Characterizing legacy nitrogen-induced time lags in riverine nitrogen reduction for the Songhuajiang River Basin: Source analysis, spatio-seasonal patterns, and impacts on future water quality improvement.

Legacy nitrogen (N) originating from net N inputs (NNI) may pose ongoing threats to riverine water quality worldwide and even cause serious time-lags between water quality restoration and NNI declines. A better understanding of legacy N effects on riverine N pollutions in different seasons is essential to improve riverine water quality. Here, we investigated contributions of legacy N on riverine dissolved inorganic N (DIN) changes in different seasons and quantified spatio-seasonal time-lags in the Songhuajiang River basin (SRB), a hotspot of NNI with four distinct seasons, by exploring long-term (1978-2020) NNI-DIN relationships. Results firstly showed a significant seasonal difference in NNI, with the highest value observed in spring (average, 2184.1 kg/km2), 1.2, 5.0, and 4.6 times higher than that in summer, autumn, and winter, respectively. Cumulative legacy N had dominated riverine DIN changes, with a relative contribution of approximately 64% in 2011-2020, causing time-lags of 11-29 years across the SRB. The longest seasonal lags existed in spring (average, 23 years) owing to greater impacts of legacy N to riverine DIN changes in this season. Mulch film application, soil organic matter accumulation, N inputs, and snow cover were identified as the key factors that strengthened seasonal time-lags by collaboratively enhancing legacy N retentions in soils. Furthermore, a machine learning-based model system suggested that timescales for water quality improvement (DIN, ≤1.5 mg/L) varied considerably (from 0 to >29 years, Improved N Management-Combined scenario) across the SRB, with greater lag effects contributing to slower recovery. These findings can provide a more comprehensive insight into sustainable basin N management in the future.

Metagenomics and network analysis elucidating the coordination between fermentative bacteria and microalgae in a novel bacterial-algal coupling reactor (BACR) for mariculture wastewater treatment.

To achieve the goal of treating mariculture wastewater economically and efficiently, a novel bacterial-algal coupling reactor (BACR) integrating acidogenic fermentation and microalgae cultivation was firstly investigated for mariculture wastewater treatment. Volatile fatty acids (VFAs) generated in the dark chamber migrated into the photo chamber for microalgal utilization, which alleviated the pH drop and feedback inhibition of the acidogenic fermentation. The maximum dry cell weight (DCW) of microalgae was 1.46 g/L, and pollutants such as chemical oxygen demand (COD), ammonium (NH4+-N) and total phosphorus (TP) in the BACR were effectively removed under the mixotrophic culture condition. Furthermore, bacterial community profiles and functional genes in the BACR and single acidogenic fermentation reactor were identified. Compared with the single acidogenic fermentation reactor, most of the fermentative bacteria (e.g., Ruminococcus, Christensenellaceae R-7 group, Exiguobacterium, Pseudomonas and Levilinea) were enriched by the BACR. From the genetic perspective, the abundances of dominant genes (ackA, acs and atoD) associated with acetic, propionic and butyric acid production were greatly enhanced in the BACR. In the fatty acid biosynthesis pathway (ko00061), three kinds of high-abundance acetyl-CoA carboxylase genes and eight kinds of downstream functional genes were up-regulated in the BACR. Finally, based on co-occurrence network analysis, the coordination between fermentative bacteria and microalgae in the BACR was revealed. This study provided a deep insight into the advantage and potential of the BACR in mariculture wastewater treatment.

Mariculture wastewater treatment with Bacterial-Algal Coupling System (BACS): Effect of light intensity on microalgal biomass production and nutrient removal.

Mariculture wastewater generated from the mariculture industry has increased public concern due to its impact on the sustainability of aquatic environments and aquaculture practices. Herein, the Bacterial-Algal Coupling System was applied for mariculture wastewater treatment. Microalgae growth in heterotrophy and mixotrophy (2000-8000 lux) was first compared. The best microalgal growth and nutrient removal were obtained at 5000 lux, where biomass productivity of microalgae was 0.465 g L-1 d-1, and 98.1% of chemical oxygen demand, 70.7% of ammonia-nitrogen, and 90.0% of total phosphorus were removed. To further understand the nutrient removal through microalgae cultivation, the enzyme activities involved in the Calvin cycle and the Tricarboxylic Acid cycle at different light intensities were determined. Under mixotrophic cultivation, there was a coordination between photosynthesis and heterotrophic metabolism in the agal cell, which resulted in a high algal biomass production and removal efficiency of nutrients. This study provided a novel insight into the bioremediation of mariculture wastewater and microalgae cultivation.

Denitrification performance evaluation and kinetics analysis with mariculture solid wastes (MSW) derived carbon source in marine recirculating aquaculture systems (RAS).

Biological denitrification using mariculture solid wastes (MSW) carbon source is a promising solution for removing nitrate (NO3--N) and disposing MSW in marine recirculating aquaculture systems (RAS). To enhance denitrification performance, heating (HT), rhamnolipid (RL), alkali (AL), thermophilic bacteria (TB) pre-treated MSW acidogenic fermentation effluents were prepared as carbon sources. Profiles of soluble organics in four types of fermentation effluents were first evaluated. The highest volatile fatty acids (VFAs) yield (52.1%) was obtained from TB treated MSW after acidification. RL and TB treated MSW acidogenic fermentation effluents showed high NO3--N removal efficiency (NRE) (around 97%). Acidogenic fermentation effluent from TB treated MSW presented a high biodegradability, with the minimum effluent chemical oxygen demand (COD) amount (35 mg/L). Denitrification kinetics parameters were also analyzed; high fraction (74.5%) of the most readily biodegradable organics (SS) demonstrated that TB treated MSW acidogenic fermentation effluent is a high-quality carbon source for enhancing denitrification.

Heterotrophic denitrification strategy for marine recirculating aquaculture wastewater treatment using mariculture solid wastes fermentation liquid as carbon source: Optimization of COD/NO3--N ratio and hydraulic retention time.

Heterotrophic denitrification using mariculture solid wastes (MSW) fermentation liquid as carbon source is an economically and environmentally sustainable strategy for NO3--N removal in marine recycling aquaculture systems (RAS). The optimization of COD/NO3--N ratio (C/N) and hydraulic retention times (HRT) with respect to MSW fermentation liquid driven denitrification for marine RAS wastewater treatment was investigated. The optimum C/N of 8 and HRT of 6 h for heterotrophic denitrification was obtained with NO3--N removal efficiency of 97.8% and 94.2%, respectively. Using MSW fermentation liquid as carbon source, the utilization of VFAs was more effective than that of carbohydrates and proteins, and effluent COD concentration decreased with an increment in HRT from 4 to 8 h. The results of high-throughput sequencing analysis showed microbial communities were enriched selectively in the reactors by optimizing C/N and HRT, which obviously enhanced the nitrogen removal in respect to MSW fermentation liquid driven denitrification.