 217 (2019) 105328 nitrous oxide reductase (nos i.e. N2O to N2) catalyze key steps and their respective genes (nirS/K and nosZ) have been used as functional genetic markers of denitrification Many polluted sites are affected by other contaminants that influence denitrification rates and gene activity
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