Continuous and Synoptic Assessment of Indian Inland Waters for Harmful Algae Blooms

Recent advances in visible light driven inactivation of bloom forming blue-green algae using novel nano-composites: Mechanism, efficiency and fabrication approaches

Over the years, algae have proved to be a water pollutant due to global warming, climate change, and the unregulated addition of organic compounds in water bodies from diffused resources. Harmful algal blooms (HABs) are severely affecting the health of humans and aquatic ecosystems. Among available anti-blooming technologies, semiconductor photocatalysis has come forth as an effective alternative. In the recent past, literature has been modified extensively with a decisive knowledge regarding algal invasion, desired preparation of nanomaterials with enhanced visible light absorption capacity and mechanisms for algal cell denaturation. The motivation behind this review article was to gather algal inactivation data in a systematic way based on various research studies, including the construction of nanoparticles and purposely to test their anti-algal activities under visible irradiation. Additionally, this article mentions variety of starting materials employed for preparation of various nano-powders with focus on their synthesis routes, analytical techniques as well as proposed mechanisms for lost cellular integrity in context of reduced chlorophyll' a' level, cell rapture, cell leakage and damages to other physiological constituents; credited to oxidative damage initiated by reactive oxidation species (ROS). Various floating and recyclable composited catalysts Ag2CO3-N: GO, Ag/AgCl@ZIF-8, Ag2CrO4-g-C3N4-TiO2/mEP proved to be game-changers owing to their enhanced VL absorption, adsorption, stability, separation and reusability. An outlook for the generalized limitations of published reports, cost estimations for practical implementation, issues and challenges faced by nano-photocatalysts and possible opportunities for future studies are also proposed. This review will be able to provide vast insights for coherent fabrication of catalysts, breakthroughs in experimental methodologies and help in elaboration of damage mechanisms.

Microplastic contamination in Indian rural and urban lacustrine ecosystems

This study investigates the microplastics (MPs) pollution of the lacustrine ecosystems of Tamil Nadu, South India. It examines the seasonal distribution, characteristics and morphology of MPs and assesses the risk posed by MPs pollution. MPs abundance in the 39 rural and urban lakes studied varies from 16 ± 2.69 to 118.17 ± 22.17 items/L (water) and 19.50 ± 4.75 to 156.23 ± 36.41 items/kg (sediment). The water and sediment of urban lakes show average MPs abundances of 88.06 items/L and 115.24 items/kg respectively, while the rural lakes exhibit average MPs abundances of 42.98 items/L and 53.29 items/kg. The results demonstrate that study areas with more residential and urban centers with higher population density and larger discharge of sewage have greater MP abundance. Urban zones have greater MP diversity integrated index (MPDII = 0.73) than rural zones (MPDII = 0.59). Fibres are the dominant group and polyethylene and polypropylene are the most commonly found polymers, possibly gaining entry through land-based plastic litter and urban activities in this region. The weathering index values, 50 % of MPs exhibit high degree of oxidation (WI >0.31) with an age of >10 years. SEM-EDAX results reveal that the weathered MPs from urban lakes have a wider variety of metal elements (Al, Cr, Mn, Co, Ni, Cu, Zn, As, Sr, Hg, Pb and Cd) than those from rural lakes (Na, Cl, Si, Mg, Al, Cu). Though PLI shows low risk (<10) in terms of abundance, PHI reflects pollution status III (10-100) and IV (100-1000) in rural areas and IV and V (>1000) in urban areas based on the toxicity score of the polymer. Ecological risk assessment shows minor risks (<150) at present. The assessment indicates the risk posed by the MPs to the lakes studied and emphasizes the necessity for best MP management practices in future.

Transmission of Microcystins in Natural Systems and Resource Processes: A Review of Potential Risks to Humans Health

The rapid rise of microcystins (MCs) poses a serious threat to global freshwater ecosystems and has become an important issue of global public health. MCs have considerable stability and are the most widely distributed hepatotoxins. It cannot only accumulate in aquatic organisms and transfer to higher nutrients and levels, but also be degraded or transferred during the resource utilization of cyanobacteria. No matter which enrichment method, it will lead to the risk of human exposure. This review summarizes the research status of MCs, and introduces the distribution of MCs in different components of aquatic ecosystems. The distribution of MCs in different aquatic organisms was summarized, and the potential risks of MCs in the environment to human safety were summarized. MCs have polluted all areas of aquatic ecosystems. In order to protect human life from the health threats caused by MCs, this paper also proposes some future research directions to promote MCs control and reduce human exposure to MCs.

Co-occurrence patterns and environmental factors associated with rapid onset of Microcystis aeruginosa bloom in a tropical coastal lagoon

The environmental factors contributing to the Microcystis aeruginosa bloom (hereafter referred to as Microcystis bloom) are still debatable as they vary with season and geographic settings. We examined the environmental factors that triggered Microcystis bloom outbreak in India's largest brackish water coastal lagoon, Chilika. The warmer water temperature (25.31-32.48 °C), higher dissolved inorganic nitrogen (DIN) loading (10.15-13.53 μmol L^-1), strong P-limitation (N:P ratio 138.47-246.86), higher water transparency (46.62-73.38 cm), and low-salinity (5.45-9.15) exerted a strong positive influence on blooming process. During the bloom outbreak, M. aeruginosa proliferated, replaced diatoms, and constituted 70-88% of the total phytoplankton population. The abundances of M. aeruginosa increased from 0.89 × 10⁴ cells L^-1 in September to 1.85 × 10⁴ cells L^-1 in November and reduced drastically during bloom collapse (6.22 × 10³ cells L^-1) by the late November of year 2017. The decrease in M. aeruginosa during bloom collapse was associated with a decline in DIN loading (2.97 μmol L^-1) and N:P ratio (73.95). Sentinel-3 OLCI-based satellite monitoring corroborated the field observations showing Cyanophyta Index (CI) > 0.01 in September, indicative of intense bloom and CI < 0.0001 during late November, suggesting bloom collapse. The presence of M. aeruginosa altered the phytoplankton community composition. Furthermore, co-occurrence network indicated that bloom resulted in a less stable community with low diversity, inter-connectedness, and prominence of a negative association between phytoplankton taxa. Variance partitioning analysis revealed that TSM (16.63%), salinity (6.99%), DIN (5.21%), and transparency (5.15%) were the most influential environmental factors controlling the phytoplankton composition. This study provides new insight into the phytoplankton co-occurrences and combination of environmental factors triggering the rapid onset of Microcystis bloom and influencing the phytoplankton composition dynamics of a large coastal lagoon. These findings would be valuable for future bloom forecast modeling and aid in the management of the lagoon.

Key hydrodynamic principles for controlling algal blooms using emergency reservoir operation strategies

Reservoir operation strategies with low cost and high efficiency have been proposed to control algal blooms. However, the key hydrodynamic principle for performing reservoir operation strategies is still unknown, posing an obstacle to practical applications. To address this challenge, we proposed short-term emergency reservoir operation strategies (EROSs), established a three-dimensional (3D) eutrophication model of the Zipingpu Reservoir, and designed six 14-day reservoir operation cases to explore the mechanism of EROSs in controlling algal blooms. Large outflows with rapid water exchange should be adopted early in EROSs to control algal blooms in the reservoir. Small variations in the surface water temperature or the mixed layer depth/euphotic layer depth (Z_mix/Z_eu) ratio were found for different EROSs, indicating that these variations might not have been responsible for the differences in the algal blooms in the reservoir. The EROSs induced high surface flow velocity (V_s) and depth-averaged velocity (V_d) values in the reservoir, thereby controlling algal blooms by inhibiting algal growth and disrupting algal accumulation in the upper water layers. The flow of V_s against the direction of the water intake was detected during the execution of the EROSs, suggesting that increasing V_s might enhance water retention in the reservoir. Increasing V_d not only promoted water exchange to disrupt algal accumulation but also enhanced V_s to inhibit algal growth. Moreover, V_d demonstrated a strong linear relationship with the inhibition ratio of algal blooms. These results demonstrate that V_d is the key hydrodynamic indicator for performing EROSs and that accelerating V_d to exceed 0.039 m s^-1 in the near-dam region can control algal blooms. Overall, in this study, we develop a novel EROS and elucidate corresponding principles for the use of EROSs to control algal blooms in reservoirs.

Microcystis sp. AE03 strain in Dal Lake harbors cylindrospermopsin and microcystin synthetase gene cluster

Cyanobacterial harmful algal blooms (CHABs) are increasing at an alarming rate in different water bodies worldwide. In India, CHAB events in water bodies such as Dal Lake have been sporadically reported with no study done to characterize the cyanobacterial species and their associated toxins. We hypothesized that this Lake is contaminated with toxic cyanobacterial species with the possibility of the presence of cyanotoxin biosynthetic genes. We, therefore, used some of the molecular tools such as 16S ribosomal DNA, PCR, and phylogenetic analysis to explore cyanobacterial species and their associated toxins. A 3-year (2018–2020) survey was conducted at three different sampling sites of Dal Lake namely, Grand Palace Gath (S1), Nigeen basin (S2), and Gagribal basin (S3). Two strains of Dolichospermum sp. AE01 and AE02 (S3 and S1 site) and one strain of Microcystis sp. AE03 (S2 site) was isolated, cultured, and characterized phylogenetically by 16S ribosomal DNA sequencing. The presence of cyanotoxin genes from the isolates was evaluated by PCR of microcystins (mcyB), anatoxins (anaC), and cylindrospermopsins (pks) biosynthesis genes. Results revealed the presence of both mcyB and pks gene in Microcystis sp. AE03, and only anaC gene in Dolichospermum sp. AE02 strain. However, Dolichospermum sp. AE01 strain was not found to harbor any such genes. Our findings, for the first time, reported the coexistence of pks and mcyB in a Microcystis AE03 strain. This study has opened a new door to further characterize the unexplored cyanobacterial species, their associated cyanotoxin biosynthetic genes, and the intervention of high-end proteomic techniques to characterize the cyanotoxins.

Quantitative prediction of toxin-producing Aphanizomenon cyanobacteria in freshwaters using Sentinel-2 satellite imagery

This study aimed to develop an empirical model to predict the spatial distribution of Aphanizomenon using the Ridiyagama reservoir in Sri Lanka with a dual-model strategy. In December 2020, a bloom was detected with a high density of Aphanizomenon and chlorophyll-a concentration. We generated a set of algorithms using in situ chlorophyll-a data with surface reflectance of Sentinel-2 bands on the same day using linear regression analysis. The in situ chlorophyll-a concentration was better regressed to the reflectance ratio of (1 + R₆₆₅)/(1-R₇₀₅) derived from B4 and B5 bands of Sentinel-2 with high reliability (R² = 0.81, p < 0.001). The second regression model was developed to predict Aphanizomenon cell density using chlorophyll-a as the proxy and the relationship was strong and significant (R² = 0.75, p<0.001). Coupling the former regression models, an empirical model was derived to predict Aphanizomenon cell density in the same reservoir with high reliability (R² = 0.71, p<0.001). Furthermore, the predicted and observed spatial distribution of Aphanizomenon was fairly agreed. Our results highlight that the present empirical model has a high capability for an accurate prediction of Aphanizomenon cell density and their spatial distribution in freshwaters, which helps in the management of toxic algal blooms and associated health impacts.