Morphological, physiological and molecular responses of Nitzschia palea under cadmium stress

Potential cellular targets of platinum in the freshwater microalgae Chlamydomonas reinhardtii and Nitzschia palea revealed by transcriptomics

Platinum group element levels have increased in natural aquatic environments in the last few decades, in particular as a consequence of the use of automobile catalytic converters on a global scale. Concentrations of Pt over tens of μg L-1 have been observed in rivers and effluents. This raises questions regarding its possible impacts on aquatic ecosystems, as Pt natural background concentrations are extremely low to undetectable. Primary producers, such as microalgae, are of great ecological importance, as they are at the base of the food web. The purpose of this work was to better understand the impact of Pt on a cellular level for freshwater unicellular algae. Two species with different characteristics, a green alga C. reinhardtii and a diatom N. palea, were studied. The bioaccumulation of Pt as well as its effect on growth were quantified. Moreover, the induction or repression factors of 16 specific genes were determined and allowed for the determination of possible intracellular effects and pathways of Pt. Both species seemed to be experiencing copper deficiency as suggested by inductions of genes linked to copper transporters. This is an indication that Pt might be internalized through the Cu(I) metabolic pathway. Moreover, Pt could possibly be excreted using an efflux pump. Other highlights include a concentration-dependent negative impact of Pt on mitochondrial metabolism for C. reinhardtii which is not observed for N. palea. These findings allowed for a better understanding of some of the possible impacts of Pt on freshwater primary producers, and also lay the foundations for the investigation of pathways for Pt entry at the base of the aquatic food web.

Effect of Cd toxicity on root morphology, ultrastructure, Cd uptake and accumulation of wheat under intercropping with Solanum nigrum L

In order to evaluate the effect of intercropping of the hyperaccumulator, Solanum nigrum L. and wheat on the absorption and accumulation of cadmium (Cd) in wheat. The experiment was conducted with three replicates, which conducted on four Cd concentrations (0, 20, 40, 60 μmol L^-1) in the Hoagland solution and using two planting patterns [monoculture wheat (MW), intercropping wheat and Solanum nigrum L. (IW，IS)]. The results showed that the addition of Cd in the solutions reduced the total root length by 19.08-55.98%, total root area by 12.35-44.48%, and total root volume by 16.01-46.00% of wheat plants. Intercropping with Solanum nigrum L. significantly reduced Cd contents by 28.3-47.2% and Cd accumulations by 10.08-32.43% in the roots of wheat. Transmission electron microscope (TEM) revealed that the root-tip cells of the monoculture wheat treated with Cd exhibited swollen spheres of intracellular mitochondria, disorderly arranged inner ridges of mitochondria, some damaged mitochondrial membranes, and deformed nuclear membranes. Many dense electron particles in the form of Cd were deposited in the cell gap, and the cell nucleus became smaller or even disappeared. Under the same Cd concentrations, root-tip cells of intercropped wheat showed much less density of electron particles, starch granules, and the damage to the nucleus and nuclear membrane by Cd.These results indicated that intercropping with Solanum nigrum L. reduced the Cd toxicity to wheat roots and decreased Cd uptake and accumulation in both the shoots and roots of the wheat.

Characterization of benthic biofilms in mangrove sediments and their variation in response to nutrients and contaminants

Diatom-dominated biofilms and associated extracellular polymeric substances (EPS) may adapt to the stress of long-term exposure to nutrients and anthropogenic contaminants. However, such interactions in contaminated mangrove sediments have rarely been reported. Based on the in situ characterization of biofilm components and environmental factors, the present study aimed to explore the key factors involved in shaping sediment biofilms through correlational and multivariate analyses. The pennate diatom Navicula is the core taxon that plays a crucial role in balancing the abundance of Nitzschia and Cyclotella, and is the main producer of bound-polysaccharides. The taxa composition shifts in a high N/P matrix, with the populations of pennate diatoms increasing but that of centric diatoms decreasing. High nutrient concentrations yield more number of diatoms and elevated levels of EPS. Bacteria are the main consumers of EPS and tend to be more symbiotic with Nitzschia than the other two diatom taxa. Some bound-polysaccharides dominated by arabinose and glucose units are transformed into the colloidal fraction, whereas other conservative ones serve as structural materials in concert with the bound-proteins. The planktonic phase of Cyclotella breaks down the structural EPS secreted by pennate diatoms in a process that directly affects the dynamic renewal of benthic biofilms. Most heavy metals as well as bisphenol A inhibit the abundance of bacteria and diatoms but enhance most EPS fractions except bound-polysaccharides. The response of structural EPS to specific contaminants varies, exhibiting increases in Co and Ni levels but decreases in nonylphenol and methylparaben levels. The present study improves our understanding of the microbial carbon loop of benthic biofilms in mangrove ecosystems under stress by nutrients and mixed contaminants.

Blue light irradiation increases the relative abundance of the diatom Nitzschia palea in co-culture with cyanobacterium Microcystis aeruginosa

Lake eutrophication is associated with cyanobacterial blooms. The pennate diatom Nitzschia palea (N. palea) inhibits the growth of the cyanobacterium Microcystis aeruginosa (M. aeruginosa); therefore, increasing the relative abundance of N. palea may contribute to the inhibition of Microcystis blooms. Several studies have demonstrated that blue light irradiation promotes diatom growth and inhibits cyanobacterial growth. In this study, we evaluated the effects of blue light irradiation on N. palea and M. aeruginosa abundance. Monocultures and co-cultures of N. palea and M. aeruginosa were exposed to blue light and fluorescent light at 32 μmol photons m-2 s-1. The relative abundance of N. palea under fluorescent light decreased gradually, whereas the abundance under blue light was relatively higher (approximately 74% and 98% under fluorescent light and blue light, respectively, at the end of the experiment). The inhibition efficiency of blue light on the growth rate of M. aeruginosa was related to the light intensity. The optimal light intensity was considered 20 μmol photons m-2 s-1 based on the inhibition efficiency of 100%. Blue light irradiation can be used to increase the abundance of N. palea to control Microcystis blooms. PRACTITIONER POINTS: The effects of blue light irradiation on N. palea abundance was discussed. Monocultures and co-cultures of N. palea and M. aeruginosa were exposed to blue light and to fluorescent light. The relative abundance of N. palea increased upon irradiation with blue light in co-culture with M. aeruginosa.

Intraspecific variation in metal tolerance modulate competition between two marine diatoms

Despite widespread metal pollution of coastal ecosystems, little is known of its effect on marine phytoplankton. We designed a co-cultivation experiment to test if toxic dose-response relationships can be used to predict the competitive outcome of two species under metal stress. Specifically, we took into account intraspecific strain variation and selection. We used 72 h dose-response relationships to model how silver (Ag), cadmium (Cd), and copper (Cu) affect both intraspecific strain selection and competition between taxa in two marine diatoms (Skeletonema marinoi and Thalassiosira baltica). The models were validated against 10-day co-culture experiments, using four strains per species. In the control treatment, we could predict the outcome using strain-specific growth rates, suggesting low levels of competitive interactions between the species. Our models correctly predicted which species would gain a competitive advantage under toxic stress. However, the absolute inhibition levels were confounded by the development of chronic toxic stress, resulting in a higher long-term inhibition by Cd and Cu. We failed to detect species differences in average Cu tolerance, but the model accounting for strain selection accurately predicted a competitive advantage for T. baltica. Our findings demonstrate the importance of incorporating multiple strains when determining traits and when performing microbial competition experiments.

Predicting chronic algal toxicity from 1- to 48-h pulsed exposures to mine site waters using time-averaged concentrations

Despite concentrations often fluctuating in aquatic systems that receive contaminant inputs, there has only been a relatively small number of studies investigating the toxicity of intermittent exposures. This is particularly the case for industrial and mine effluents that may contain complex mixtures of contaminants and other stressors. The lack of information is impeding the regulation of such contaminant exposures, whose risk is often assessed by comparison to continuous exposures in whole effluent toxicity (direct toxicity assessment) testing. The current study compared the toxicity from continuous (72-h) and pulsed (1- to 48-h) exposures of two neutralised mine waters (NMWs) to the freshwater algae, Chlorella sp. When the algal toxicity of the different exposures was related to the time-averaged concentration (TAC) of contaminants, it was found that the TAC was a good predictor of toxicity in any given test, with variability in toxicity between tests mainly related to differences in contaminant concentrations from the neutralisation of the acidic mine waters. When the data from tests on two samples were combined on a whole-effluent TAC basis, the EC50 values (95% confidence intervals) for the continuous and pulsed exposures were 0.68% (0.36-1.3) and 0.63% (0.38-1.1) respectively, for NMW sample one, while the corresponding EC50 values for NMW sample two were 1.3% (1.0-1.7) and 1.9% (1.6-2.2), respectively. The toxicity of the second water was strongly influenced by the zinc, and probably copper, concentrations, while the toxicity of the first appeared to be related to additive or synergistic toxicity from Al, Cd, Mn and Pb. The findings are discussed in relation to using a contaminant TAC-approach to revise water quality guideline values derived for continuous exposures for application to pulsed exposures, where higher concentrations may be permissible for short durations.

Diatom Deformities and Tolerance to Cadmium Contamination in Four Species

The relative tolerance of four diatoms (Nitzschia palea, Pinnularia mesolepta, Mayamaea atomus, and Gomphonema truncatum) to Cd was evaluated, including their proneness to deformities, and the severity of the abnormalities in relation to Cd concentration. The indirect effect of Cd on photosynthetic capacities was assessed during a short time exposure experiment using a dose-response approach to evaluate the relative tolerance of the four diatom species. The EC25 were 9 (3, 23), 606 (348, 926), 1179 (1015, 1349) and 2394 (1890, 2896) µg/L for P. mesolepta, G. truncatum, N. palea, and M. atomus respectively. P. mesolepta was by far the most Cd sensitive species while M. atomus was the most tolerant. In addition, diatoms were exposed to a single concentration of Cd comparable to a heavily contaminated environment for a longer duration to evaluate the effect of Cd on growth kinetics and the deformities induced. N. palea, P. mesolepta, and M. atomus were able to grow when cultivated with Cd while G. truncatum was not. Cadmium strongly affected the effective quantum yield in G. truncatum (4.8 ± 5.9% of the control) and P. mesolepta cultures (29.2 ± 6.9% of the control). The effects were moderate for N. palea (88.3 ± 0.7% of the control) and no impact was observed for M. atomus. The results from the two approaches were in accordance since they identified N. palea and M. atomus as the two most tolerant species to Cd, while P. mesolepta and G. truncatum were the most sensitive. The microscopy analyses revealed that P. mesolepta was more impacted by Cd than N. palea and M. atomus considering both the quantity of abnormal cells and the severity of the deformities. Overall, this research shows that not all deformities can be considered equal for a water quality bio-assessment. The work highlights a need to take into account metal-tolerance/sensitivity of the species and the severity of the deformities.