The role of sulfur cycle and enzyme activity in dissimilatory nitrate reduction processes in heterotrophic sediments

Key role of macrophyte-dominated habitats over cyanobacteria-dominated habitats in stability of microbial sulfur cycling in freshwater lakes

Despite extensive studies on sulfate-reducing bacteria (SRB) and sulfur-oxidizing bacteria (SOB) in marine and deep stratified ecosystems, their spatiotemporal dynamics and ecological roles in shallow freshwater lakes remain poorly understood, especially with habitat shifts driven by eutrophication and climate change. Here, we monitored seasonal changes in sediment and porewater chemical parameters, and applied high-throughput sequencing and co-occurrence network analysis to compare SRB and SOB communities in surface sediments from cyanobacteria-dominated (ZSB) and macrophyte-dominated (XKB) habitats of Lake Taihu across four seasons. In ZSB, seasonal variations in acid volatile sulfide (AVS), dissolved sulfide (∑H2S), and Fe(II) were the primary drivers of sulfur bacterial communities. In contrast, SRB in XKB were mainly influenced by total nitrogen (TN) and total phosphorus (TP), while SOB were regulated by spatial heterogeneity, with AVS as the key driver. Spore-forming SRB such as Desulfotomaculum were distinctly enriched in ZSB, indicating an adaptive strategy to environmental fluctuations. Notably, Cupriavidus, a genus rarely linked to sulfur oxidation, was the dominant SOB genus in both habitats. Co-occurrence network analysis showed that dominant genera in ZSB acted as network hubs, indicating greater vulnerability to environmental changes in cyanobacteria-dominated habitats. Conversely, XKB displayed evenly distributed interaction networks without dominant network hubs, enhancing resilience to environmental fluctuations. These findings provide new insight into how macrophyte-dominated habitats enhance the resilience of shallow freshwater lakes to algal bloom expansion via stabilized sulfur cycling. Hydrological and hydrodynamic factors should be considered in future to better elucidate sulfur cycling in freshwater lakes.

Combining effects of submerged macrophytes and lanthanum-modified bentonite on sediment enzyme activity: Evidence from mesocosm study

Lanthanum-modified bentonite (LMB) combined with submerged macrophytes (SM) has been a conventional means of eutrophication management in lakes in recent years, and is one of the most important methods for P removal. However, trends in nutrients and sediment enzymes at the water-sediment interface during this process have not been systematically assessed, and there are still some gaps in how abiotic properties drive changes in enzyme activity. Here, we show changes in aquatic environmental conditions under the action of different ratios of modified bentonite (0, 10%, 20%, and 30%) in combination with SM (Vallisneria natans, Potamogeton lucens, and Hydrilla verticillate) and quantify their effects on sediment enzyme activities. The results showed that the nutrient cycling at the water-sediment interface was facilitated by the combined effect of SM and LMB, which effectively reduced the overlying water nutrient concentration, increased the sediment enzyme activity and enhanced the N cycling process. Partial least squares structural equation model (PLS-SEM) showed that sediment parameters strongly influenced changes in enzyme activity, with NO3-N as the main controlling factors. Our study fills in the process of changing environmental conditions in lake water under geoengineered materials combined with macrophyte measures, especially the changes in biological properties enzyme activities, which contributes to a clearer understanding of nutrient fluxes during the management of eutrophication in lakes.

Ditch level-dependent N removal capacity of denitrification and anammox in the drainage system of the Ningxia Yellow River irrigation district

Drainage networks, consisting of different levels of ditches, play a positive role in removing reactive nitrogen (N) via self-purification before drainage water returns to natural water bodies. However, relatively little is known about the N removal capacity of irrigation agricultural systems with different drainage ditch levels. In this study, we employed soil core incubation and soil slurry 15N paired tracer techniques to investigate the N removal rate (i.e., N2 flux), denitrification, anaerobic ammonium oxidation (anammox), and dissimilatory nitrate reduction to ammonium (DNRA) rates in the Ningxia Yellow River irrigation district at various ditch levels, including field ditches (FD), paddy field ditches (PFD), lateral ditches (LD1 and LD2), branch ditches (BD1, BD2, BD3), and trunk ditches (TD). The results indicated that the N removal rate ranged from 44.7 to 165.22 nmol N g-1 h-1 in the ditches, in the following decreasing order: trunk ditches > branch ditches > paddy field ditches > lateral ditches > field ditches. This result suggested that the N removal rate in drainage ditches is determined by the ditch level. In addition, denitrification and anammox were the primary pathways for N removal in the ditches, contributing 68.40-76.64 % and 21.55-30.29 %, respectively, to the total N removal. In contrast, DNRA contributed only 0.82-2.15 % to the total nitrate reduction. The N removal rates were negatively correlated with soil EC and pH and were also constrained by the abundances of denitrification functional genes. Overall, our findings suggest that the ditch level should be considered when evaluating the N removal capacity of agricultural ditch systems.

Effects of microplastics on denitrification and associated N2O emission in estuarine and coastal sediments: insights from interactions between sulfate reducers and denitrifiers

Global estuarine and coastal zones are facing severe microplastics (MPs) pollution. Sulfate reducers (SRB) and denitrifiers (DNB) are two key functional microorganisms in these zones, exhibiting intricate interactions. However, whether and how MPs modulate the interactions between SRB and DNB, with implications for denitrification and associated N2O emissions, remains poorly understood. Here, we simultaneously investigated the spatial response patterns of SRB-DNB interactions and denitrification and associated N2O emissions to different MPs exposure along an estuarine gradient in the Yangtze Estuary. Spatial responses of denitrification to polyvinyl chloride (PVC) and polyadipate/butylene terephthalate (PBAT) MPs exposure were heterogeneous, while those of N2O emissions were not. Gradient-boosted regression tree and multiple regression model analyses showed that sulfide, followed by nitrate (NO3-), controlled the response patterns of denitrification to MPs exposure. Further mechanistic investigation revealed that exposure to MPs resulted in a competitive and toxic (sulfide accumulation) inhibition of SRB on DNB, ultimately inhibiting denitrification at upstream zones with high sulfide but low NO3- levels. Conversely, MPs exposure induced a competitive inhibition of DNB on SRB, generally promoting denitrification at downstream zones with low sulfide but high NO3- levels. These findings advance the current understanding of the impacts of MPs on nitrogen cycle in estuarine and coastal zones, and provide a novel insight for future studies exploring the response of biogeochemical cycles to MPs in various ecosystems.

Simultaneous nitrate and sulfate biotransformation driven by different substrates: comparison of carbon sources and metabolic pathways at different C/N ratios

Nitrate (NO₃^-) and sulfate (SO₄^2-) often coexist in organic wastewater. The effects of different substrates on NO₃^- and SO₄^2- biotransformation pathways at various C/N ratios were investigated in this study. This study used an activated sludge process for simultaneous desulfurization and denitrification in an integrated sequencing batch bioreactor. The results revealed that the most complete removals of NO₃^- and SO₄^2- were achieved at a C/N ratio of 5 in integrated simultaneous desulfurization and denitrification (ISDD). Reactor Rb (sodium succinate) displayed a higher SO₄^2- removal efficiency (93.79%) with lower chemical oxygen demand (COD) consumption (85.72%) than reactor Ra (sodium acetate) on account of almost 100% removal of NO₃^- in both Ra and Rb. Ra produced more S^2- (5.96 mg L^-1) and H₂S (25 mg L^-1) than Rb, which regulated the biotransformation of NO₃^- from denitrification to dissimilatory nitrate reduction to ammonium (DNRA), whereas almost no H₂S accumulated in Rb which can avoid secondary pollution. Sodium acetate-supported systems were found to favor the growth of DNRA bacteria (Desulfovibrio); although denitrifying bacteria (DNB) and sulfate-reducing bacteria (SRB) were found to co-exist in both systems, Rb has a greater keystone taxa diversity. Furthermore, the potential carbon metabolic pathways of the two carbon sources have been predicted. Both succinate and acetate could be generated in reactor Rb through the citrate cycle and the acetyl-CoA pathway. The high prevalence of four-carbon metabolism in Ra suggests that the carbon metabolism of sodium acetate is significantly improved at a C/N ratio of 5. This study has clarified the biotransformation mechanisms of NO₃^- and SO₄^2- in the presence of different substrates and the potential carbon metabolism pathway, which is expected to provide new ideas for the simultaneous removal of NO₃^- and SO₄^2- from different media.

Microbial communities and sediment nitrogen cycle in a coastal eutrophic lake with salinity and nutrients shifted by seawater intrusion

Coastal waters are often influenced by seawater intrusion and terrestrial emissions because of its special location. In this study, the dynamics of microbial community with the role of nitrogen cycle in sediment in a coastal eutrophic lake were studied under a warm season. The water salinity gradually increased from 0.9‰ in June to 4.2‰ in July and 10.5‰ in August because of seawater invasion. Bacterial diversity of surface water was positively related with salinity and nutrients of total nitrogen (TN) as well as total phosphorus (TP), but eukaryotic diversity had no relationship with salinity. In surface water, algae belonging to Cyanobacteria and Chlorophyta were dominant phyla in June with the relative abundances of >60%, but Proteobacteria became the largest bacterial phylum in August. The variation of these predominant microbes had strong relationship with salinity and TN. In sediment, the bacterial and eukaryotic diversity was greater than that of water, and a significantly different microbial community was observed with dominant bacterial phyla Proteobacteria and Chloroflexi, and dominant eukaryotic phyla Bacillariophyta, Arthropoda, and Chlorophyta. Proteobacteria was the only enhanced phylum in the sediment with the highest relative abundance of 54.62% ± 8.34% due to seawater invasion. Denitrifying genera (29.60%-41.81%) were dominant in surface sediment, then followed by microbes related to nitrogen fixation (24.09%-28.87%), assimilatory nitrogen reduction (13.54%-19.17%), dissimilatory nitrite reduction to ammonium (DNRA, 6.49%-10.51%) and ammonification (3.07%-3.71%). Higher salinity caused by seawater invasion enhanced the accumulation of genes involved in dentrificaiton, DNRA and ammonification, but decreased genes related to nitrogen fixation and assimilatory nitrogen reduction. Significant variation of dominant genes of narG, nirS, nrfA, ureC, nifA and nirB mainly caused by the changes in Proteobacteria and Chloroflexi. The discovery of this study would be helpful to understand the variation of microbial community and nitrogen cycle in coastal lake under seawater intrusion.