Beyond Microcystins: Cyanobacterial Extracts Induce Cytoskeletal Alterations in Rice Root Cells

Current status and prognosis of Raphidiopsis raciborskii distribution in Bulgaria as part of the southeastern region of Europe

The cyanobacterial species Raphidiopsis raciborskii (Wołoszyńska) Aguilera et al. has a high invasiveness potential, which in less than a century leads to its cosmopolitan spread. In the temperate climate of Europe, R. raciborskii has been reported in many countries, but there is still a lack of detailed information about the current status of its distribution in lakes of Bulgaria, as a part of the southeastern range of its spread in Europe. We investigated the distribution of the species using data on the phytoplankton of 122 lakes surveyed during 13-years period (2009-2022). The species was found in 33 lakes (up to 324 m asl), and 14 new localities were registered during the studied period. The results reveal that the number of lakes with the presence of R. raciborskii (27 % of all research lakes) and its contribution to the total phytoplankton biomass, has increased significantly over the last decade. The species has successfully adapted and dominated the phytoplankton in 9 lakes, forming a bloom in 8 of them. The dominant position of R. raciborskii causes loss of species and functional diversity of phytoplankton and displaces the native bloom-forming cyanobacteria. Lakes with and without the species were compared based on the available data on bioclimatic and local environmental variables. Statistically significant differences were established with respect to water transparency, conductivity, maximum depth and maximum air temperature in the warmest month. Species distribution models (SDMs) were used to identify lakes in high risk of future invasion by R. raciborskii. The results of the SDMs implementation confirmed the high maximum air temperature and low water transparency to be important predictors of the occurrence of R. raciborskii in freshwater lakes in Bulgaria. In the areas with high summer temperatures the most suitable for R. raciborskii development were found to be shallow polymictic or medium deep lakes with small surface area and low water transparency. In areas with a suitable climate, the large, deep reservoirs with high transparency as well as macrophyte dominated lakes have a low probability of occurrence of R. raciborskii. Future colonization of lakes above 500 m asl (but most likely below 700 m asl) is also possible, especially in the conditions of global warming. SDMs account for climatic and biogeographic differences of lakes and could help in elucidating the underlying factors that control the occurrence and adaptation of R. raciborskii in a given area.

First report on adverse effects of cyanobacterial anabaenopeptins, aeruginosins, microginin and their mixtures with microcystin and cylindrospermopsin on aquatic plant physiology: An experimental approach

Cyanobacteria produce a variety of oligopeptides beyond microcystins and other metabolites. Their biological activities are not fully recognized especially to aquatic plants. Acute toxicity tests on Spirodela polyrhiza and Lemna minor exposed to a range of concentrations of cyanobacterial metabolites: anabaenopeptins (ANA-A, ANA-B), aeruginosins 98 (Aer-A, Aer-B), microginin-FR1 (MG-FR1), microcystin-LR (MC-LR) and cylindrospermopsin (Cyl) were carried out to compare their influence on plant physiology. Effects of their binary mixtures were determined by isobole approach and calculation of the combination index (CI) that indicates a type of metabolites' interaction. Cyclic oligopeptides microcystin-LR and anabaenopeptin-A revealed the strongest inhibition of S. polyrhiza growth while other metabolites appeared less toxic. Oxygen evolution was inhibited by Cyl, MC-LR, ANA-A, ANA-B, while both variants of aeruginosins and MG-FR1 did not affect this process. Photosynthetic pigments' contents decreased in S. polyrhiza exposed to ANA-A and Cyl, while MC-LR and Aer-A caused their slight increase. 96 h-EC50 values showed that the growth of L. minor was more sensitive to MC-LR, ANA-A, MG-FR1 and Cyl than the growth of S. polyrhiza. In S. polyrhiza synergistic effects of all the binary mixtures of peptides with MC-LR on oxygen evolution were observed, while antagonistic one on the growth of S. polyrhiza exposed to the mixtures with aeruginosins and ANA-A. The mixtures of MC-LR and MG-FR1 with cylindrospermopsin revealed synergistic effects on the growth but antagonistic one to the O2 evolution. Quadruple mixtures (ANA-A + MC-LR + MG-FR1+Cyl) did not reveal any inhibitive effect on the plant growth and very slight on the oxygen evolution, irrespectively of their total concentrations. Various effects caused by ANA-A and ANA-B suggest the importance of molecule structures of metabolites for toxicity. Composition of the mixtures of cyanobacterial metabolites was essential for the observed effects.

Microcystin-LR and cyanobacterial extracts alter the distribution of cell wall matrix components in rice root cells

Cyanobacterial toxins (known as cyanotoxins) disrupt the plant cytoskeleton (i.e. microtubules and F-actin), which is implicated in the regulation of cell wall architecture. Therefore, cyanotoxins are also expected to affect cell wall structure and composition. However, the effects of cyanobacterial toxicity on plant cell wall have not been yet thoroughly studied. Accordingly, the alterations of cell wall matrix after treatments with pure microcystin-LR (MC-LR), or cell extracts of one MC-producing and one non-MC-producing Microcystis strain were studied in differentiated Oryza sativa (rice) root cells. Semi-thin transverse sections of variously treated LR-White-embedded roots underwent immunostaining for various cell wall epitopes, including homogalacturonans (HGs), arabinogalactan-proteins (AGPs), and hemicelluloses. Homogalacturonan and arabinan distribution patterns were altered in the affected roots, while a pectin methylesterase (PME) activity assay revealed that PMEs were also affected. Elevated intracellular Ca²⁺ levels, along with increased callose and mixed linkage glucans (MLGs) deposition, were also observed after treatment. Xyloglucans appeared unaffected and lignification was not observed. The exact mechanism of cyanobacterial toxicity against the cell wall is to be further investigated.

Microcystins in Water: Detection, Microbial Degradation Strategies, and Mechanisms

Microcystins are secondary metabolites produced by some cyanobacteria, a class of cyclic heptapeptide toxins that are stable in the environment. Microcystins can create a variety of adverse health effects in humans, animals, and plants through contaminated water. Effective methods to degrade them are required. Microorganisms are considered to be a promising method to degrade microcystins due to their high efficiency, low cost, and environmental friendliness. This review focuses on perspectives on the frontiers of microcystin biodegradation. It has been reported that bacteria and fungi play an important contribution to degradation. Analysis of the biodegradation mechanism and pathway is an important part of the research. Microcystin biodegradation has been extensively studied in the existing research. This review provides an overview of (1) pollution assessment strategies and hazards of microcystins in water bodies and (2) the important contributions of various bacteria and fungi in the biodegradation of microcystins and their degradation mechanisms, including mlr gene-induced (gene cluster expressing microcystinase) degradation. The application of biodegradable technology still needs development. Further, a robust regulatory oversight is required to monitor and minimize MC contamination. This review aims to provide more references regarding the detection and removal of microcystins in aqueous environments and to promote the application of biodegradation techniques for the purification of microcystin-contaminated water.

Microcystin-leucine arginine exhibits adverse effects on human aortic vascular smooth muscle cells in vitro

Microcystin-leucine arginine (MC-LR) is a kind of toxin produced by cyanobacteria, which can do harm to human and livestock health. MC-LR can easily enter tissues and organs through the blood circulation and accumulate in certain target organs. Vessels are prone to contact with MC-LR during growth and development. Previous study had demonstrated that MC-LR had potential vascular toxicity. However, it is not clear whether MC-LR has adverse effects on vascular smooth muscle cells. In this study, we evaluated the cytotoxicity of MC-LR exposure (0.01, 0.05, 0.1, 0.5, and 1 μM) on human aortic vascular smooth muscle cells (HAVSMCs) in vitro. The data showed that MC-LR exposure inhibited the HAVSMC proliferation and migration, induced HAVSMC apoptosis, cytoskeleton destruction, S-phase arrest, mitochondrial transmembrane potential (MMP) loss, and reactive oxygen species (ROS) production. In addition, MC-LR exposure resulted in the imbalance between oxidants and antioxidants, increased the caspase-3 and caspase-9 activities, and down-regulated the gene expressions (integrin β1, Rho, ROCK, MLC). Taken together, MC-LR could induce the generation of ROS in HAVSMCs, leading to apoptosis by the mitochondrial signaling pathway. MC-LR could also induce cytoskeletal disruption by integrin-mediated FAK/ROCK signaling pathway, leading to cell cycle arrest and the inhibition of HAVSMCs proliferation and migration. The current findings facilitate an understanding of the mechanism of MC-LR toxicity involved in angiocardiopathy.

Cyanobacteria in hot pursuit: Characterization of cyanobacteria strains, including novel taxa, isolated from geothermal habitats from different ecoregions of the world

Well-studied thermal spring microbial mat systems continue to serve as excellent models from which to make discoveries of general importance to microbial community ecology in order to address comprehensively the question of "who is there" in a microbial community. Cyanobacteria are highly adaptable and an integral part of many ecosystems including thermal springs. In this context, we sampled disparate thermal springs, spanning from Iceland and Poland to Greece and Tajikistan. Thirteen (13) strains were isolated and characterised with taxonomic indices and molecular markers (16S-23S rRNA region and cpcBA gene), whilst their thermotolerance was evaluated. Screening for the presence of genes encoding three heat shock proteins, as well as non-ribosomal peptide synthetases (NRPSs) and polyketide synthases (PKSs) was performed. This approach resulted in the description of two new genera (Hillbrichtia and Amphirytos) and their type species (Hillbrichtia pamiria and Amphirytos necridicus) representing Oscillatoriales and Synechococcales orders, respectively. We also found unique lineages inside the genus Thermoleptolyngbya, describing a novel species (T. hindakiae). We described the presence of sub-cosmopolitan taxa (such as Calothrix, Desertifilum, and Trichormus). Strains were diverse concerning their thermophilic ability with the strains well adapted to high temperatures possessing all three investigated genes encoding heat shock proteins as well as studied PKS and NRPS genes. In this work, we show novel cyanobacteria diversity from thermal springs from disparate environments, possible correlation of thermotolerance and their genetic background, which may have implications on strategic focusing of screening programs on underexploited taxa in these habitats.

Plant-cyanobacteria interactions: Beneficial and harmful effects of cyanobacterial bioactive compounds on soil-plant systems and subsequent risk to animal and human health

Plant-cyanobacteria interactions occur in different ways and at many different levels, both beneficial and harmful. Plant-cyanobacteria interactions, as a beneficial symbiosis, have long been demonstrated in rice-growing areas (Poaceae) where the most efficient nitrogen-fixing cyanobacteria are present in paddies. Moreover, cyanobacteria may in turn produce and/or secrete numerous bioactive compounds that have plant growth-promoting abilities or that may make the plant more resistant to abiotic or biotic stress. In recent years, there has been a growing worldwide interest in the use of cyanobacterial biomass as biofertilizers to replace chemical fertilizers, in part to overcome increasing organic-farming demands. However, the potential presence of harmful cyanotoxins has delayed the use of such cyanobacterial biomass, which can be found in large quantities in freshwater ecosystems around the world. In this review, we describe the existing evidence for the positive benefit of plant-cyanobacteria interactions and discuss the use of cyanobacterial biomass as biofertilizers and its growing worldwide interest. Although mass cyanobacterial blooms and scums are a current and emerging threat to the degradation of ecosystems and to animal and human health, they may serve as a source of numerous bioactive compounds with multiple positive effects that could be of use as an alternative to chemical fertilizers in the context of sustainable development.

Role of Rhizospheric Microbiota as a Bioremediation Tool for the Protection of Soil-Plant Systems from Microcystins Phytotoxicity and Mitigating Toxin-Related Health Risk

Frequent toxic cyanoblooms in eutrophic freshwaters produce various cyanotoxins such as the monocyclic heptapeptides microcystins (MCs), known as deleterious compounds to plant growth and human health. Recently, MCs are a recurrent worldwide sanitary problem in irrigation waters and farmland soils due to their transfer and accumulation in the edible tissues of vegetable produce. In such cases, studies about the persistence and removal of MCs in soil are scarce and not fully investigated. In this study, we carried out a greenhouse trial on two crop species: faba bean (Vicia faba var. Alfia 321) and common wheat (Triticum aestivum var. Achtar) that were grown in sterile (microorganism-free soil) and non-sterile (microorganism-rich soil) soils and subjected to MC-induced stress at 100 µg equivalent MC-LR L⁻¹. The experimentation aimed to assess the prominent role of native rhizospheric microbiota in mitigating the phytotoxic impact of MCs on plant growth and reducing their accumulation in both soils and plant tissues. Moreover, we attempted to evaluate the health risk related to the consumption of MC-polluted plants for humans and cattle by determining the estimated daily intake (EDI) and health risk quotient (RQ) of MCs in these plants. Biodegradation was liable to be the main removal pathway of the toxin in the soil; and therefore, bulk soil (unplanted soil), as well as rhizospheric soil (planted soil), were used in this experiment to evaluate the accumulation of MCs in the presence and absence of microorganisms (sterile and non-sterile soils). The data obtained in this study showed that MCs had no significant effects on growth indicators of faba bean and common wheat plants in non-sterile soil as compared to the control group. In contrast, plants grown in sterile soil showed a significant decrease in growth parameters as compared to the control. These results suggest that MCs were highly bioavailable to the plants, resulting in severe growth impairments in the absence of native rhizospheric microbiota. Likewise, MCs were more accumulated in sterile soil and more bioconcentrated in root and shoot tissues of plants grown within when compared to non-sterile soil. Thereby, the EDI of MCs in plants grown in sterile soil was more beyond the tolerable daily intake recommended for both humans and cattle. The risk level was more pronounced in plants from the sterile soil than those from the non-sterile one. These findings suggest that microbial activity, eventually MC-biodegradation, is a crucial bioremediation tool to remove and prevent MCs from entering the agricultural food chain.

Subcellular Alterations Induced by Cyanotoxins in Vascular Plants-A Review

Phytotoxicity of cyanobacterial toxins has been confirmed at the subcellular level with consequences on whole plant physiological parameters and thus growth and productivity. Most of the data are available for two groups of these toxins: microcystins (MCs) and cylindrospermopsins (CYNs). Thus, in this review we present a timely survey of subcellular cyanotoxin effects with the main focus on these two cyanotoxins. We provide comparative insights into how peculiar plant cellular structures are affected. We review structural changes and their physiological consequences induced in the plastid system, peculiar plant cytoskeletal organization and chromatin structure, the plant cell wall, the vacuolar system, and in general, endomembrane structures. The cyanotoxins have characteristic dose-and plant genotype-dependent effects on all these structures. Alterations in chloroplast structure will influence the efficiency of photosynthesis and thus plant productivity. Changing of cell wall composition, disruption of the vacuolar membrane (tonoplast) and cytoskeleton, and alterations of chromatin structure (including DNA strand breaks) can ultimately lead to cell death. Finally, we present an integrated view of subcellular alterations. Knowledge on these changes will certainly contribute to a better understanding of cyanotoxin–plant interactions.