Environmental DNA metabarcoding reveals the effect of environmental selection on phytoplankton community structure along a subtropical river.

The Dongjiang River, a major tributary of the Pearl River system that supplies water to more than 40 million people in Guangdong Province and neighboring regions of China, harbors rich biodiversity, including many endemic and endangered species. However, human activities such as urbanization, agriculture, and industrialization have posed serious threats to its water quality and biodiversity. To assess the status and drivers of phytoplankton diversity, which is a key indicator of aquatic ecosystem health, this study used Environmental DNA (eDNA) metabarcoding combined with machine learning methods to explore spatial variations in the composition and structure of phytoplankton communities along the Dongjiang River, including its estuary. The results showed that phytoplankton diversity exhibited spatial distribution patterns, with higher community structure similarity and lower network complexity in the upstream than in the downstream regions. Environmental selection was the main mechanism shaping phytoplankton community composition, with natural factors driving the dominance of Pyrrophyta, Ochrophyta, and Cryptophyta in the upstream regions and estuaries. In contrast, the downstream regions was influenced by high concentrations of pollutants, resulting in increased abundance of Cryptophyta. The random forest model identified temperature, dissolved oxygen, chlorophyll a, NO2-, and NH4+ as the main factors influencing the primary phytoplankton communities and could be used to predict changes during wet periods. This study provides valuable insights into the factors influencing phytoplankton diversity and community composition in the Dongjiang River, and demonstrates the application value of eDNA metabarcoding technique in large-scale, long-distance river biodiversity monitoring.

Insights into the effects of bromide at fresh water levels on the radical chemistry in the UV/peroxydisulfate process.

Bromide (Br-) is a typical scavenger to sulfate radical (SO4•-) and hydroxyl radical (HO•), which simultaneously forms secondary reactive bromine species (RBS) such as Br• and Br2•-. This study investigated the effects of Br- at fresh water levels (~µM) on the radical chemistry in the UV/peroxydisulfate (UV/PDS) process by combining the degradation kinetics of probe compounds (nitrobenzene, metronidazole, and benzoate) with kinetic model. Br- at 1 - 50 µM promoted the conversion from SO4•- to HO• and RBS in the UV/PDS process. At pH 7, the concentration of SO4•- monotonically decreased by 31.5 - 94.8% at 1 - 50 µM Br-, while that of HO• showed an increasing and then decreasing pattern, with a maximum increase by 171.7% at 5 µM Br-. The concentrations of Br• and Br2•- (10-12 - 10-10 M) were 2 - 3 orders of magnitude higher than SO4•- and HO•. Alkaline condition promoted the conversion from SO4•- to HO•, and drove the transformation from RBS to HO•, resulting in much lower concentrations of RBS at pH 10. Br- at 1 µM and 5 µM decreased the pseudo-first-order reaction rates (k's) of 15 pharmaceuticals and personal care products (PPCPs) by 15.2 - 73.9%, but increased k's of naproxen and ibuprofen by 13.7 - 57.3% at pH 7. The co-existence of 10 - 1000 µM Cl- with 5 µM Br- further promoted the conversion from SO4•- to HO• compared to Br- alone. Bicarbonate consumed SO4•- and HO• but slightly affected RBS, while natural organic matter (NOM) exerted scavenging effects on HO• and RBS more significantly than SO4•-. This study demonstrated that Br- at fresh water levels significantly altered the radical chemistry of the UV/PDS process, especially for promoting the formation of HO•.

Kinetics and mechanisms of the degradation of PPCPs by zero-valent iron (Fe°) activated peroxydisulfate (PDS) system in groundwater.

The abatement of pharmaceuticals and personal care products (PPCPs), including carbamazepine (CBZ), acetaminophen (ACP) and sulfamethoxazole (SMX), by zero-valent iron (Fe°) activated peroxydisulfate (PDS) system (Fe°/PDS) in pure water and groundwater was investigated. The removal rates of CBZ, ACP and SMX were 85.4%, 100% and 73.1%, respectively, within 10 min by Fe°/PDS in pure water. SO4•-, •OH and O2•- were identified in the Fe°/PDS system, and O2•- was indicated to play an important role in the ACP degradation. The degradation of PPCPs increased with increasing dosages of Fe° and PDS or with decreasing pH and initial PPCP concentrations. Interestingly, the degradation of PPCPs by Fe°/PDS was significantly enhanced in groundwater compared with that in pure water, which was partially attributed to SO42- and Cl-. The first-order constants of CBZ, ACP and SMX increased from 0.021, 0.242 and 0.013 min-1 to 0.239, 2.536 and 0.259 min-1, and to 0.172, 1.516 and 0.197 min-1, respectively, with increasing the concentrations of SO42- and Cl- to 100 mg/L and 10 mg/L, respectively. This study firstly reports the unexpected enhancement of groundwater matrix on the degradation of micropollutants by Fe°/PDS, demonstrating that Fe°/PDS can be an efficient technology for groundwater remediation.