Comparative assessment of algaecide performance on freshwater phytoplankton: Understanding differential sensitivities to frame cyanobacteria management

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.

Unveiling the susceptibility mechanism of Microcystis to consecutive sub-lethal oxidative stress-Enhancing oxidation technology for cyanobacterial bloom control

The use of H2O2 to mitigate cyanobacterial blooms has gained popularity due to its selectivity. Previous research has shown that consecutive low-dose H2O2 are far more effective in suppressing cyanobacteria than a single higher dose, minimizing damage to co-existing organisms in the aquatic ecosystem. However, the underlying mechanism remains unclear. This study aimed to investigate the mechanism underlying this sensitivity by monitoring the progression from oxidative stress to cell death in Microcystis induced by consecutive low doses of H2O2 (3 + 5 mg/L, with an interval time of 4 h). The initial application of H2O2 (3 mg/L) resulted in a rapid increase in the transcription of antioxidant genes (gpx, 2-cys prx, trxA and sod) within 1 h, and returned to baseline levels within 8 h. The addition of a second H2O2 led to a significant increase in glutathione peroxidase (gene and product) and glutathione within 24 h. The cell death following consecutive H2O2 stress was classified as regulated cell death (RCD), characterized by the upregulated metacaspase genes, increased caspase-like activity, modulation of the mazEF system, DNA fragmentation, cell vacuolization, and membrane disruption. Interestingly, the RCD process coincided with the fluctuation of glutathione cycle. Validation experiments demonstrated that exogenous glutathione can promote the gene expression and activity of metacaspase, while inhibition of glutathione biosynthesis led to decreased intracellular glutathione and suppressed metacaspase activity and gene expression. Therefore, glutathione may play a vital role in the connection between oxidative stress and RCD during consecutive H2O2 treatment. These results reveal the inherent vulnerability of Microcystis to consecutive oxidative stress, providing a biological mechanism for a sustainable strategy to mitigate cyanobacterial bloom.

Using total adenosine triphosphate (tATP) measurements for cyanobacterial bloom monitoring and response assessment during algaecide treatments

Total adenosine triphosphate (tATP) was investigated for its potential as a rapid indicator of cyanobacterial growth and algaecide effectiveness. tATP and other common bloom monitoring parameters were measured over the growth cycles of cyanobacteria and green algae in laboratory cultures and examined at a drinking water source during an active bloom. Strong correlations (R2>0.78) were observed between tATP and chlorophyll-a in cyanobacteria cultures. tATP offered greater sensitivity by increasing two orders of magnitude approximately 7 d before changes in chlorophyll-a or optical density were observed in Lyngbya sp. and Dolichospermum sp. cultures. Increases in tATP per cell coincided with the onset of exponential growth phases in lab cultures and increase in cell abundance in field samples, suggesting that ATP/cell is a sensitive indicator that may be used to identify the development of blooms. Bench-scale trials using samples harvested during a bloom showed that tATP exhibited a clear dose-response during copper sulfate (CuSO4) and hydrogen peroxide (H2O2) treatment compared to chlorophyll-a and cell counts, indicating that cellular production and storage of ATP decreases even when live and dead cells cannot be distinguished. During Copper (Cu) algaecide application at a reservoir used as a drinking water source, tATP and cell counts decreased following initial algaecide application; however, the bloom rebounded within 10 d showing that the Cu algaecide only has limited effectiveness. In this case, tATP was a sensitive indicator to bloom rebounding after algaecide treatments and correlated positively with cell counts (R2=0.7). These results support the use of tATP as a valuable complementary bloom monitoring tool for drinking water utilities to implement during the monitoring and treatment of cyanobacterial blooms.

Unleashing the power of acetylacetone: Effective control of harmful cyanobacterial blooms with ecological safety

Harmful algal blooms resulting from eutrophication pose a severe threat to human health. Acetylacetone (AA) has emerged as a potential chemical for combatting cyanobacterial blooms, but its real-world application remains limited. In this study, we conducted a 42-day evaluation of AA's effectiveness in controlling blooms in river water, with a focus on the interplay between ecological community structure, organism functional traits, and water quality. At a concentration of 0.2 mM, AA effectively suppressed the growth of Cyanobacteria (88 %), Bacteroidia (49 %), and Alphaproteobacteria (52 %), while promoting the abundance of Gammaproteobacteria (5.0 times) and Actinobacteria (7.2 times) that are associated with the degradation of organic matter. Notably, after dosing of AA, the OD680 (0.07 ± 0.02) and turbidity (8.6 ± 2.1) remained at a satisfactory level. AA induced significant disruptions in two photosynthesis and two biosynthesis pathways (P < 0.05), while simultaneously enriching eight pathways of xenobiotics biodegradation and metabolism. This enrichment facilitated the reduction of organic pollutants and supported improved water quality. Importantly, AA treatment decreased the abundance of two macrolide-related antibiotic resistance genes (ARGs), ereA and vatE, while slightly increased the abundance of two aminoglycoside-related ARGs, aacA and strB. Overall, our findings establish AA as an efficient and durable algicide with favorable ecological safety. Moreover, this work contributes to the development of effective strategies for maintaining and restoring the health and resilience of aquatic ecosystems impacted by harmful algal blooms.