The secondary outbreak risk and mechanisms of Microcystis aeruginosa after H2O2 treatment.

The secondary outbreak of cyanobacteria after algicide treatment has been a serious problem to water ecosystems. Hydrogen peroxide (H2O2) is an algaecide widely used in practice, but similar re-bloom problems are inevitably encountered. Our work found that Microcystis aeruginosa (M. aeruginosa) temporarily hibernates after H2O2 treatment, but there is still a risk of secondary outbreaks. Interestingly, the dormant period was as long as 20 and 28 days in 5 mg L-1 and 20 mg L-1 H2O2 treatment groups, respectively, but the photosynthetic activity was both restored much earlier (within 14 days). Subsequently, a quantitative imaging flow cytometry-based method was constructed and confirmed that the re-bloom had undergone two stages including first recovery and then re-division. The expression of ftsZ and fabZ genes showed that M. aeruginosa had active transcription processes related to cell division protein and fatty acid synthesis during the dormancy stat. Furthermore, metabolomics suggested that the recovery of M. aeruginosa was mainly by activating folate and salicylic acid synthesis pathways, which promoted environmental stress resistance, DNA synthesis, and cell membrane repair. This study reported the comprehensive mechanisms of secondary outbreak of M. aeruginosa after H2O2 treatment. The findings suggest that optimizing the dosage and frequency of H2O2, as well as exploring the potential use of salicylic acid and folic acid inhibitors, could be promising directions for future algal control strategies.

Comparative study about ozonation to treat Microcystis-laden source water at the development and maintenance stage.

The outbreak of toxic cyanobacteria blooms is hazardous to water safety. Ozonation has been used to treat cyanobacteria-laden source water. Generally, cyanobacterial blooms enter into a long-term maintenance stage from the bloom development, but how the changed bloom stage affects ozonation is still unknow. Herein, influences of ozonation on cell inactivation and microcystin removal of Microcystis at the development and maintenance stage, were investigated. Then, ozonation-assisted coagulation for Microcystis removal at the two stages was compared. Results showed no significant difference in the photosynthetic inactivation of Microcystis at both stages. Microcystis at the maintenance stage exhibited a lower loss of membrane integrity (268-480 M-1 s-1) than that at the development stage (413-596 M-1 s-1). However, the extracellular microcystin increased by 30-410% at the maintenance stage at a lower ratio of [O3: DOC] (0.10-0.80) compared to the development stage (0.21-1.68), mainly ascribed to a decrease in the ozonation efficiency for microcystin removal. This finding might result from the elevated biomass and N-containing organics as competitors to reduce microcystin ozonation. Meanwhile, it was possible to generate fewer hydroxyl radicals to oxidize microcystin at the maintenance stage than that at the development stage. Besides, the removal ratio of Microcystis after ozonation-assisted coagulation, was reduced by 46-230% at the maintenance stage, due to the insufficient modification of cellular surface or elevated organics of 3-30 kDa. This work indicated that ozonation is effective to treat Microcystis at the development stage of a bloom whist pre-ozonation might be an inappropriate choice at the long-term maintenance stage.

Dynamic characteristics of total and microcystin-producing Microcystis in a large deep reservoir.

Microcystis, one of the common cyanobacteria, often causes blooms in reservoirs, which has seriously threatened the safety of drinking water worldwide. To identify the growth characteristic of total and microcystin-producing Microcystis in large deep reservoirs, we used Quantitative PCR (qPCR) to measure the cell density of total and microcystin-producing Microcystis and monitored water quality in the water samples collected in Dongzhang Reservoir once a month. Microcystis blooms occurred in Dongzhang Reservoir in April 2017, which was composed of microcystin-producing and non-microcystin-producing Microcystis. Water temperature, dissolved oxygen, pH, and chlorophyll-a showed significant vertical stratification during Microcystis blooms. Total and microcystin-producing Microcystis grew rapidly under the high concentration of total phosphorus and rising water temperatures. Nitrate-nitrogen had a significant linear correlation with the abundance of microcystin-producing Microcystis. Our results indicated that nutrients and water temperature could be key triggers of Microcystis blooms and nitrate-nitrogen potentially regulates the competition between microcystin-producing and non-microcystin-producing Microcystis. This study improves our understanding of the characteristics of Microcystis blooms and the competition between microcystin-producing and non-microcystin-producing Microcystis in large deep reservoirs.

Microbiological risks increased by ammonia-oxidizing bacteria under global warming: The neglected issue in chloraminated drinking water distribution system.

A rising outbreak of waterborne diseases caused by global warming requires higher microbial stability in the drinking water distribution system (DWDS). Chloramine disinfection is gaining popularity in this context due to its good persistent stability and fewer disinfection byproducts. However, the microbiological risks may be significantly magnified by ammonia-oxidizing bacteria (AOB) in distribution systems during global warming, which is rarely noticed. Hence, this work mainly focuses on AOB to explore its impact on water quality biosafety in the context of global warming. Research indicates that global warming-induced high temperatures can directly or indirectly promote the growth of AOB, thus leading to nitrification. Further, its metabolites or cellular residues can be used as substrates for the growth of heterotrophic bacteria (e.g., waterborne pathogens). Thus, biofilm may be more persistent in the pipelines due to the presence of AOB. Breakpoint chlorination is usually applied to control such situations. However, switching between this strategy and chloramine disinfection would result in even more severe nitrification and other adverse effects. Based on the elevated microbiological risks in DWDS, the following aspects should be paid attention to in future research: (1) to understand the response of nitrifying bacteria to high temperatures and the possible association between AOB and pathogenic growth, (2) to reveal the mechanisms of AOB-mediated biofilm formation under high-temperature stress, and (3) to develop new technologies to prevent and control the occurrence of nitrification in drinking water distribution system.

Higher sensitivity to Cu2+ exposure of Microcystis aeruginosa in late lag phase is beneficial to its control.

Cyanobacterial blooms are always treated in exponential phase, which demands high dosages of algicides (e.g., CuSO4). Actually, cyanobacterial blooms in late lag phase exhibit low cell-density and specific physiological/biochemical characteristics, implying the possibility of controlling blooms in a more efficient and economical way with CuSO4 treatment if cyanobacterial cells in late lag phase can be treated. In this study, the outbreakof a Microcystis bloom was simulated, and Microcystis samples in late lag and exponential phases were treated with CuSO4. The results showed that M. aeruginosa in late lag phase had a higher ratio of dividing-cells, Fv/Fm and intracellular total organic carbon content (TOC) than that in exponential phase, indicating that its metabolic activity was vigorous. M. aeruginosa in late lag phase could more easily be blocked, since a higher decrease in chlorophyll-a, Fv/Fm and membrane integrity occurred under the same dosages of CuSO4 exposure compared to M. aeruginosa in exponential phase. Meanwhile, microcystin release in late lag phase was less than that in exponential phase. Moreover, higher sensitivity in late lag phase was confirmed at the individual level, as the photosynthesis related genes psaB and rbcL were more down-regulated than those in exponential phase. In general, cyanobacteria in late lag phase exhibited higher sensitivity to CuSO4, indicating that CuSO4 treatments in late lag phase can achieve a higher control efficiency and fewer release of microcystin with low-dosages algicide. Hence, it is a more environmentally friendly strategy to control cyanobacterial blooms than the traditional strategy applied in exponential phase.

Recovery of microbiological quality of long-term stagnant tap water in university buildings during the COVID-19 pandemic.

Stagnant water can cause water quality deterioration and, in particular, microbiological contaminations, posing potential health risks to occupants. University buildings were unoccupied with little water usage during the COVID-19 pandemic. It's an opportunity to study microbiological quality of long-term stagnant water (LTSW) in university buildings. The tap water samples were collected for three months from four types of campus buildings to monitor water quality and microbial risks after long-term stagnation. Specifically, the residual chlorine, turbidity, and iron/zinc were disqualified, and the heterotrophic plate counts (HPC) exceeded the Chinese national standard above 100 times. It took 4-54 days for these parameters to recover to the routine levels. Six species of pathogens were detected with high frequency and levels (101-105 copies/100 mL). Remarkably, L. pneumophilia occurred in 91% of samples with turbidity > 1 NTU. The absence of the culturable cells for these bacteria possibly implied their occurrence in a viable but non-culturable (VBNC) status. The bacterial community of the stagnant tap water differed significantly and reached a steady state in more than 50 days. Furthermore, a high concentration of endotoxin (>10 EU/mL) was found in LTSW, which was in accordance with the high proportion of dead bacteria. The results suggested that the increased microbiological risks require more attention and the countermeasures before the building reopens should be taken.