Physiology and molecular basis of thallium toxicity and accumulation in Arabidopsis thaliana

Thallium (Tl) is a non-essential metal mobilized through industrial processes which can lead to it entering the environment and exerting toxic effects. Plants are fundamental components of all ecosystems. Therefore, understanding the impact of Tl on plant growth and development is of great importance for assessing the potential environmental risks of Tl. Here, the responses of Arabidopsis thaliana to Tl were elucidated using physiological, genetic, and transcriptome analyses. Thallium can be absorbed by plant roots and translocated to the aerial parts, accumulating at comparable concentrations throughout plant parts. Genetic evidence supported the regulation of Tl uptake and movement by different molecular compartments within plants. Thallium primarily caused growth inhibition, oxidative stress, leaf chlorosis, and the impairment of K homeostasis. The disturbance of redox balance toward oxidative stress was supported by significant differences in the expression of genes involved in oxidative stress and antioxidant defense under Tl exposure. Reduced GSH levels in cad2-1 mutant rendered plants highly sensitive to Tl, suggesting that GSH has a prominent role in alleviating Tl-triggered oxidative responses. Thallium down-regulation of the expression of LCHII-related genes is believed to be responsible for leaf chlorosis. These findings illuminate some of the mechanisms underlying Tl toxicity at the physiological and molecular levels in plants with an eye toward the future environment management of this heavy metal.

Similar but different: Characterization of dddD gene-mediated DMSP metabolism among coral-associated Endozoicomonas

Endozoicomonas are often predominant bacteria and prominently important in coral health. Their role in dimethylsulfoniopropionate (DMSP) degradation has been a subject of discussion for over a decade. A previous study found that Endozoicomonas degraded DMSP through the dddD pathway. This process releases dimethyl sulfide, which is vital for corals coping with thermal stress. However, little is known about the related gene regulation and metabolic abilities of DMSP metabolism in Endozoicomonadaceae. In this study, we isolated a novel Endozoicomonas DMSP degrader and observed a distinct DMSP metabolic trend in two phylogenetically close dddD-harboring Endozoicomonas species, confirmed genetically by comparative transcriptomic profiling and visualization of the change of DMSP stable isotopes in bacterial cells using nanoscale secondary ion spectrometry. Furthermore, we found that DMSP cleavage enzymes are ubiquitous in coral Endozoicomonas with a preference for having DddD lyase. We speculate that harboring DMSP degrading genes enables Endozoicomonas to successfully colonize various coral species across the globe.

High-resolution spatial and genomic characterization of coral-associated microbial aggregates in the coral Stylophora pistillata

Bacteria commonly form aggregates in a range of coral species [termed coral-associated microbial aggregates (CAMAs)], although these structures remain poorly characterized despite extensive efforts studying the coral microbiome. Here, we comprehensively characterize CAMAs associated with Stylophora pistillata and quantify their cell abundance. Our analysis reveals that multiple Endozoicomonas phylotypes coexist inside a single CAMA. Nanoscale secondary ion mass spectrometry imaging revealed that the Endozoicomonas cells were enriched with phosphorus, with the elemental compositions of CAMAs different from coral tissues and endosymbiotic Symbiodiniaceae, highlighting a role in sequestering and cycling phosphate between coral holobiont partners. Consensus metagenome-assembled genomes of the two dominant Endozoicomonas phylotypes confirmed their metabolic potential for polyphosphate accumulation along with genomic signatures including type VI secretion systems allowing host association. Our findings provide unprecedented insights into Endozoicomonas-dominated CAMAs and the first direct physiological and genomic linked evidence of their biological role in the coral holobiont.

Sedimentary processes dominate nitrous oxide production and emission in the hypoxic zone off the Changjiang River estuary

Coastal oceans, known as the major nitrous oxide (N₂O) source to the atmosphere, are increasingly subject to eutrophication and concurrent near-bottom hypoxia. The natural nitrogen cycle is likely to be altered markedly in hypoxic coastal oceans. However, the processes responsible for N₂O production and emission remain elusive because of lacking field rate measurements simultaneously conducted in the water column and sediment. Here, we quantified N₂O production rates using a ¹⁵N-labeled technique in the water-column and surface sediments off the Changjiang (Yangtze) River estuary, the largest hypoxic zone in the Pacific margins. Our results showed that the estuarine surface sediments were the major source for N₂O production, accounting for approximately 90% of the total water-column accumulation and consequent efflux of N₂O in the hypoxic zone, whereas the water-column nitrification and denitrification combined only contributed <10%. More importantly, the coupling of nitrification and denitrification at the presence of abundant supply and remineralization of labile organic matter was the main driver of the N₂O release from the sediment-water interface in this region. This study highlights the dominant role of benthic processes occurring at the sediment-water interface controlling the coastal N₂O budget, as the anthropogenic eutrophication and hypoxia are expanding in coastal oceans.

Assessment of indium toxicity to the model plant Arabidopsis

The use of indium in semiconductor products has increased markedly in recent years. The release of indium into the ecosystem is inevitable. Under such circumstances, effective and accurate assessment of indium risk is important. An indispensable aspect of indium risk assessment is to understand the interactions of indium with plants, which are fundamental components of all ecosystems. Physiological responses of Arabidopsis thaliana exposed to indium were investigated by monitoring toxic effects, accumulation and speciation of indium in the plant. Indium can be taken up by plants and is accumulated mainly in roots. Limited indium root-to-shoot translocation occurs because of immobilization of indium in the root intercellular space and blockage of indium by the Casparian band in the endodermis. Indium caused stunted growth, oxidative stress, anthocyanization and unbalanced phosphorus nutrition. Indium jeopardizes phosphate uptake and translocation by inhibiting the accumulation of phosphate transporters PHOSPHATE TRANSPORTER1 (PHT1;1/4), responsible for phosphate uptake, and PHOSPHATE1 (PHO1), responsible for phosphate xylem loading. Organic acid secretion is stimulated by indium exposure. Secreted citrate could function as a potential detoxifier to lower indium uptake. Our findings provide insights into the potential fate and effects of indium in plants and will aid the evaluation of risks with indium contamination.

Metagenomic, phylogenetic, and functional characterization of predominant endolithic green sulfur bacteria in the coral Isopora palifera

BACKGROUND

Endolithic microbes in coral skeletons are known to be a nutrient source for the coral host. In addition to aerobic endolithic algae and Cyanobacteria, which are usually described in the various corals and form a green layer beneath coral tissues, the anaerobic photoautotrophic green sulfur bacteria (GSB) Prosthecochloris is dominant in the skeleton of Isopora palifera. However, due to inherent challenges in studying anaerobic microbes in coral skeleton, the reason for its niche preference and function are largely unknown.

RESULTS

This study characterized a diverse and dynamic community of endolithic microbes shaped by the availability of light and oxygen. In addition, anaerobic bacteria isolated from the coral skeleton were cultured for the first time to experimentally clarify the role of these GSB. This characterization includes GSB's abundance, genetic and genomic profiles, organelle structure, and specific metabolic functions and activity. Our results explain the advantages endolithic GSB receive from living in coral skeletons, the potential metabolic role of a clade of coral-associated Prosthecochloris (CAP) in the skeleton, and the nitrogen fixation ability of CAP.

CONCLUSION

We suggest that the endolithic microbial community in coral skeletons is diverse and dynamic and that light and oxygen are two crucial factors for shaping it. This study is the first to demonstrate the ability of nitrogen uptake by specific coral-associated endolithic bacteria and shed light on the role of endolithic bacteria in coral skeletons.

Fractionation of otolith nitrogen stable isotopes measured by peroxodisulfate oxidation-bacterial conversion and isotope ratio mass spectrometry

RATIONALE

Otoliths are usually used to estimate the age of fish and the chemical composition such as nitrogen stable isotope ratios (δ¹⁵ N values) may record environmental information and ecological role of the fish. However, the isotopic fractionation of δ¹⁵ N values between diets and otoliths has rarely been investigated and remains unclear.

METHODS

Nitrogen isotopic fractionation between five different diets (δ¹⁵ N_diet values) and otoliths (δ¹⁵ N_oto values) were elucidated in tilapia Oreochromis mossambica reared in controlled feeding experiments. The otoliths were dissolved with hydrogen chloride and peroxodisulfate was used to oxidize the total organic materials to nitrate, which was further converted into N₂ O gas by denitrification bacteria before the measurement of δ¹⁵ N_oto values by isotope ratio mass spectrometry. The δ¹⁵ N values of muscles, gills, scales and livers of the tilapias were also measured by isotope ratio mass spectrometry.

RESULTS

The peroxodisulfate oxidation-bacterial conversion method reduced the minimum mass of the otoliths required for analysis to as low as 2 mg, unlike past methods, which have required masses of 8-155 mg. The δ¹⁵ N_oto values were not significantly different from the δ¹⁵ N_diet values of the five diets. Furthermore, the somatic growth rate had no effect on the δ¹⁵ N_oto values. Nevertheless, the δ¹⁵ N values of metabolically active tissues were significantly different from each other and higher than the δ¹⁵ N_diet values, due to the deamination of these tissues.

CONCLUSIONS

These results suggest that diet was the main source of amino acids for the otolith organic matrix and there was no biochemical transamination during the assimilation of dietary amino acids to otoliths. The δ¹⁵ N_oto value can be used as a proxy of nitrogen sources of fishes and may have potential application in ecological studies such as the detection of diet shift, migration, trophic levels and environmental changes experienced by the fish population.

Assimilation of nitrogen and carbon isotopes from fish diets to otoliths as measured by nanoscale secondary ion mass spectrometry

RATIONALE

Nitrogen and carbon stable isotope ratios (δ¹⁵ N and δ¹³ C values) of carbonate-bound organic materials in otoliths can provide information to address the biological and ecological functions of fish. Correct interpretation of otolith δ¹⁵ N and δ¹³ C profiles requires knowledge of the metabolic routes of nitrogen and carbon isotopes. However, the isotopic assimilation of δ¹⁵ N and δ¹³ C compositions from diets to otoliths has rarely been investigated.

METHODS

This study traced the daily nitrogen and carbon isotopic assimilation between diets and otoliths using nanoscale secondary ion mass spectrometry (NanoSIMS). Isotopically labeled algae (Tetraselmis chui) were fed to tilapia (Oreochromis niloticus) for 14-17 days. NanoSIMS and conventional isotope ratio mass spectrometry were used to measure δ¹⁵ N and δ¹³ C variations in the otoliths and fish muscle, respectively.

RESULTS

Otolith δ¹⁵ N values abruptly surged from natural abundance levels by 1000-2300‰ after the fish ate ¹⁵ N-spiked algae with δ¹⁵ N values of approximately 2200‰. However, the δ¹⁵ N values of fish muscle increased to only approximately 500‰ at the end of the feeding experiment. Much higher δ¹⁵ N values (3700-14 000‰) and moderate δ¹³ C values (60-200‰) were detected in the otoliths after the tilapia ate ¹⁵ N- and ¹³ C-spiked algae with a δ¹⁵ N value of 36667‰ and a δ¹³ C value of 272‰. Mapping analysis showed sub-micrometer-scale distribution of ¹⁵ N embedded in the otolith growth increments with a low-to-high δ¹⁵ N signal after the tilapia shifted diets from non-spiked to ¹⁵ N-labeled algae.

CONCLUSIONS

These results suggest that otolith nitrogen and carbon isotopes from food were directly assimilated on the same day. Food is the major and in some cases only source of otolith nitrogen isotopes but makes only a partial contribution to otolith carbon isotopes. Therefore, the δ¹⁵ N values recorded in the sclerochronological layers of the otoliths can be used to determine the trophic levels, food sources and diet changes of fish.

Isotopic evidence of nitrogen sources and nitrogen transformation in arsenic-contaminated groundwater

High concentrations of naturally occurring arsenic (As) are typically found in young alluvial and deltaic deposits, and high concentrations of ammonium (NH₄⁺) and nitrate (NO₃^-) are often present in groundwater affected by anthropogenic activities. In this study, on the basis of physicochemical characteristics of groundwater and the nitrogen and oxygen isotope composition of NO₃^-, it was inferred that the main sources of NO₃^- in the proximal fan of the Choushui River alluvial fan are likely to be ammonium fertilizers, manure, and septic waste; that in the mid-fan and the distal fan, the possible sources are nitrate fertilizers and marine nitrate. In the proximal fan, the oxidative state obviously promotes microbial nitrification. Denitrification occurs from the upstream region to the downstream region of the Choushui River, and therefore, the decrease in NO₃^- concentration along streams connecting the Choushui River to the ocean appears plausible. High DO concentrations and relatively low values of δ¹⁸O_NO3 in the deeper aquifer of the proximal fan may be attributed to unconfined granular nature and groundwater pumping by agricultural activities. In the mid-fan, NO₃^- assimilation is the dominant response to NO₃^- attenuation, and denitrification is insignificant; however, high concentrations of As, NH4⁺ and Fe and depletion of δ¹⁵N_NO3 imply the occurrence of feammox process. By contrast, denitrification evidently occurs in the distal fan, through assimilation, mineralization, and dissimilatory NO₃^- reduction to NH₄⁺, resulting in depletion of NO₃^- and increase in NH₄⁺ in groundwater. Feammox in the mid-fan and denitrification in the distal fan may be the main processes leading to the release of As from As-bearing Fe oxyhydroxides into groundwater.