Sinking loss should be taken into account while studying the dynamics of Microcystis under light-availability control

Spatio-temporal variation of toxin-producing gene abundance in Microcystis aeruginosa from Poyang Lake

Microcystis aeruginosa (M. aeruginosa) causes massive blooms in eutrophic freshwater and releases microcystin. Poyang Lake is the largest freshwater lake in China and has kept a mid-nutrient level in recent years. However, there is little research on microcystin production in Poyang Lake. In this study, water and sediment samples from ten sampling sites in Poyang Lake were collected from May to December in 2020, and from January to April in 2021 respectively. Microcystis genes (mcyA, mcyB, 16 s rDNA) were quantified by real-time fluorescence quantitative PCR analysis, and then the spatial and temporal variation of mcy genes, physicochemical factors, and bacterial population structure in the lake was analyzed. The relationship between the abundance of mcy genes and physicochemical factors in water column was also revealed. Results indicated that the microcystin-producing genes mcyA and mcyB showed significant differences in spatial and temporal levels as well, which is closely related to the physicochemical factors especially the water temperature (p < 0.05) and the nitrogen content (p < 0.05). The abundance of mcy genes in the sediment in December affected the abundance of mcy genes in the water column in the next year, while the toxic Microcystis would accumulate in the sediment. In addition to the toxic Microcystis, we also found a large number of non-toxic Microcystis in the water column and sediment, and the ratio of toxic to non-toxic species can also affect the toxicity production of M. aeruginosa. Overall, the results showed that M. aeruginosa toxin-producing genes in Poyang Lake distributed spatially and temporally which related to the physicochemical factors of Poyang Lake.

Spatial and seasonal change in algal community structure and its interaction with nutrient dynamics in a gravel-bed urban river

Harmful algal blooms frequently occur in urban rivers due to intense human activities. However, little is known about the change in algal community structure and its interactions with nutrient dynamics in gravel-bed urban rivers. In present study, water samples were collected from a gravel-bed River Xin'an, China for five months over four seasons and a rainy month to measure algal community structure, dissolved nitrogen gas (N₂) and Argon (Ar) concentrations, and other water quality parameters. The results showed that the harmful Cyanophyta accounted for 31.6 ± 24.1% of the total community in the hot season while Bacillariophyta contributed more than 60% to the community in the other three seasons. The N₂ was supersaturated in the moderate and cold seasons but it was unsaturated in the hot season, along with high concentrations of nitrogen-fixing cyanobacteria (Anabaena), indicating that the nitrogen fixation capacity was strong and even stronger than denitrification and anammox in the hot season. However, nitrogen fixation was not the main source of nitrogen in the water column. The concentrations of nutrients and Chla in the downstream river were significantly higher than those in the upstream river (p < 0.001 for nutrients and p = 0.029 for Chla), suggesting that human activities along the river greatly affected nutrient concentrations, as well as algal growth. Our study provides new insights into the algal community succession in a gravel-bed urban river and puts forward effective measures such as controlling exogenous nutrient input and dredging organic sediment for mitigating the harmful algal blooms in urban rivers.

Retrieval of water quality parameters from hyperspectral images using a hybrid feedback deep factorization machine model

Environmental protection of water resources is of critical importance to daily life of human beings. In recent years, monitoring the variation of water quality using remote sensing techniques has become prevalent. Unmanned aerial vehicle (UAV) based remote sensing techniques have been applied to quantitative retrieval of concentrations of water quality parameters including phosphorus, nitrogen, chemical oxygen demand (COD), biochemical oxygen demand (BOD), and chlorophyll a (Chl-a), successfully and efficiently. In this study, a novel method with deep factorization machine, spatial distribution pattern analysis, and probabilistic analysis engaged, named hybrid feedback deep factorization machine (HF-DFM), has been developed to quantitatively estimate concentrations of water quality parameters based on hyperspectral reflectance data on large scale effectively. Our proposed method is a unified model for quantifying concentrations of water quality parameters with an end to end structure, which integrates UAV based optical remote sensing techniques and deep learning to estimate concentrations of water quality parameters. Furthermore, our proposed model was applied to real-time quantitative monitoring the variation of water quality of Mazhou River, Shenzhen, Guangdong, China. Finally, we evaluate the performance of proposed model on a real-world dataset in terms of root of mean squared error (RMSE), mean absolute percent error (MAPE), and coefficient of determination (R²). The experimental results show that our proposed model outperforms other state-of-the-art models with respect to RMSE, MAPE, and R², where resulting MAPEs for quantifying all water quality parameters range from 8.78% to 12.36%, and resulting R²s range from 0.81 to 0.93. It can serve as a useful tool for decision makers in effectively monitoring water quality of urban rivers.

Nitrogen availability affects the dynamics of Microcystis blooms by regulating the downward transport of biomass

Nitrogen availability is one of the key factors affecting the dynamics of non-diazotrophic cyanobacterial blooms in eutrophic lakes. While previous studies mainly focused on the promoting effect of nitrogen on the growth of cyanobacteria, this study aimed to investigate the role of nitrogen availability in the downward transport of biomass and its effects on the dynamics of Microcystis blooms. We performed field enclosure experiments which demonstrated that nitrogen availability negatively affects the downward transport of biomass. With a nitrogen loading of 0.02 g N m^-2 d^-1, the Microcystis biomass in the water column decreased by 56.2% over a 4-day period. During the same period of time, the average sinking ratio was 0.23 d^-1; moreover, the termination of biomass growth was detected. At the notably higher nitrogen loading of 0.5 g N m^-2d^-1, the downward transport of biomass could still compensate for the biomass growth, although the average sinking ratio was lower at 0.16 d^-1. Additional laboratory culture experiments demonstrated that the increase in the downward transport of Microcystis occurred in parallel to an increase in the carbohydrate content and a decrease in gas vesicle content. Further proteomic analysis indicated that the carbohydrate accumulation induced by nitrogen deficiency was a result of the slowing down of catabolic consumption, especially the downregulation of glycolysis. Thus, our study suggests that increased intracellular carbohydrate accumulation at low nitrogen availability causes a higher sinking ratio of Microcystis, indicating that nitrogen limits the duration of Microcystis blooms; thus, decreased nitrogen availability may lead to increased sinking of biomass out of the water column, accelerating the dissipation of Microcystis blooms.

Nitrogen limitation affects the sinking property of Microcystis by promoting carbohydrate accumulation

Nitrogen limitation has been proven to inhibit Microcystis proliferation, and the significant decline in Microcystis blooms in late summer or autumn has been considered to be related to the nitrogen depletion in water. Sinking loss is another factor that influences the dynamics of cyanobacteria in lakes. However, to date, it is still unclear how the sinking property of Microcystis responds to nitrogen availability. Our results suggest that nitrogen limitation would directly influence sinking property of Microcystis, through a significant increase in the specific density of cells. In the short term, carbohydrate accumulation was mainly responsible for the high specific density, showing a high correlation among the NO₃^--N concentration, specific density and carbohydrate content. Furthermore, carbohydrates could rapidly accumulate after one light/dark cycle, which was mainly due to the reduction in carbohydrate consumption in the darkness under nitrogen limitation. Under nitrogen-light coupling conditions, the specific density ranged from 1.060 to 1.068, except for the treatment with high-nitrogen plus low-light, which showed the value of 1.032. More importantly, when coupled with low nitrogen, the low light did not decrease the carbohydrate content and the specific density, which implied that the sinking cells could not migrate back to the surface. Accordingly, a hypothesis was proposed that the carbohydrate accumulation induced by low nitrogen availability caused an increase in specific density, which invalidates the buoyancy regulation, and cells sink continually out of the water column. This study explores a new understanding on the disappearance mechanisms of Microcystis blooms in the late summer and fall.

The secretion of organics by living Microcystis under the dark/anoxic condition and its enhancing effect on nitrate removal

Recent studies indicated that the algal decomposition produces particulate and dissolved organic carbon (DOC), and can enhance denitrification in eutrophic lakes. However, the effects of the living cyanobacteria on nitrogen cycling in eutrophic lakes were still an unknown question. This study explores a new underlying mechanism of nitrate removal which is driven by living Microcystis. The results suggested that living Microcystis significantly enhanced the nitrate removal at sediment-water interface, with a nitrate removal rate of 0.54 d^-1, which was 2.57 times higher than the nitrate removal rate in the treatment without the addition of Microcystis. Measurements of Chl a and Fv/Fm confirmed that Microcystis was tolerant to the dark/anoxic condition, and the recovery experiments suggested that Microcystis could survive under such stress conditions for at least seven days. Meanwhile, DOC secreted by living Microcystis reached to 4.55 mg C mg^-1 Chl a. These secretions were biodegradable hydrophilic and contained carbohydrates and proteins. Our study indicated that during blooms, sinking Microcystis cells could directly provide DOC as carbon source, then consequently enhanced the denitrification at sediment-water interface, and the interactive relationship between living cyanobacteria and permanent nitrate removal should be taken into account while studying nitrogen cycling in aquatic ecosystem.

The production of cyanobacterial carbon under nitrogen-limited cultivation and its potential for nitrate removal

Harmful cyanobacterial blooms (CyanoHABs) represent a serious threat to aquatic ecosystems. A beneficial use for these harmful microorganisms would be a promising resolution of this urgent issue. This study applied a simple method, nitrogen limitation, to cultivate cyanobacteria aimed at producing cyanobacterial carbon for denitrification. Under nitrogen-limited conditions, the common cyanobacterium, Microcystis, efficiently used nitrate, and had a higher intracellular C/N ratio. More importantly, organic carbons easily leached from its dry powder; these leachates were biodegradable and contained a larger amount of dissolved organic carbon (DOC) and carbohydrates, but a smaller amount of dissolved total nitrogen (DTN) and proteins. When applied to an anoxic system with a sediment-water interface, a significant increase of the specific NO_X^--N removal rate was observed that was 14.2 times greater than that of the control. This study first suggests that nitrogen-limited cultivation is an efficient way to induce organic and carbohydrate accumulation in cyanobacteria, as well as a high C/N ratio, and that these cyanobacteria can act as a promising carbon source for denitrification. The results indicate that application as a carbon source is not only a new way to utilize cyanobacteria, but it also contributes to nitrogen removal in aquatic ecosystems, further limiting the proliferation of CyanoHABs.