Exploring the relative changes in dissolved organic matter for assessing the water quality of full-scale drinking water treatment plants using a fluorescence ratio approach

Authors

Tahir Maqbool, Yanling Qin, Quang Viet Ly, Jiaxing Zhang, Chengyue Li, Muhammad Bilal Asif, Zhenghua Zhang

This study aims to extend and demonstrate the application of fluorescence spectroscopy for monitoring the water quality of three differently operated full-scale drinking water treatment plants located in the Shenzhen city (China). A ratio of fluorescent dissolved organic matter (FDOM), which describes relative changes in humic-like to protein-like fluorescence, was used to explain mechanisms behind the physicochemical processes. The fluorescence components obtained through individual and combined parallel factor analysis (PARAFAC) modeling revealed the presence of humic-like (C1) and protein-like (C2) structures in the DOM. The C1/C2 ratio provided a direct relationship between the seasonal variations and DOM composition. Wet season generated DOM enriched with humic-like fluorescence, while dry season caused a higher release of protein-like fluorescence. The fluorescence ratio presented unique patterns of DOM in treatment trains. The chemical pretreatment and disinfection unit processes showed a higher tendency to remove the humic-like fluorescence. However, the C1/C2 ratio increased during physical treatment processes such as coagulation-precipitation and sand filtration, indicating preferential removal of protein-like fluorescence. The DOM composition in influent directly (R² = 0.77) influenced the relative intensities of fluorescence components in the treated water. Compared to the dry season, the wet season caused significant changes in DOM composition and produced treated water enriched with humic-like fluorescence. This fluorescence ratio offers an approach to explore the role of different treatment units and determine the factors affecting the composition of DOM in the surface water and drinking water treatment plants.

Disentangling the drivers of Microcystis decomposition: Metabolic profile and co-occurrence of bacterial community

Authors

Shengnan Chen, Miaomiao Yan, Tinglin Huang, Hui Zhang, Kaiwen Liu, Xin Huang, Nan Li, Yutian Miao, Raju Sekar

In aquatic ecosystems, water microbial communities can trigger the outbreak or decline of cyanobacterial blooms. However, the microbiological drivers of Microcystis decomposition in reservoirs remain unclear. Here, we explored the bacterial community metabolic profile and co-occurrence dynamics during Microcystis decomposition. The results showed that the decomposition of Microcystis greatly altered the metabolic characteristics and composition of the water bacterial community. Significant variations in bacterial community composition were observed: the bacterial community was mainly dominated by Proteobacteria, Actinobacteria, Planctomycetes, and Bacteroidetes during Microcystis decomposition. Additionally, members of Exiguobacterium, Rhodobacter, and Stenotrophomonas significantly increased during the terminal stages. Dissolved organic matters (DOM) primarily composed of fulvic-like, humic acid-like, and tryptophan-like components, which varied distinctly during Microcystis decomposition. Additionally, the metabolic activity of the bacterial community showed a continuous decrease during Microcystis decomposition. Functional prediction showed a sharp increase in the cell communication and sensory systems of the bacterial communities from day 12 to day 22. Co-occurrence networks showed that bacteria responded significantly to variations in the dynamics of Microcystis decomposition through close interactions between each other. Redundancy analysis (RDA) indicated that Chlorophyll a, nitrate nitrogen (NO₃⁻-N), dissolved oxygen (DO), and dissolved organic carbon (DOC) were crucial drivers for shaping the bacterial community structure. Taken together, these findings highlight the dynamics of the water bacterial community during Microcystis decomposition from the perspective of metabolism and community composition, however, further studies are needed to understand the algal degradation process associated with bacteria.

Measuring photosynthesis of both oxygenic and anoxygenic photosynthetic organisms using pulse amplitude modulation (PAM) fluorometry in wastewater ponds

Authors

P. Chandaravithoon, R. J. Ritchie & J. W. Runcie

Oxygenic photosynthesis can be measured easily using O₂ or CO₂ gas exchange, oxygen electrodes, Winkler titration, ¹⁴CO₂-fixation and by PAM (pulse amplitude modulation) fluorometry. PAM estimates the photosynthetic electron transport rate (ETR) by measuring the variable fluorescence of chlorophyll (Chl) a (> 695 nm) induced by absorption of blue or red light. Anoxygenic photosynthetic bacteria (APB) do not use water as an electron source and are typically photoheterotrophic rather than photoautotrophic and so ¹⁴CO₂ fixation is a misleading estimate of photosynthetic electron transport in APB photosynthesis. In vivo bacteriochlorophyll a (BChl a) absorbs blue light similar to Chl a but its characteristic longer-wavelength absorption is in the infrared and fluorescence is at > 800 nm. Blue light-induced PAM fluorescence can be used to measure the ETR in purple non-sulphur anoxygenic photobacteria and purple sulphur photobacteria because their RC-2 type BChl a complexes fluoresce similarly to PSII but at longer wavelengths than Chl a. Conventional PAM fluorometers using blue light cannot readily distinguish between oxygenic and RC-2 type anoxygenic photosynthesis because they use a simple > 700 nm highpass filter in front of the detector diode. We modified one fluorometer to use a 695–750-nm bandpass filter to measure Chl a fluorescence from PS-II, representing oxygenic photosynthesis. Similarly, we modified another fluorometer to use a highpass filter (> 830 nm) to measure BChl a fluorescence, representing anoxygenic photosynthesis. However, the fluorescence bands of Chl a and BChl a were found to be too wide to unambiguously distinguish between oxygenic and anoxygenic photosynthesis purely by fluorometry. Treatment with the specific PS-II inhibitor DCMU (Diuron) did enable discrimination of the two types of photosynthesis in a mixture of oxygenic and anoxygenic organisms. Ecological niches made up of both oxygenic and anoxygenic organisms such as microbial mats and hypereutrophic environments such as sewage ponds, wastewater ponds and prawn farm ponds are much more common than often realized. Anoxygenic photosynthesis in such systems is significant yet largely unquantified.

 

Relative importance of top‐down vs. bottom‐up control of lake phytoplankton vertical distributions varies among fluorescence‐based spectral groups

Authors

Mary E. Lofton, Taylor H. Leach, Beatrix E. Beisner, Cayelan C. Carey

The relative importance of top‐down vs. bottom‐up control of phytoplankton biomass in aquatic ecosystems has been long debated and studied. However, few studies have considered the relative importance of top‐down vs. bottom‐up control on phytoplankton vertical distributions and characteristics of deep chlorophyll maxima (DCMs), and fewer still have investigated the importance of these drivers for multiple phytoplankton groups. We examined depth profiles of four phytoplankton spectral groups and a suite of top‐down (zooplankton) and bottom‐up (nutrients, temperature, and light) drivers from 51 north temperate lakes varying on gradients of size, trophic state, light availability, and thermal stratification. We used regression trees to identify the most important drivers of different vertical distribution metrics for each phytoplankton spectral group. The relative importance of top‐down vs. bottom‐up control varied across spectral groups and was related to the characteristics of the dominant taxa within each spectral group, as assessed by microscope counts. Zooplankton biomass was the most important driver of brown algae vertical distributions, likely because this group contained highly edible taxa (primarily chrysophytes), while thermal stratification predicted vertical distributions of buoyancy‐regulating cyanobacteria. Our work highlights the importance of examining phytoplankton community composition to improve understanding of DCM characteristics and top‐down vs. bottom‐up control of phytoplankton in aquatic systems.

Regional-scale investigation of dissolved organic matter and lead binding in a large impacted lake with a focus on environmental risk assessment

Authors

Xiaokai Zhang, Boling Li, Jianming Deng, Boqiang Qin, Mona Wells, Boris Tefsen

Environmental risk assessment (ERA) increasingly relies on speciation modeling of bioavailability. Heavy metals are the most prevalent pollutants globally, and dissolved organic matter (DOM) plays an important role in speciation and bioavailability of heavy metals. Due to the variation of DOM properties in natural aquatic systems, improvements to the standard one-size-fits-all approach to modeling metal-DOM interactions are needed for ERA. In this study, we investigate variations in DOM and lead (Pb)-DOM binding in Lake Tai (Taihu), a large, impacted lake in eastern China that is characterized by a complex drainage network and is an important water resource at a regional level, and we assess implications of our findings within the context of ERA needs. In our study, DOM in water samples collected from across the 2,400 km² area of Taihu was characterized using three-dimensional excitation-emission matrix and synchronous fluorescence spectroscopy spectra, the latter being used to calculate conditional stability constants for metal binding. Parallel factor analysis and peak picking were used to assess contributions of protein- and humic-like components of DOM, and fluorescence indices indicative of diagenetic processes were calculated. These quantities calculated from spectroscopic studies, in addition to water quality parameters, were analyzed by bivariate and multivariate analysis. Results show that different DOM components are highly variable across different regions of Taihu, and bivariate and multivariate analyses confirm that water quality and DOM characterization parameters are strongly interrelated. This reflects the different inputs, diagenetic and transport processes across the large expanse of Taihu. We find that the conditional stability constant of Pb-DOM binding is strongly affected by the water chemical properties and composition of DOM, though the conditional stability constant is not itself a parameter that differentiates lake water properties in different regions of the lake. The variability of DOM composition and Pb-DOM binding strength across Taihu is consistent with prior findings that a one-size-fits-all approach to metal-DOM binding may lead to inaccuracies in commonly used speciation models, and therefore such generalized approaches need improvement for regional-level ERA in complex watersheds. The approach taken here to obtain site-specific metal-DOM conditional stability constants for use in increasing the accuracy of speciation modeling is fit-for-purpose for ERA applications at regional levels because the approach is relatively simple, inexpensive, and amenable to high throughput analysis.

 

The effect of low frequency ultrasonic treatment on the release of extracellular organic matter of Microcystis aeruginosa

Authors

Yang-Rui Huang, Huai-Zheng Li, Xin-Min Wei, Deng-Hui Wang, Yu-Ting Liu, Lei Li

The effect of low frequency ultrasonic treatment on the growth of cyanobacteria and the behavior of extracellular organic matter (EOM) were investigated in this study. The growth of cyanobacteria can be controlled effectively by low frequency ultrasonic treatment, and the inhibitory effect of ultrasound on the photosynthetic ability of cells was more significant than on the cells density (the minimum values of cells density and chlorophyll-a was obtained at day 3 and day 1 respectively). Significant rupture of cells was observed when irradiated by a higher intensity ultrasound (ultrasonic dosage ≥ 1.5 J/mL). Tyrosine-like components were mainly distributed in the soluble EOM and loosely bound EOM fractions; both of the tryptophan-like components and polyaromatic humic-like components were distributed in the soluble EOM, loosely bound EOM, and tightly bound EOM fractions; and polycarboxylate humic-like components were mainly distributed in soluble EOM fraction. Given that the production of hydroxyl radicals was measured, negligible due to the limit of ultrasonic treatment duration (≤1 min). On the other hand, intensive shock waves and shear forces of collapsing acoustic cavitation bubbles were considered as the main reason for the rupture of cells and the changes of the behavior of EOM. A higher intensity ultrasonic treatment resulted in a greater release of EOM, including bound EOM (ultrasonic dosage ≥ 0.1 J/mL), and intracellular organic matter (ultrasonic dosage ≥ 1.5 J/mL). Moreover, the determination of protein and polysaccharide suggested that the bonding of polysaccharides to cells surface is stronger than that of proteins to cells surface, and a portion of the polysaccharides in other fractions was transferred to the tightly bound state during the first serval days after ultrasonic treatment.

An enhancement approach of fluorescence signatures in excitation emission matrixes for water contaminant analysis

Authors

Yingtian Hu, Dongdong Zhao, Yali Qin, Xiaoping Wang

Three-dimensional fluorescence spectroscopy has been widely used for the analysis of water contaminants. However, the problems of weak signals and overlapping fluorescence peaks remain unresolved. In this work, we studied the impact of absorption on the spectral shape of fluorescence and found that it is a major cause of overlapping peaks and weak signals. An approach is proposed to purify fluorescent signals and enhance fluorescence signatures based on the theory of fluorescence quantum yield. Using this theory, the problems of noise amplification and singularity points were identified, and an optimization algorithm was proposed related to Wiener filtering. For practical application to multiple compounds, three overlapping cases were discussed theoretically. The effectiveness of this procedure in subsequent parallel factor analysis was assessed and compared with original data by conducting experiments with six typical compounds and real water samples. The results indicate that overlapping along the excitation wavelength axis can be reduced despite the existence of multiple compounds, and the sensitivity of weak fluorescent signals can be significantly improved. The proposed method can enhance fluorescence signatures for the separation and analysis of fluorescent components in water contaminants.

Quantification of bacteria in water using PLS analysis of emission spectra of fluorescence and excitation-emission matrices

Authors

Amir Nakar, Ze’ev Schmilovitch, Dalit Vaizel-Ohayon, Yulia Kroupitski, Mikhail Borisover, Shlomo Sela (Saldinger)

Bacterial contamination of drinking water is a considerable concern for public health. Tryptophan-like fluorescence (TLF) has been widely suggested to enable fast and inexpensive monitoring and quantification of bacterial contamination of water. Typically, TLF is determined at a certain excitation (ex)/emission (em) wavelengths pair. The aim of this study was to assess fluorescence spectroscopy supported with partial least squares (PLS) algorithms as a tool for a rapid evaluation of the microbial quality of water, by comparing the use of a single ex/em wavelengths pair, of the spectrum of emission obtained at a single excitation wavelength to that of whole excitation-emission matrices (EEMs). For that, laboratory-grown Escherichia coli, Bacillus subtilis and Pseudomonas aeruginosa were studied as the model systems, as well as 90 groundwater samples from 6 different wells in Israel. The groundwater samples were characterized for fluorescence emission, coliforms, fecal coliforms, fecal streptococci and heterotrophic plate counts. The PLS analysis of emission spectra and, especially, of EEMs was capable of meaningfully reducing the detection limit of microorganisms in model systems, as compared with the single ex/em wavelengths pair-based determination commonly used, reaching a detection threshold as low as 10 CFU/ml. Use of PLS-analyzed EEMs becomes beneficial also in terms of correlation and similarity between the actual and predicted bacterial concentrations. Similarly, improved detection of bacteria was also achieved in groundwater samples. Furthermore, at a level of >10⁴ CFU/ml, use of EEMs coupled with PLS enabled discrimination between E. coli, B. subtilis and P. aeruginosa.

Characterization of chromophoric dissolved organic matter in lakes across the Tibet-Qinghai Plateau using spectroscopic analysis

Authors

Kaishan Song, Sijia Li, Zhidan Wen, Lili Lyu, Yingxin Shang

Fluorescent dissolved organic matter (FDOM) has a significant influence on global inland water carbon cycling and biogeochemical processes. The alpine lakes on Tibet-Qinghai Plateau (TQP) are exposed to high-intensity ultraviolet radiation, long retention time with less anthropogenic influences. Yet, limited studies were conducted on FDOM of lakes on the highest plateau in the world with unique aquatic environments. Spatiotemporal variations of FDOM components from 63 lakes (32 fresh lakes, N = 135; 31 brackish lakes, N = 109) were grouped according to salinity across the TQP collected during 2014–2017. FDOM were examined using excitation-emission matrix spectra (EEM) and fluorescence regional integration (FRI). CDOM absorption and FDOM fluorescence indices, average fluorescence intensities of the five fluorescent components (φ_i, i = I, II, III, IV, V) and total fluorescence intensities (φ_T) are significantly different between brackish and freshwater lakes (p < 0.05). High correlations (−0.51^** < r < −0.86^**, p < 0.01) were revealed between FDOM and dissolved organic carbon (DOC) concentrations with salinity > 10‰. A comparatively prolonged retention time and terrestrial allochthonous inputs caused higher DOC accumulation in brackish water. Strong UV radiation resulted in an important effect on CDOM photo-absorption characteristics, and that contributed to DOC variability and fate. Likewise, average CDOM spectroscopic indices in each basin with different land covers showed a moderate correlation between DOC and normalized humic-like φ_V for 20 basins (r = −0.46^**, p < 0.01). The results suggest that studies on the highest alpine lakes across the TQP should focus on the impact of the evapo-concentration, ultraviolet irradiance and landscape features within different basins on FDOM characteristics. It demonstrated that the FDOM was dominated by the allochthonous sources in brackish lakes from the TQP and had a similarity to those observed in marine environments.

Evaluating putative ecological drivers of microcystin spatiotemporal dynamics using metabarcoding and environmental data

Authors

A. Banerji, M.J. Bagley, J.A. Shoemaker, D.R. Tettenhorst, C.T. Nietch, H.J. Allen, J.W. Santo Domingo

Microcystin is a cyanobacterial hepatotoxin of global concern. Understanding the environmental factors that cause high concentrations of microcystin is crucial to the development of lake management strategies that minimize harmful exposures. While the literature is replete with studies linking cyanobacterial production of microcystin to changes in various nutrients, abiotic stressors, grazers, and competitors, no single biotic or abiotic factor has been shown to be reliably predictive of microcystin concentrations in complex ecosystems. We performed random forest regression analyses with 16S and 18S rRNA gene sequencing data and environmental data to determine which putative ecological drivers best explained spatiotemporal variation in total microcystin and several individual congeners in a eutrophic freshwater reservoir. Model performance was best for predicting concentrations of the congener MC-LR, with ca. 88% of spatiotemporal variance explained. Most of the variance was associated with changes in the relative abundance of the cyanobacterial genus Microcystis. Follow-up RF regression analyses revealed that factors that were the most important in predicting MC-LR were also the most important in predicting Microcystis population dynamics. We discuss how these results relate to prevailing ecological hypotheses regarding the function of microcystin.