[Departement_IRSTEA]Eaux [ADD1_IRSTEA]Systèmes aquatiques soumis à des pressions multiples | International audience | Understanding how organisms cope with global change is a major question in many fields of biology. Mainly, understanding the molecular mechanisms supporting rapid phenotypic changes of organisms in response to stress and linking stress-induced molecular events to adaptive or adverse outcomes at the individual or population levels remain a major challenge in evolutionary biology, ecology or ecotoxicology. In this view, the present study aimed to test (i) whether environmental factors, especially pollutants, can trigger changes in the activity of retrotransposons (RTs) in wild fish and (ii) if changes in RT DNA methylation or transcription levels can be linked to modifications at the individual level. RTs are genetic elements that have the ability to replicate and integrate elsewhere in the genome. Although RTs are mainly quiescent during normal development, they can be experimentally activated under life-threatening conditions, affecting the fitness of their host. Wild eels were collected in four sampling sites presenting differing levels of contamination. The methylation level and the transcriptional activity of two RTs and two genes involved in development and cell differentiation were analyzed in fish liver in addition to the determination of fish contaminants levels and diverse growth and morphometric indices. An up-regulation of RTs associated to lower methylation levels and lower growth indices were observed in highly contaminated fish. Our results suggest that RT activation in fish experiencing stress conditions could have both detrimental and beneficial implications, affecting fish growth but promoting resistance to environmental stressors such as pollutants. Retrotransposons could represent interesting environment-sensitive molecular markers allowing to link stress-induced molecular events to adverse outcomes at higher levels.

Both inorganic and organic fertilizers are widely used to increase rice yield. However, these fertilizers are also found to aggravate mercury methylation and methylmercury (MeHg) accumulation in paddy fields. The aim of this study was to reveal the mechanisms of inorganic and organic fertilizers on MeHg accumulation in rice grains, which are not yet well understood. Potting cultures were conducted in which different fertilizers were applied to a paddy soil. The results showed that both inorganic and organic fertilizers increased MeHg concentrations rather than biological accumulation factors (BAFs) of MeHg in mature rice grains. Inorganic fertilizers, especially nitrogen fertilizer, enhanced the bioavailability of mercury and the relative amount Hg-methylating microbes and therefore intensified mercury methylation in paddy soil and MeHg accumulation in rice grains. Unlike inorganic fertilizers, organic matter (OM) in organic fertilizers was the main reason for the increase of MeHg concentrations in rice grains, and it also could immobilize Hg in soil when it was deeply degraded. The enhancement of MeHg concentrations in rice grains induced by inorganic fertilizers (5.18–41.69%) was significantly (p < 0.05) lower than that induced by organic fertilizers (80.49–106.86%). Inorganic fertilizers led to a larger increase (50.39–99.28%) in thousand-kernel weight than MeHg concentrations (5.18–41.69%), resulting in a dilution of MeHg concentrations in mature rice grains. Given the improvement of soil properties by organic fertilizer, increasing the proportion of inorganic fertilizer application may be a better option to alleviate MeHg accumulation in rice grains and guarantee the rice yield in the agricultural production.

Salt marsh estuaries serve as sources and sinks for nutrients and elements to and from estuarine water, which enhances and alleviates watershed fluxes to the coastal ocean. We assessed sources and sinks of mercury in the intertidal Plum Island Sound estuary in Massachusetts, the largest salt marsh estuary of New England, using 25-km spatial water sampling transects. Across all seasons, dissolved (FHg) and total (THg) mercury concentrations in estuarine water were highest and strongly enhanced in upper marshes (1.31 ± 0.20 ng L⁻¹ and 6.56 ± 3.70 ng L⁻¹, respectively), compared to riverine Hg concentrations (0.86 ± 0.17 ng L⁻¹ and 0.88 ± 0.34 ng L⁻¹, respectively). Mercury concentrations declined from upper to lower marshes and were lowest in ocean water (0.38 ± 0.10 ng L⁻¹ and 0.56 ± 0.25 ng L⁻¹, respectively). Conservative mixing models using river and ocean water as endmembers indicated that internal estuarine Hg sources strongly enhanced estuarine water Hg concentrations. For FHg, internal estuarine Hg contributions were estimated at 26 g yr⁻¹ which enhanced Hg loads from riverine sources to the ocean by 44%. For THg, internal sources amounted to 251 g yr⁻¹ and exceeded riverine sources six-fold. Proposed sources for internal estuarine mercury contributions include atmospheric deposition to the large estuarine surface area and sediment re-mobilization, although sediment Hg concentrations were low (average 23 ± 2 μg kg⁻¹) typical of uncontaminated sediments. Soil mercury concentrations under vegetation, however, were ten times higher (average 200 ± 225 μg kg⁻¹) than in intertidal sediments suggesting that high soil Hg accumulation might drive lateral export of Hg to the ocean. Spatial transects of methylated Hg (MeHg) showed no concentration enhancements in estuarine water and no indication of internal MeHg sources or formation. Initial mass balance considerations suggest that atmospheric deposition may either be in similar magnitude, or possibly exceed lateral tidal export which would be consistent with strong Hg accumulation observed in salt marsh soils sequestering Hg from current and past atmospheric deposition.

Rice consumption is the major pathway for human methylmercury (MeHg) exposure in inland China, especially in mercury (Hg) contaminated regions. MeHg production, a microbially driven process, depends on both the chemical speciation of inorganic divalent mercury, Hg(II), that determines Hg bioavailability for methylation. Studies have shown that Hg(II) speciation in contaminated paddy soils is mostly controlled by natural organic matter and sulfide levels, which are typically thought to limit Hg mobility and bioavailability. Yet, high levels of MeHg are found in rice, calling for reconsideration of the nature of Hg species bioavailable to methylators in paddy soils. Here, we conducted incubation experiments using a multi-isotope tracer technique including ¹⁹⁸Hg(NO₃)₂, natural organic matter bond Hg(II) (NOM-¹⁹⁹Hg(II)), ferrous sulfide sorbed Hg(II) (≡FeS-²⁰⁰Hg(II)), and nanoparticulate mercuric sulfide (nano-²⁰²HgS), to investigate the relative importance of geochemically diverse yet relevant Hg(II) species on Hg methylation in paddy soils across a Hg concentration gradient. We show that methylation rates for all Hg(II) species tested decreased with increasing Hg concentrations, and that methylation rates using NOM-¹⁹⁹Hg(II) and nano-²⁰²HgS as substrates were similar or greater than rates obtained using the labile ¹⁹⁸Hg(NO₃)₂ substrate. ≡FeS-²⁰⁰Hg(II) yielded the lowest methylation rate in all sites, and thus the formation of FeS is likely a sink for labile ¹⁹⁸Hg(NO₃)₂ in sulfide-rich paddy soils. Moreover, the variability in the methylation data for a given site (1 to 5-fold variation depending on the Hg species) was smaller than what was observed across the Hg concentration gradient (10³–10⁴ fold variation between sites). These findings emphasize that at broad spatial scales, site-specific characteristics, such as microbial community structure, need to be taken into consideration, alongside the nature of the Hg substrate available for methylation, to determine net MeHg production. This study highlights the importance of developing site-specific strategies for remediating Hg pollution.

Under the climate change context, warming Southern Ocean waters may allow mercury (Hg) to become more bioavailable to the Antarctic marine food web (i.e., ice-stored Hg release and higher methylation rates by microorganisms), whose biomagnification processes are poorly documented. Biomagnification of Hg in the food web of the Antarctic Peninsula, one of the world's fastest-warming regions, was examined using carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotope ratios for estimating feeding habitat and trophic levels, respectively. The stable isotope signatures and total Hg (T-Hg) concentrations were measured in Antarctic krill Euphausia superba and several Antarctic predator species, including seabirds (gentoo penguins Pygoscelis papua, chinstrap penguins Pygoscelis antarcticus, brown skuas Stercorarius antarcticus, kelp gulls Larus dominicanus, southern giant petrels Macronectes giganteus) and marine mammals (southern elephant seals Mirounga leonina). Significant differences in δ¹³C values among species were noted with a great overlap between seabird species and M. leonina. As expected, significant differences in δ¹⁵N values among species were found due to interspecific variations in diet-related to their trophic position within the marine food web. The lowest Hg concentrations were registered in E. superba (0.007 ± 0.008 μg g⁻¹) and the highest values in M. giganteus (12.090 ± 14.177 μg g⁻¹). Additionally, a significant positive relationship was found between Hg concentrations and trophic levels (reflected by δ¹⁵N values), biomagnifying nearly 2 times its concentrations at each level. Our results support that trophic interaction is the major pathway for Hg biomagnification in Southern Ocean ecosystems and warn about an increase in the effects of Hg on long–lived (and high trophic level) Antarctic predators under climate change in the future.

Microorganisms are essential for modifying arsenic morphology, mobility, and toxicity. Still, knowledge of the microorganisms responsible for arsenic metabolism in specific arsenic-contaminated fields, such as metallurgical plants is limited. We sampled on-field soils from three depths at 70 day intervals to explore the distribution and transformation of arsenic in the soil. Arsenic-metabolizing microorganisms were identified from the mapped gene sequences. Arsenic metabolism pathways were constructed with metagenomics and AsChip analysis (a high-throughput qPCR chip for arsenic metabolism genes). It has been shown in the result that 350 genera of arsenic-metabolizing microorganisms carrying 17 arsenic metabolism genes in field soils were identified, as relevant to arsenic reduction, arsenic methylation, arsenic respiration, and arsenic oxidation, respectively. Arsenic reduction genes were the only genes shared by the 10 high-ranking arsenic-metabolizing microorganisms. Arsenic reduction genes (arsABCDRT and acr3) accounted for 73.47%–78.11% of all arsenic metabolism genes. Such genes dominated arsenic metabolism, mediating the reduction of 14.11%–19.86% of As(V) to As(III) in 0–100 cm soils. Arsenic reduction disrupts microbial energy metabolism, DNA replication and repair and membrane transport. Arsenic reduction led to a significant decrease in the abundance of 17 arsenic metabolism genes (p < 0.0001). The critical role of arsenic-reducing microorganisms in the migration and transformation of arsenic in metallurgical field soils, was emphasized with such results. These results were of pronounced significance for understanding the transformation behavior of arsenic and the precise regulation of arsenic in field soil.

The present study aims to explore the bioaccumulation and biotic transformations of inorganic (iHg) and monomethyl mercury (MMHg) by natural pico-nanoplankton community from eutrophic lake Soppen, Switzerland. Pico-nanoplankton encompass mainly bacterioplankton, mycoplankton and phytoplankton groups with size between 0.2 and 20 μm. Species-specific enriched isotope mixture of ¹⁹⁹iHg and ²⁰¹MMHg was used to explore the accumulation, the subcellular distribution and transformations occurring in natural pico-nanoplankton sampled at 2 different depths (6.6 m and 8.3 m). Cyanobacteria, diatoms, cryptophyta, green algae and heterotrophic microorganisms were identified as the major groups of pico-nanoplankton with diatoms prevailing at deeper samples. Results showed that pico-nanoplankton accumulated both iHg and MMHg preferentially in the cell membrane/organelles, despite observed losses. The ratios between the iHg and MMHg concentrations measured in the membrane/organelles and cytosol were comparable for iHg and MMHg. Pico-nanoplankton demethylate added ²⁰¹MMHg (~4 and 12% per day depending on cellular compartment), although the involved pathways are to further explore. Comparison of the concentrations of ²⁰¹iHg formed from ²⁰¹MMHg demethylation in whole system, medium and whole cells showed that 82% of the demethylation was biologically mediated by pico-nanoplankton. No significant methylation of iHg by pico-nanoplankton was observed. The accumulation of iHg and MMHg and the percentage of demethylated MMHg correlated positively with the relative abundance of diatoms and heterotrophic microorganisms in the pico-nanoplankton, the concentrations of TN, Mg²⁺, NO₃⁻, NO₂⁻, NH₄⁺ and negatively with the concentrations of DOC, K⁺, Na⁺, Ca²⁺, SO₄²⁻. Taken together the results of the present field study confirm the role of pico-nanoplankton in Hg bioaccumulation and demethylation, however further research is needed to better understand the underlying mechanisms and interconnection between heterotrophic and autotrophic microorganisms.

Fomesafen (FSA) is widely used in soybean fields for weed control. However, the persisting characteristics of FSA in the agricultural soil or water may become a hidden danger causing environmental pollution and phytotoxicity to succession crops. In this study, the growth and physiological responses of rice to FSA were investigated. It was found that the growth of rice seedlings was obviously inhibited by FSA exposure especially at over 0.1 mg L⁻¹. To gain an insight into the molecular mechanisms for the potential ecotoxicology, four libraries of rice roots and shoots exposed to FSA were created and subjected to the global RNA-sequencing (RNA-Seq) combined with HRLC-Q-TOF-MS/MS analytical technologies to comprehensively characterize the biochemical processes and catalytic reactions involved in FSA metabolism in rice. Compared with those without FSA, 499 and 450 up-regulated genes in roots and shoots with FSA were detected. Many of them were closely correlated with the tolerance to environmental stress, detoxification of xenobiotics and molecular metabolism process including cytochrome P450, glutathione S-transferases and acetyltransferase. A total of eight metabolites and fourteen conjugates in the reactive pathways of hydrolysis, substitution, reduction, methylation, glycosylation, acetylation, and malonylation were characterized by HRLC-Q-TOF-MS/MS. The relationship between the metabolized derivatives of FSA and enhanced expression the corresponding enzymatic regulators was established. This study will help understand the mechanisms and pathways of FSA metabolism and inspire the further research on FSA degradation in the paddy crops and environmental or health risks.

Root cell wall (RCW) modification is a widespread important defense strategy of plant to cope with trace metals. However, mechanisms underlying its remolding in cadmium (Cd) accumulation are still lacking in hyperaccumulators. In this study, changes of RCW structures and components between nonhyperaccumulating ecotype (NHE) and hyperaccumulating ecotype (HE) of Sedum alfredii were investigated simultaneously. Under 25 μM Cd treatment, RCW thickness of NHE is nearly 2 folds than that of HE and the thickened cell wall of NHE was enriched in low-methylated pectin, leading to more Cd trapped in roots tightly. In the opposite, large amounts of high-methylated pectin were assembled around RCW of HE with Cd supply, in this way, HE S. alfredii decreased its root fixation of Cd and enhanced Cd migration into xylem. TEM and AFM results further confirmed that thickened cell wall was caused by the increased amounts of cellulose and lignin while root tip lignification was resulted from variations of sinapyl (S) and guaiacyl (G) monomers. Overall, thickened cell wall and methylated pectin have synchronicity in spatial location of roots, and their coordination contributed to Cd accumulation in S. alfredii.

Excessive exposure to cobalt (Co) is known to make adverse impact on the nervous system, but its detailed mechanisms of neurotoxicity have yet to be determined. In this study, C57BL/6 mice (0, 4, 8, 16 mg/kg CoCl₂, 30 days) and human neuroblastoma H4 cells (0, 100, 400, 600 μM CoCl₂) were used as in vivo and in vitro models. Our results revealed that CoCl₂ intraperitoneal injection caused significant impairments in learning and memory, as well as pathological damage in the nervous system. We further certificated the alteration of m⁶A methylation induced by CoCl₂ exposure. Our findings demonstrate for the first time, significant differences in the degree of m⁶A modification, the biological function of m⁶A-modified transcripts between cortex and H4 cell samples. Specifically, MeRIP-seq and RNA-seq elucidate that CoCl₂ exposure results in differentially m⁶A-modified and expressed genes, which were enriched in pathways involving synaptic transmission, and central nervous system (CNS) development. Mechanistic analyses revealed that CoCl₂ remarkably changed m⁶A modification level by affecting the expression of m⁶A methyltransferase and demethylase, and decreasing the activity of demethylase. We observed variation of m⁶A modification in neurodegenerative disease-associated genes upon CoCl₂ exposure and identified regulatory strategy between m⁶A and potential targets mRNA. Our novel findings provide novel insight into the functional roles of m⁶A modification in neurodegenerative damage caused by environmental neurotoxicants and identify Co-mediated specific RNA regulatory strategy for broadening the epigenetic regulatory mechanism of RNA induced by heavy metals.