Short-term toxicity of hexavalent-chromium to epipsammic diatoms of a microtidal estuary (Río de la Plata): responses from the individual cell to the community structure

Declining salinity and increasing temperature reduce the diversity and resilience of benthic diatoms

Climate change will modify the marine ecosystem in several ways, but the effects of changing climate on benthic diatoms, which are one of the most important photosynthesizing organism groups in benthic habitats, are poorly studied. We conducted a mesocosm experiment to investigate the effects of increasing temperature and decreasing salinity on the taxonomic and functional diversity of benthic diatoms. We showed that decreasing salinity affects the taxonomic and functional composition of communities, and the threshold salinity for community composition is ~5. This indicates that when climate change leads to decreasing salinity in brackish systems, the most pronounced changes in communities occur in areas where salinity decreases from >5 to <5. We also showed that both increasing temperature and decreasing salinity exert stress on communities and, hence, lead to the decrease of the alpha and beta diversity of communities. This indicates that climate change reduces the size of the species pool of diatoms. Our results show that, along with the changing climate, we can expect benthic diatom communities to become less diverse and less resilient.

Qualitative and quantitative assessment of diatom deformities and protoplasmic condition under metal and metalloid stress

Metals and metalloids are toxic, persistent, and non-biodegradable and can be biomagnified (e.g., Hg), and therefore pose a serious threat to the algal flora of aquatic ecosystems. This laboratory study tested the effects of metals (Zn, Fe, and Hg) and a metalloid (As) on the cell wall morphology and protoplasmic content of living cells of six widespread diatom genera over 28 days. Diatoms exposed to Zn and Fe had a higher frequency of deformed diatom frustules (> 1%) compared to the As, Hg, and control treatments (< 1%). Deformities in the valve outline and striae were found in all treatments, including the control, whereas deformed raphes and more than one type of deformity were more prevalent under Zn and Hg stress. The order of toxicity is as follows: Zn > Fe > Hg≈As. Deformities were more frequent in Achnanthes and Diploneis (adnate forms) than in the motile genera of Nitzschia and Navicula. The correlation between the % healthy diatoms and % deformities in all six genera showed a negative relationship with the integrity of protoplasmic content (i.e., greater alteration in protoplasmic content was associated with greater frustule deformation). We conclude that diatom deformities can be a good indicator of metal and metalloid stress in waterbodies and are very useful in the rapid biomonitoring of aquatic ecosystems.

Exploring a unique water ecosystem under long-term exposure to hexavalent chromium - An in situ study of natural diatom (Bacillariophyceae) communities

This paper reports the third in a series of three studies of a unique aquatic phytobial consortium that developed in a polluted ditch situated near an old chromium waste landfill. The ditch is a specific ecological niche having increased loads of several chemical compounds, including extreme hexavalent chromium concentrations up to two thousand times the allowed limit (0.02 mg dm-3 in Poland) in the more polluted section B; the moderate concentrations in section A are twice the limit. We focus on the microscopic algae group of diatoms, one of the most important components of the phytobial consortium, and continue our novel attempt to analyze the bioremediation potential of the entire consortium under those environmental conditions. We used numerical methods to analyze differences in diatom biodiversity between sections A and B, and assessed the relations between diatoms and selected water chemistry properties, including hexavalent chromium, chlorides and sulphates, the latter two known to positively influence the resistance of algae to the impact of hexavalent chromium. We noted 37 diatom taxa in section A of the ditch and 30 in section B. The most frequently observed diatoms were cosmopolitan taxa and/or characteristic taxa of saline waters. Sulphates were the most important factor influencing the diatom composition in the ditch, explaining 52% of the total variability, followed by chlorides (30%) and hexavalent chromium (9%). Gomphonema acuminatum, Melosira varians and Nitzschia frustulum var. frustulum were found to be most resistant to hexavalent chromium and were selected for further experimental studies on their biotechnological usefulness.

Preliminary assessment of microbial mats in seawater metal remediation

Microbial mats have a special role in the removal of metals in marine environment. This study aimed to experimentally determine the efficiency of Cr removal from seawater by microbial mats. The effect of Cr on the microphytobenthic community and the influence of an aerated condition on the removal of metal and microorganisms were also considered. Hence, microbial mat samples were subdivided in 4 groups: submerged in Cr (chromium 2 mg/L without aeration), Cr + O₂ (chromium 2 mg/L with aeration), SW + O₂ (filtered seawater with aeration), and a control: SW (filtered seawater with neither chromium nor aeration). Water and microbial mat subsamples were used to determine Cr concentrations, organic matter content, granulometry, physicochemical parameters, chlorophyll a, phaeopigments, and quantitative analysis of the microphytobenthic community. Cr removal efficiency from seawater was 95% for the Cr treatment and 99% for the Cr + O₂ treatment. The abundance of cyanobacteria tended to decrease between the initial and final days of the assay, whereas the opposite trend was observed for diatoms. Two aspects are worth noting in the paper: microbial mats efficiently removed Cr from seawater at a concentration of 2 mg Cr/L and the removal of Cr by microbial mats was more efficient with water aeration.

Exposure of suckling rats to hexavalent chromium (CrVI) alters bone formation at the base of the alveolus causing a delay in tooth eruption

OBJECTIVES

Hexavalent chromium (CrVI)-exposure of suckling rats has been shown to delay tooth eruption. However, the effects of CrVI-exposure on bone formation at the base of the alveolus, which provides a motive force for tooth eruption in the early stages of the process, remain unknown. The present work sought to evaluate the effect of CrVI-exposure on bone formation at the base of the alveolus during the intraosseous stage of tooth eruption in suckling rats, using histomorphometric and immunohistochemical studies.

METHODS

Experimental animals received 12.5 mg/kg-bw/day of potassium dichromate dissolved in saline solution by gavage starting on day 4 of the experiment; controls similarly received an equivalent volume of saline. All the animals were euthanized at the age of 9 days. The base of the developing alveolus at the level of the mesial root of the first lower molar was analyzed histomorphometrically and immunohistochemically. Data were statistically analyzed using student's t-test, with statistical significance set at p <0.05.

RESULTS

CrVI-exposed animals showed lower bone volume and height at the base of the alveolus, a significant preponderance of bone rest on the surface adjacent to the dental follicle, and a significantly lower percentage of positive Runt-related transcription factor-2 (RUNX2+) osteoblasts and a significantly higher number of mesenchymal-like RUNX2+ cells at the latter site.

CONCLUSION

CrVI-exposure during lactation affects bone formation at the base of the developing alveolus, delaying tooth eruption. These findings underscore the importance of controlling drinking water levels of toxic substances since their effects can alter the growth and development of individuals exposed during early childhood.

Resilience of a microphytobenthos community from the Severn Estuary, UK, to chlorination: A mesocosm approach

The Severn Estuary is a large macrotidal estuary which includes an extensive mudflat with microphytobenthos (MPB) playing a key role in the ecosystem. This study evaluated the impact of chlorination at two different dosing levels (0.05 and 0.5 mg/l as total residual oxidants, TRO, representative of potential concentrations in the mixing zone and within the cooling water systems of a power station) on a MPB community representative of the Severn Estuary. Biomass and diversity were not negatively impacted while physiology was partially affected at the beginning of the experiment, and it recovered towards the end of the experiment. Further investigations for diversity are needed to consolidate our findings. In conclusion our results show that MPB is resilient to chlorination up to a concentration of 0.5 mg/l which is much higher (>10 times) than what might be expected near the chlorinated discharges for most coastal power stations.

Diatom microalgae as smart nanocontainers for biosensing wastewater pollutants: recent trends and innovations

Microalgae have been recognized as one of the most efficient microorganisms to remediate industrial effluents. Among microalgae diatoms are silica shelled unicellular eukaryotes, found in all types of water bodies and flourish very well even in wastewater. They have their silica cell wall made up of nano arrayed pores arranged in a uniform fashion. Therefore, they act as smart nanocontainers to adsorb various trace metals, dyes, polymers, and drugs which are hazardous to human as well to aquatic life. The beautiful nanoarchitecture in diatoms allows them to easily bind to ligands of choice to form a nanocomposite structure with the pollutants which can be a chemical or biological component. Such naturally available diatom nanomaterials are economical and highly sensitive compared to manmade artificial silica nanomaterials to help in facile removal of the toxic pollutants from wastewater. This review is thus focused on employing diatoms to remediate various pollutants such as heavy metals, dyes, hydrocarbons detected in the wastewater. It also includes different microalgae as biosensors for determination of pollutants in effluents and the perspectives for nanotechnological applications in the field of remediating pollutants through microalgae. The review also discusses in length the hurdles and perspectives of employing microalgae in wastewater remediation.

A review on water treatment technologies for the management of oxoanions: prospects and challenges

Oxoanions are a class of contaminants that are easily released into the aquatic systems either through natural or anthropogenic activities. Depending on their oxidation states, they are highly mobile, resulting in the contamination of underground water. Above the permissible level in groundwater, they pose as threats to mammals when the contaminated water is consumed. Some of the health challenges caused are cancer, neurological, cardiac, gastrointestinal, and skin disorders. Several treatment technologies have been adopted over the years for the management of these oxoanions present in the aquatic systems. However interesting these treatment technologies might be, they also have their limitations such as cost-effectiveness, the complexity of the process, and generation of secondary pollutants. This work focused on some of the water treatment technologies applied for the removal of oxoanions. Some of the advantages and disadvantages of these treatment technologies are also highlighted. Amongst all the treatment technologies, adsorption is the most applied method for the removal of oxoanions. However, photocatalysis has a higher prospect since it is non-selective and secondary pollutants are not generated after the treatment process. Also, photocatalysis can simultaneously reduce and oxidise oxoanions as well as organic pollutants respectively.

Diatoms recovery from wastewater: Overview from an ecological and economic perspective

Alarming water pollution is toxic to the aquatic ecosystem leading to a sharp decline in species diversity. Diatoms have great potency to survive in contaminated water bodies, hence they can be compelling bioindicators to monitor the change in the environmental matrices effectively. Around the globe, researchers are intended to evaluate the impact of pollution on the diatoms recovery and techniques used for the assessment. The diatoms are precious for futuristic need viz. value-added products, energy generation, pharmaceuticals, and aquaculture feedstocks. All these applications led to a significant rise in diatoms research among the scientific community. This review presents different isolation practices, cultivation, and other challenges associated with the diatoms. A precise focus is given to diatoms isolation techniques from highly polluted water bodies with the main thrust towards obtaining an axenic culture to elucidate the significance of pure diatom cultures. Recovery of "jewels of the sea" from polluted water signifies the prospective ecological and economic aspects.

Diatom motility and nuclear alterations are affected by sediment elutriates of agricultural streams

Diatoms respond to toxicants in lotic systems, and they are commonly considered to be sensitive indicators in environmental safety assessment. In addition to the structural characteristics of the algal populations, recent studies have shown that endpoints such as nuclear anomalies or diatom motility measures can be affected quickly by environmental changes. We sought to determine if cell density, cell size, nuclear anomalies and motility of the diatom Nitzschia palea were useful indicators of sediment quality from agricultural streams. For this purpose, we exposed cultures of the diatom to elutriates from sediments of a stream that flows through an intensive agricultural area, and measured the responses of the populations for 7 days in laboratory tests. The bioassays showed that motility measures in Nitzschia palea and the condition of their nuclear membranes rapidly reflected the effects of sediment quality after only 48 h of exposure; mean cell density and length were affected by day 7. The sediment elutriates affected cell movements by shortening the total path length and decreasing cell velocity; they also increased the number of cells with nuclear membrane breakage. Our results from these bioassays show that diatom motility measurements and the condition of the nuclei might be indicators that respond faster to impacts than the traditional structural parameters, such as cell density, specific composition of the assemblage or diversity metrics of the algal communities more often used in biomonitoring.

Effects of hexavalent chromium on phytoplankton and bacterioplankton of the Río de la Plata estuary: an ex-situ assay

We examined the responses of the phytoplankton and the bacterioplankton of the freshwater zone of the Río de la Plata estuary when exposed to an addition of hexavalent chromium (Cr⁺⁶). The planktonic community from a coastal site was exposed to a chromium increase of 80 μg L^-1 for 72 h in laboratory conditions. The results showed a decrease in the concentration of Cr⁺⁶ by 33% in the treatments, along with significant decreases in chlorophyll-a (63%), the chlorophyll-a:pheophytin-a ratio (33%), oxygen production (37%), and in the total density of the phytoplankton (15%). The relative abundance of chlorophytes and diatoms decreased, while the cyanobacteria thrived. Finally, the total bacterial density and the density of viable bacteria decreased. These results show that even small increments in Cr⁺⁶ can cause significant effects on the phytoplankton and bacterioplankton, which could potentially affect other trophic levels of the community, risking alterations of the entire ecosystem.

The use of diatoms in ecotoxicology and bioassessment: Insights, advances and challenges

Diatoms are regularly used for bioassessment and ecotoxicological studies in relation to environmental and anthropogenic disturbances. Traditional taxonomical diatom parameters (cell counts, biovolume estimates, species richness, diversity indices and metrics using sensitive and tolerant diatom species) are regularly used for these studies. In the same context, very less focus was given on new endpoints of diatoms (life-forms, nuclear anomalies, alteration in photosynthetic apparatus shape, motility, lipid bodies, size reduction and deformities), in spite of their numerous merits, such as, their easiness, quickness, cheapness, global acceptation and no especial training in diatom taxonomy. In this review we analyzed 202 articles (from lab and field studies), with the aim to investigate the bioassessment and ecotoxicological advancement taken place in diatom research especially in terms of exploring new endpoints along with the traditional taxonomical parameters in a perspective which can greatly enhance the evaluation of fluvial ecosystem quality for biomonitoring practices.

Effects of pulse and press additions of salt on biofilms of nutrient-rich streams

Chronic and pulse increments of salinity can cause different consequences on the aquatic communities, and its effects are related to factors such as the magnitude, frequency and ionic composition, as well as on the baseline salt concentrations in the water. The aim of this study was to explore the responses of the biofilms from a nutrient-rich stream to both pulse and chronic additions of salt, along with their recovery after the stressor had been removed. For this purpose, a microcosm study was conducted exposing biofilms to water enriched with sodium chloride in two treatments (press and pulse), and comparing the changes in the biofilm with control microcosms without salt additions. The experiment lasted 72h, and the variables measured included bacterial density, chlorophyll-a concentration, community composition, total carbohydrate content, oxygen consumption and the percentage of nuclear alterations in diatoms. Both treatments resulted in a decrease in the bacterial density of the biofilm and in oxygen consumption; the chronic treatment in particular also caused an increased percentage of nuclear abnormalities in the diatom assemblage. The biofilm recovered to control levels after the treatments had been discontinued for 72h. We concluded that the biofilms can be altered significantly under both chronic and pulse additions of salt even after a short-term exposure, and that the community can recover if the stressor is withdrawn.

Effects of organic pollution on biological communities of marine biofilm on hard substrata

We examined the effect of organic enrichment on diatom and bacterial assemblages of marine epilithic biofilms on two locations in the Mediterranean, one situated in Spain and the other in Greece. Total organic carbon, total organic nitrogen, stable isotopes (δ(13)C and δ(15)N) and chlorophyll a indicated significant incorporation of organic wastes, increased primary production and trophic niche modifications on the biofilms close to the organic enrichment source. In Spain, where the organic load was higher than in Greece, diatom and, to some extent, bacterial assemblages varied following the organic enrichment gradient. The taxonomic richness of diatom and bacterial communities was not influenced by organic enrichment. Classical community parameters showed consistent patterns to organic pollution in both locations, whereas community assemblages were only influenced when organic pollution was greatest. The successional patterns of these communities were similar to other epilithic communities. The modification of community assemblages induced by organic pollution may affect ecological functions.