Variations in pH significantly affect cadmium uptake in grafted muskmelon (Cucumis melo L.) plants and drive the diversity of bacterial communities in a seedling substrate

Association between bacterial community and cadmium distribution across Colombian cacao crops

Assessing the bacterial community composition across cacao crops is important to understand its potential role as a modulator of cadmium (Cd) translocation to plant tissues under field conditions; Cd mobility between soil and plants is a complex and multifactorial problem that cannot be captured only by experimentation. Although microbes have been shown to metabolize and drive the speciation of Cd under controlled conditions, regardless of the link between soil bacterial community (SBC) dynamics and Cd mobilization in the rhizosphere, only a few studies have addressed the relationship between soil bacterial community composition (SBCC) and Cd content in cacao seeds (Cdseed). Therefore, this study aimed to explore the association between SBCC and different factors influencing the distribution of Cd across cacao crop systems. This study comprised 225 samples collected across five farms, where we used an amplicon sequencing approach to characterize the bacterial community composition. The soil Cd concentration alone (Cdsoil) was a poor predictor of Cdseed. Still, we found that this relationship was more apparent when the variation within farms was controlled, suggesting a role of heterogeneity within farms in modulating Cd translocation and, thus, seed Cd content. Our results provide evidence of the link between soil bacterial communities and the distribution of Cd across Colombian cacao crops, and highlight the importance of incorporating fine-spatial-scale studies to advance the understanding of factors driving Cd uptake and accumulation in cacao plants.

IMPORTANCE

Cadmium (Cd) content in cacao crops is an issue that generates interest due to the commercialization of chocolate for human consumption. Several studies provided evidence about the non-biological factors involved in its translocation into the cacao plant. However, factors related to this process, including soil bacterial community composition (SBCC), still need to be addressed. It is well known that soil microbiome could impact compounds' chemical transformation, including Cd, on the field. Here, we found the first evidence of the link between soil bacterial community composition and Cd concentration in cacao soils and seeds. It highlights the importance of including the variation of bacterial communities to assess the factors driving the Cd translocation into cacao seeds. Moreover, the results highlight the relevance of the spatial heterogeneity within and across cacao farms, influencing the variability of Cd concentrations.

Transport Behavior of Cd2+ in Highly Weathered Acidic Soils and Shaping in Soil Microbial Community Structure

The mining and smelting site soils in South China present excessive Cd pollution. However, the transport behavior of Cd in the highly weathered acidic soil layer at the lead-zinc smelting site remains unclear. Here, under different conditions of simulated infiltration, the migration behavior of Cd2+ in acid smelting site soils at different depths was examined. The remodeling effect of Cd2+ migration behavior on microbial community structure and the dominant microorganisms in lead-zinc sites soils was analyzed using high-throughput sequencing of 16S rRNA gene amplicons. The results revealed a specific flow rate in the range of 0.3-0.5 mL/min that the convection and dispersion have no obvious effect on Cd2+ migration. The variation of packing porosity could only influence the migration behavior by changing the average pore velocity, but cannot change the adsorption efficiency of soil particles. The Cd has stronger migration capacity under the reactivation of acidic seepage fluid. However, in the alkaline solution, the physical properties of soil, especially pores, intercept the Cd compounds, further affecting their migration capacity. The acid-site soil with high content of SOM, amorphous Fe oxides, crystalline Fe/Mn/Al oxides, goethite, and hematite has stronger ability to adsorb and retain Cd2+. However, higher content of kaolinite in acidic soil will increase the potential migration of Cd2+. Besides, the migration behavior of Cd2+ results in simplified soil microbial communities. Under Cd stress, Cd-tolerant genera (Bacteroides, Sphingomonas, Bradyrhizobium, and Corynebacterium) and bacteria with both acid-Cd tolerance (WCHB 1-84) were distinguished. The Ralstonia showed a high enrichment degree in alkaline Cd2+ infiltration solution (pH 10.0). Compared to the influence of Cd2+ stress, soil pH had a stronger ability to shape the microbial community in the soil during the process of Cd2+ migration.

Assessing the suitability of municipal sewage sludge and coconut bran as breeding medium for Oryza sativa L. seedlings and developing a standardized substrate

The utilization of organic solid waste (OSW) for preparing standardized seedling substrates is a main challenge due to its temporal and spatial variability. This study aims to form models based on data from the literature and validate them through experiments to explore a standardized seedling substrate. The typical OSW in Hainan Province, including municipal sewage sludge (MSS), coconut bran (CB), seaweed mud (SM), and municipal sewage sludge biochar (MSSB), was used as raw material. A series of six mixing ratios was tested, namely: T1 (0% MSS: 90% CB), T2 (10% MSS: 80% CB), T3 (30% MSS: 60% CB), T4 (50% MSS: 40% CB), T5 (70% MSS: 20% CB), and T6 (90% MSS: 0% CB). SM and MSSB were added as amendment materials at 5% (w/w) for each treatment. The physicochemical properties of substrates, agronomic traits of rice seedlings and microbial diversity were analyzed. The results showed that the four kinds of OSW played an active role in providing rich sources of nutrients. The dry weight of the above-ground part was 2.98 times greater in T3 than that of the commercial substrate. Furthermore, the microbial analysis showed a higher abundance of Actinobacteria in T3, representing the stability of the composted products. Finally, the successful fitting of the results with the linear regression models could establish relationship equations between the physicochemical properties of the substrate and the growth characteristics of seedlings. The relevant parameters suitable for the growth of rice seedlings were as follows: pH (6.46-7.01), EC (less than 2.12 mS cm-1), DD (0.13-0.16 g cm-3), and TPS (65.68-82.73%). This study proposed relevant parameters and models for standardization of seedling substrate, which would contribute to ensuring the quality of seedlings and OSW resource utilization.

Phosphate-solubilizing bacteria abate cadmium absorption and restore the rhizospheric bacterial community composition of grafted watermelon plants

The grafting of watermelon plants to rootstocks is common due to the strong capacity of rootstocks to adapt to abiotic and biotic stresses. However, the effect of phosphate-solubilizing bacteria (PSB) on grafted watermelon plant growth and bacterial structures in root soil is unclear. In this study, the growth and hormone levels of grafted plants were measured, and the bacterial communities under cadmium (Cd) stress and inoculation with PSB were sequenced in three treatments (S1, control; S2, 50 μmol Cd [CdCl₂]; and S3, 50 μmol Cd plus inoculation with the Cd-resistant PSB strain 'N3'). The results showed that inoculation with PSB significantly (P < 0.05) improved the total dry weight of the grafted plants. Typically, inoculation with PSB significantly (P < 0.05) reduced Cd content in scions and roots. The level of the phytohormone jasmonic acid increased in treatment S2, but decreased in treatment S3 under inoculation with PSB. The functional annotation of prokaryotic taxa showed that Cd decreased the abundance of nitrogen respiration and chloroplast functional groups. Nevertheless, inoculation with PSB helped restore bacterial community structures. These findings provide a new understanding of the effect of PSB on the promotion of seedling growth and bacterial communities in grafted watermelon plants under Cd stress.

Drivers of cadmium accumulation in Theobroma cacao L. beans: A quantitative synthesis of soil-plant relationships across the Cacao Belt

Elevated cadmium (Cd) concentrations in cacao and cocoa-based products (e.g., chocolate) present a potentially serious human health risk. While recent regulatory changes have established a threshold of 0.8 mg kg^-1 for Cd content of cocoa-based products, the biophysical factors (e.g., climatic or edaphic conditions) that determine the amount of soil-derived Cd in the cacao bean are poorly understood and have yet to be quantitatively assessed across diverse production contexts. To determine the primary drivers of cacao bean Cd, we used the scientific literature to systematically compile a database of climatic, edaphic, and plant data from across the Cacao Belt, which is approximately 20 degrees latitude on either side of the equator. From this compiled dataset, we then used boosted regression trees to quantitatively synthesize and evaluate these drivers of cacao bean Cd. Total soil Cd concentration, soil pH, and leaf Cd were the best predictors of bean Cd content. Notably, we found that both available soil Cd and soil organic carbon (SOC) content had negligible effects on bean Cd. However, soil pH and SOC decreased the degree of bioconcentration of total soil Cd in the bean Cd concentration. Thus, given the difficulty in remediating soil Cd enriched soils, our results suggest that Cd mitigation strategies targeting plant physiology-based approaches (e.g., breeding, rootstocks) have a higher probability of success than soil-based strategies (e.g., remediation).

Cadmium (Cd) chloride affects the nutrient uptake and Cd-resistant bacterium reduces the adsorption of Cd in muskmelon plants

This study investigated the effect of cadmium (Cd) chloride on the uptake of N, P, and K and evaluate the effect of Cd-resistant bacterium “N3” on reducing the adverse effect of Cd in grafted and nongrafted plants. The shoot and total dry weights of the nongrafted muskmelon plants decreased under 50 and 100 µM Cd treatments. The scion and shoot dry weights of the grafted plants increased significantly, whereas their root dry weight increased by nearly onefold compared with those of the CK-grafted plants regardless of Cd concentration. The N, P, and K contents in the nongrafted plants decreased under Cd treatments but increased under 50 µM Cd treatment when inoculated with “N3”. The N, P, and K contents in the grafted plants were lower than those treated with only Cd. The grafted and nongrafted plants exhibited low Cd accumulation in the scion or shoot part compared with the root tissues. “N3” inoculation reduced the Cd concentration in all tissues of the grafted and nongrafted plants. Our results demonstrated great variation in Cd accumulation in the grafted and nongrafted muskmelon plants, thereby promoting food safety under Cd contamination conditions.

Change of rhizospheric bacterial community of the ancient wild tea along elevational gradients in Ailao mountain, China

The rhizospheric microbial community is one of the major environmental factors affecting the distribution and fitness of plants. Ancient wild tea plants are rare genetic resource distributed in Southwest China. In this study, we investigated that rhizospheric bacterial communities of ancient wild tea plants along the elevational gradients (2050, 2200, 2350 and 2500 m) in QianJiaZhai Reserve of Ailao Mountains. According to the Illumina MiSeq sequencing of 16 S rRNA gene amplicons, Proteobacteria, Acidobacteria and Actinobacteria were the dominant phyla with the relative abundance 43.12%, 21.61% and 14.84%, respectively. The Variibacter was the most dominant genus in rhizosphere of ancient wild tea plant. Phylogenetic null modeling analysis suggested that rhizospheric bacterial communities of ancient wild tea plants were more phylogenetically clustered than expected by chance. The bacterial community at 2050 m was unique with the highest alpha diversity, tend to cluster the nearest taxon and simple co-occurrence network structure. The unique bacterial community was correlated to multiple soil factors, and the content soil ammonium nitrogen (NH₄⁺-N) was the key factor affecting the diversity and distribution of bacterial community along the elevational gradients. This study provided the necessary basic information for the protection of ancient tea trees and cultivation of tea plants.