ABSTRACT
Anaerobic digestion (AD) plays a significant role in renewable energy recovery. Upgrading AD from thermophilic (50-57 °C) to mesophilic (30-38 °C) conditions to enhance process stability and reduce energy input remains challenging due to the high sensitivity of thermophilic microbiomes to temperature fluctuations. Here we compare the effects of two decreasing-temperature modes from 55 to 35 °C on cell viability, microbial dynamics, and interspecies interactions. A sharp transition (ST) is a one-step transition by 20 °C d-1, while a mild transition (MT) is a stepwise transition by 1 °C d-1. We find a greater decrease in methane production with ST (88.8%) compared to MT (38.9%) during the transition period. ST mode overproduced reactive oxygen species by 1.6-fold, increased membrane permeability by 2.2-fold, and downregulated microbial energy metabolism by 25.1%, leading to increased apoptosis of anaerobes by 1.9-fold and release of intracellular substances by 2.9-fold, further constraining methanogenesis. The higher (1.6 vs. 1.1 copies per gyrA) metabolic activity of acetate-dependent methanogenesis implied more efficient methane production in a steady mesophilic, MT-mediated system. Metagenomic binning and network analyses indicated that ST induced dysbiosis in keystone species and greatly enhanced microbial functional redundancy, causing loss of microbial syntrophic interactions and redundant metabolic pathways. In contrast, the greater microbial interconnections (average degrees 44.9 vs. 22.1) in MT at a steady mesophilic state suggested that MT could better maintain necessary system functionality and stability through microbial syntrophy or specialized pathways. Adopting MT to transform thermophilic digesters into mesophilic digesters is feasible and could potentially enhance the further optimization and broader application of practical anaerobic engineering.
ABSTRACT
Polychlorinated dibenzothiophenes (PCDTs) are a class of compounds structurally similar to dioxins that possess various toxicological impacts on living organisms. Unfortunately, information on the levels of PCDTs in freshwater lakes in China is still scarce. In this work, the occurrence of 14 congeners of PCDTs in different matrices (i.e., sediment, suspended particulate matter (SPM), and water) of Chaohu Lake was investigated. It was determined that the concentrations of 14 PCDTs (Σ14PCDTs) in the sediment, SPM, and surface water were 0.40-3.55 ng g-1 (dry weight, d.w.), 0.38-2.95 ng·g-1 d.w., and 0.34-2.61 ng L-1, respectively. The dominant congener found in sediments was 1,2,3,4,7-penta-CDT (19.54%), and 1,3,9-tri-CDT was the predominant congener in SPM (19.13%) and water (20.08%). Medium- and high-chlorinated PCDTs were detected as the major compounds in sediments and SPM. The low-chlorinated PCDTs (e.g., mono-CDTs) have higher relative percentages in the water than those detected in the sediment samples. The annual Σ14PCDT input of the eight main tributaries to Chaohu Lake was 19.90 kg. A strong linear correlation between the Σ14PCDT levels and the organic carbon (OC) content demonstrated that OC had an important influence on the PCDT redistribution in Chaohu Lake. In addition, the organic carbon normalized partitioning coefficient (logKOC) of PCDTs in the SPM-water system in Chaohu Lake was 1.95-2.49 mL g-1, and correlations between logKOC and other typical environment-related properties of PCDTs were established. This study provided useful data on the evaluation of ecological risks of PCDTs in Chaohu Lake.
