The change of amino acids samples under metalimnetic oxygen minimum condition: Characterization and mechanism

Molecular response and adaptation mechanism of Microcystis aeruginosa under metalimnetic oxygen minimum conditions

Reservoirs are important drinking water sources. The metalimnetic oxygen minimum (MOM) usually occurs periodically in summer and autumn in deep-water reservoirs due to algae blooms and thermal stratification. This study aimed to explore the physiological and molecular responses of Microcystis aeruginosa (M. aeruginosa) under MOM conditions (darkness coupled with low dissolved oxygen (DO) concentration, hydrostatic pressure, and nutrient starvation). The comprehensive response of M. aeruginosa suggested that MOM conditions led to an immediate collapse of gas vesicles. This was followed by a gradual inhibition of photosynthesis by disturbing the electron transport chain and a significant downregulation of energy metabolism and carbohydrate metabolism. The active cells were approximately 5 % and > 45 % under MOM aerobic (3.0-7.0 mg/L DO) and anaerobic conditions (< 0.5 mg/L DO), respectively, for 20 days. In addition, a single exposure to darkness or pressure accelerated the decay of M. aeruginosa cells; however, MOM conditions with a low DO concentration had the opposite effect. The survival of M. aeruginosa cells under MOM conditions could be attributed to stringent response and the activation of HIF-1 signal when DO concentration decreased to < 2.0 mg/L by promoting the formation of cellular quiescence and resource redistribution. This study sheds light on the molecular response and adaptation mechanism of M. aeruginosa under MOM conditions.

Response of dissolved organic matter and disinfection by-product precursors to algal blooms and thermal stratification in deep reservoirs

Algal bloom contribute substantially to dissolved organic matter (DOM) and disinfection by-product (DBP) precursors in deep reservoirs, threatening drinking water safety. However, the variations in DOM and DBP precursors in deep-water reservoirs during algal bloom remain unclear. UV and fluorescence spectroscopy and chlorination experiments were used to analyze the variations in DOM and DBP precursors during algal bloom in the Sanhekou Reservoir. Before algal bloom, the DOM and DBP precursors decreased due to biodegradation. After algal bloom, the DOM and DBP precursors increased by 48.3% and 86.9% due to algae producing protein-like compounds. Notably, the algal bloom produced a range of nitrogenous compounds that significantly promote the formation of trichloronitromethane, a major contributor to the mammalian cytotoxicity associated with DBPs. In addition, the heterogeneous matrix led to the stratification of DOM and DBP precursors. The surface water (0-5 m) was more vulnerable to algae, with protein-like components being much higher than in other layers, while humic and fulvic-like components were much lower. However, high temperatures and sufficient oxygen conditions accelerated the biodegradation of DOM and DBP precursors, resulting in significantly lower levels of DOM and DBP precursors in the surface water compared to other layers (p < 0.05). This study provides insights into the variations and the drivers in DOM and DBP precursors during algal bloom, essential for developing water intake strategies in similar water reservoirs.

Changes of characteristics and disinfection by-products formation potential of intracellular organic matter with different molecular weight in metalimnetic oxygen minimum

Metalimnetic oxygen minimum (MOM) occurs in reservoirs or lakes due to stratification and algal blooms, which has low dissolved oxygen (DO) levels and leads to the deterioration of water quality. The transformation mechanism and the impact on the water quality of intracellular organic matter (IOM) derived from algae are poorly understood under MOM conditions. In this study, IOM extracted by Microcystis aeruginosa was divided into five components according to molecular weight (MW), and the changes of characteristics and correlated disinfection by-products formation potential (DBPFP) were analyzed and compared under MOM conditions. The removal efficiency of dissolved organic carbon (DOC) in the <5 kDa fraction (66.6%) was higher than that in the >100 kDa fraction (41.8%) after a 14-day incubation under MOM conditions. The same tendency also occurred in Fmax and DBPFP. The decrease in Fmax was mainly due to the decline in tryptophan-like and tyrosine-like for all IOM fractions. The diversity of microorganisms degrading the MW > 100 kDa fraction was lower than others. Besides low MW fractions, these findings indicated that more attention should be paid to high MW fractions which were resistant to biodegradation under MOM conditions during water treatment.