Site-specific response of sediment microbial community to supplementation of polyhydroxyalkanoates as biostimulants for PCB reductive dechlorination

Unveiling organohalide respiration potential in River Nile sediments via 16S rRNA gene amplicon sequencing of endogenous bacterial communities

BACKGROUND

Industrial waste, agricultural runoff and untreated sewage contaminate the Nile, leaving a toxic legacy in its sediments. Organohalides-polluted sediment in particular poses serious public health risks and detrimental effects on aquatic life. Sediment microbiomes may harbor bacterial strains that could be utilized in bioremediation of such toxic pollutants.

MATERIAL AND METHODS

Two microbiomes from polluted River Nile sediments were analyzed by using 16S rRNA gene amplicon sequencing. In addition, PICRUSt analysis based on 16S rRNA data was used to explore the organohalide respiring bacteria (OHRB) genera and their corresponding organohalide respiration (OHR) activity. Microcosm studies were performed to validate the potential for dechlorination activity of River Nile sediment. Dechlorination of the parent chloroethenes into daughter end product were detected by gas chromatography coupled with flame ionization detection analysis.

RESULTS

Analysis of 16S rRNA gene amplicon sequences using the EZ-biocloud server identified Proteobacteria as the dominant phylum in both microbiomes, with Bacteroidetes and Chloroflexi prevalent in RNS1 sediment and Chlorobi in RNS2 sediment. EZ-biocloud and PCR analyses detected several potential OHRB genera, including Dehalococcoides, Dehalogenimonas, Desulfomonile, Desulfovibrio, and Geobacter, suggesting potential OHR activity. Further evidence for potential OHR activity was provided by PICRUSt functional prediction analysis, which suggested the presence of reductive dehalogenases as functional biomarkers associated with OHR in the sediment samples. Specifically, PICRUSt analysis predicted the presence of potential genes of tetrachloroethene reductive dehalogenase and 3-chloro-4-hydroxyphenylacetate reductive dehalogenase, previously linked to OHR. Microcosm studies confirmed the dechlorination potential of tetrachloroethene to dichloroethene.

CONCLUSION

This study demonstrates that River Nile sediment in industrialized area harbors distinct microbiomes enclosing various OHRB genera, providing substantial evidence for potential reductive dechlorination activity. It also provides potential functional biomarkers for OHR activity.

Chemical-physical parameters and microbial community changes induced by electrodes polarization inhibit PCB dechlorination in a marine sediment

Microbial reductive dechlorination of organohalogenated pollutants is often limited by the scarcity of electron donors, that can be overcome with microbial electrochemical technologies (METs). In this study, polarized electrodes buried in marine sediment microcosms were investigated to stimulate PCB reductive dechlorination under potentiostatic (-0.7 V vs Ag/AgCl) and galvanostatic conditions (0.025 mA·cm-2-0.05 mA·cm-2), using graphite rod as cathode and iron plate as sacrificial anode. A single circuit and a novel two antiparallel circuits configuration (2AP) were investigated. Single circuit polarization impacted the sediment pH and redox potential (ORP) proportionally to the intensity of the electrical input and inhibited PCB reductive dechlorination. The effects on the sediment's pH and ORP, along with the inhibition of PCB reductive dechlorination, were mitigated in the 2AP system. Electrodes polarization stimulated sulfate-reduction and promoted the enrichment of bacterial clades potentially involved in sulfate-reduction as well as in sulfur oxidation. This suggested the electrons provided were consumed by competitors of organohalide respiring bacteria and specifically sequestered by sulfur cycling, which may represent the main factor limiting the applicability of METs for stimulating PCB reductive dechlorination in marine sediments.

Enrichment of Aerobic and Anaerobic Hydrocarbon-Degrading Bacteria from Multicontaminated Marine Sediment in Mar Piccolo Site (Taranto, Italy)

Marine sediments act as a sink for the accumulation of various organic contaminants such as polychlorobiphenyls (PCBs). These contaminants affect the composition and activity of microbial communities, particularly favoring those capable of thriving from their biodegradation and biotransformation under favorable conditions. Hence, contaminated environments represent a valuable biological resource for the exploration and cultivation of microorganisms with bioremediation potential. In this study, we successfully cultivated microbial consortia with the capacity for PCB removal under both aerobic and anaerobic conditions. The source of these consortia was a multicontaminated marine sediment collected from the Mar Piccolo (Taranto, Italy), one of Europe's most heavily polluted sites. High-throughput sequencing was employed to investigate the dynamics of the bacterial community of the marine sediment sample, revealing distinct and divergent selection patterns depending on the imposed reductive or oxidative conditions. The aerobic incubation resulted in the rapid selection of bacteria specialized in oxidative pathways for hydrocarbon transformation, leading to the isolation of Marinobacter salinus and Rhodococcus cerastii species, also known for their involvement in aerobic polycyclic aromatic hydrocarbons (PAHs) transformation. On the other hand, anaerobic incubation facilitated the selection of dechlorinating species, including Dehalococcoides mccartyi, involved in PCB reduction. This study significantly contributes to our understanding of the diversity, dynamics, and adaptation of the bacterial community in the hydrocarbon-contaminated marine sediment from one sampling point of the Mar Piccolo basin, particularly in response to stressful conditions. Furthermore, the establishment of consortia with biodegradation and biotransformation capabilities represents a substantial advancement in addressing the challenge of restoring polluted sites, including marine sediments, thus contributing to expanding the toolkit for effective bioremediation strategies.

The Application of Arbuscular Mycorrhizal Fungi as Microbial Biostimulant, Sustainable Approaches in Modern Agriculture

Biostimulant application can be considered an effective, practical, and sustainable nutritional crop supplementation and may lessen the environmental problems related to excessive fertilization. Biostimulants provide beneficial properties to plants by increasing plant metabolism, which promotes crop yield and improves the quality of crops; protecting plants against environmental stresses such as water shortage, soil salinization, and exposure to sub-optimal growth temperatures; and promoting plant growth via higher nutrient uptake. Other important benefits include promoting soil enzymatic and microbial activities, changing the architecture of roots, increasing the solubility and mobility of micronutrients, and enhancing the fertility of the soil, predominantly by nurturing the development of complementary soil microbes. Biostimulants are classified as microbial, such as arbuscular mycorrhizae fungi (AMF), plant-growth-promoting rhizobacteria (PGPR), non-pathogenic fungi, protozoa, and nematodes, or non-microbial, such as seaweed extract, phosphite, humic acid, other inorganic salts, chitin and chitosan derivatives, protein hydrolysates and free amino acids, and complex organic materials. Arbuscular mycorrhizal fungi are among the most prominent microbial biostimulants and have an important role in cultivating better, healthier, and more functional foods in sustainable agriculture. AMF assist plant nutrient and water acquisition; enhance plant stress tolerance against salinity, drought, and heavy metals; and reduce soil erosion. AMF are proven to be a sustainable and environmentally friendly source of crop supplements. The current manuscript gives many examples of the potential of biostimulants for the production of different crops. However, further studies are needed to better understand the effectiveness of different biostimulants in sustainable agriculture. The review focuses on how AMF application can overcome nutrient limitations typical of organic systems by improving nutrient availability, uptake, and assimilation, consequently reducing the gap between organic and conventional yields. The aim of this literature review is to survey the impacts of AMF by presenting case studies and successful paradigms in different crops as well as introducing the main mechanisms of action of the different biostimulant products.