Soil salinity, due to natural phenomena or human activities, alters the water potential, which in turn affects plant growth, negatively influencing their nutrient and water uptake. Plant-growth-promoting rhizobacteria (PGPR) can be used to counteract these negative effects, especially in glycophytes. The aim of our study was to characterize physiologically, genetically, and biochemically the novel halotolerant/halophilic bacteria isolated in our previous work. We evaluated the plant-growth-promoting (PGP) features and NaCl regulation’s roles in them. In this regard, analysis based on 16S rDNA sequences confirmed that our isolates are distinct bacterial strains, probably belonging to new species, which we named Bacillus sp. M21 and M23 and Halomonas sp. QH23 and QH24. In literature, it is known that many species of Halomonas and Bacillus genera produce factors regulating plant development, such as indole-3-acetic acid (IAA), ammonium, and siderophores; and their efficiency in promoting plant growth and productivity was also demonstrated in vivo. We demonstrated that the newly isolated strains exhibit different PGP activities, highlighting how the latter are regulated by NaCl and in a strain-dependent manner. In particular, the main results showed that NaCl negatively affects the production of IAA in QH23, M21 and M23, whereas it promotes it in QH24, where it is strictly salt-dependent. Both Halomonas strains produce ammonium only in the presence of NaCl, directly correlated to its concentration. The opposite occurs in Bacillus strains, where salt reduces its production up to ten times. Overall, the results underline how halotolerance is a preferable, but not sufficient, condition for considering a PGPR potentially useful in applications aimed at improving the tolerance and productivity of crops in saline soils.

Starting from the Industrial Revolution in the 18th century to date, urban areas have faced increasing environmental challenges due to the diffusion of harmful substances, resulting from vehicular traffic, the activities of different industries, waste, and building construction, etc. These pollutants are dangerous, since they pose a threat to both the environment and human health. Phytoremediation is an environmentally friendly and low-cost technique that uses plants and their associated microorganisms to clean-up contaminated sites. In this review, we explore its main applications in urban settings. Specifically, we investigate how phytoremediation works, highlighting the most effective plants for its success in a city context. Moreover, we also describe the main factors influencing its effectiveness, such as soil, climate, and pollutants. In this regard, several case studies, conducted worldwide, have reported on how phytoremediation can successfully reclaim contaminated areas, transforming them into reusable city green spaces, with reduced costs compared to traditional remediation techniques (e.g., soil replacement, soil washing, etc.). Moreover, by integrating it into urban planning, cities can address environmental pollution, while promoting biodiversity, enhancing the landscape, and increasing its social acceptance. This nature-based solution offers a practical path toward more sustainable and resilient urban environments, especially in regard to the climate change framework.

Rare Earth Elements (REE), also known as Lanthanides (Ln³⁺), are a group of 17 elements showing peculiar physical and chemical properties. Unlike technological applications, very little is known about the physiological role and toxicity of Ln³⁺ on biological systems, in particular on microorganisms (e.g., bacteria), which represent the most abundant domains on our planet. Up to now, very limited studies have been conducted due to Ln³⁺ precipitation with some anions commonly present in the culture media. Therefore, the development of a minimal medium is essential to allow the study of Ln³⁺-microbial interactions, limiting considerably the precipitation of insoluble salts. In this regard, a new minimal culture medium capable of solubilizing large amounts of Ln³⁺ and allowing the growth of different microbial taxa was successfully developed. Assays have shown that the medium is capable of solubilizing Ln³⁺ up to 100 times more than other common culture media and allowing the growth of 63 bacteria and 5 fungi. The kinetic growth of one yeast and one Gram-positive bacterium has been defined, proving to support superior growth and biomass compared to other commonly used minimal media. Moreover, the sensitivity and uptake/absorption of a Bacillus stratosphericus strain were tested, highlighting its capability to tolerate concentrations up to 10 mM of either Cerium, Gadolinium or Lanthanum and accumulate different quantities of the three.

Rare Earth Elements (REE), also known as Lanthanides (Ln3+), are a group of 17 elements showing peculiar physical and chemical properties. Unlike technological applications, very little is known about the physiological role and toxicity of Ln3+ on biological systems, in particular on microorganisms (e.g., bacteria), which represent the most abundant domains on our planet. Up to now, very limited studies have been conducted due to Ln3+ precipitation with some anions commonly present in the culture media. Therefore, the development of a minimal medium is essential to allow the study of Ln3+-microbial interactions, limiting considerably the precipitation as insoluble salts. In this regard, a new minimal culture medium capable of solubilizing large amounts of Ln3+, and allowing the growth of different microbial taxa, was successfully developed. Assays have shown that the medium is capable of solubilizing Ln3+ up to 100 times more than other common culture media and allowed the growth of 63 bacteria and 5 fungi. The kinetic growth of one yeast and one gram-positive bacterium has been defined, proving to support superior growth and biomass compared to other commonly used minimal media. Moreover, the sensitivity and uptake/absorption of a Bacillus stratosphericus strain was tested, highlighting its capability to tolerate concentrations up to 10 mM of either Cerium, Gadolinium or Lanthanum and accumulate different quantities of the three.