Exiguobacterium sp. as a bioinoculant for plant-growth promotion and Selenium biofortification strategies in horticultural plants

Plant growth-promoting rhizobacteria (PGPR) have a positive effect on plant development and being a promising way to enhance crop productivity and as substitution of chemical fertilizers. Selenium (Se) is an important trace element and its intake is usually lower than the daily minimum amount required for humans; hence, there is a demand on the design of Se biofortification strategies. Here, the genetic traits known to be associated with Plant-Growth Promotion (PGP) and Se biotransformation of Exiguobacterium sp. S17 were evaluated through genome analysis. Its growth-promoting capacity was tested through plant-growth promotion assays in laboratory and field conditions, using Brassica juncea (indian mustard), Beta vulgaris (chard), and Lactuca sativa (lettuce). Additionally, the Se biotransformation ability of Exiguobacterium sp. S17 was evaluated and the obtained selenized bacteria were tested in mustard plants. The sequenced bacteria genome revealed the presence of multiple genes involved in important functions regarding soil and plant colonization, PGP and Se biotransformation. Moreover, it was demonstrated that Exiguobacterium sp. S17 enhanced plant growth and could be useful to produce Se accumulation and biofortification in accumulator plants such as mustard. Thereby, Exiguobacterium sp. S17 might be used for developing new, sustainable, and environmentally friendly agro-technological strategies.

Biogenic selenium nanoparticles with antifungal activity against the wood-rotting fungus Oligoporus pelliculosus

Selenium nanoparticles (SeNPs) have antimicrobial and antifungal activity. SeNPs using Se resistant bacteria is a low cost and eco-friendly technology. Fungal contamination of wood during drying is one of the main causes of economic losses in the wood industry. The bacterium Delftia sp. 5 resistance to Se and its ability to produce SeNPs able to inhibit the growth of the wood brown-rotting fungus Oligoporus pelliculosus was analyzed. The strain showed an optimal SeNPs production when selenite concentration was 160 mg L ^-1. The SeNPs were spherical with an average size 192.33 ± 8.6 nm and a zeta potential of -41.4 ± 1.3 nm. The SeNPs produced by Delftia sp. 5 (33.6 ± 0.1 mg L ^-1 Se) inhibited the growth of O. pelliculosus in agar plates and in Nothofagus pumilio (Lenga) wood samples. Delftia sp. 5 SeNPs could be used for embedding lenga wood prior to drying for preventing the growth of the deteriorating fungi O. pelliculosus.

Arsenic metabolism in high altitude modern stromatolites revealed by metagenomic analysis

Modern stromatolites thrive only in selected locations in the world. Socompa Lake, located in the Andean plateau at 3570 masl, is one of the numerous extreme Andean microbial ecosystems described over recent years. Extreme environmental conditions include hypersalinity, high UV incidence, and high arsenic content, among others. After Socompa's stromatolite microbial communities were analysed by metagenomic DNA sequencing, taxonomic classification showed dominance of Proteobacteria, Bacteroidetes and Firmicutes, and a remarkably high number of unclassified sequences. A functional analysis indicated that carbon fixation might occur not only by the Calvin-Benson cycle, but also through alternative pathways such as the reverse TCA cycle, and the reductive acetyl-CoA pathway. Deltaproteobacteria were involved both in sulfate reduction and nitrogen fixation. Significant differences were found when comparing the Socompa stromatolite metagenome to the Shark Bay (Australia) smooth mat metagenome: namely, those involving stress related processes, particularly, arsenic resistance. An in-depth analysis revealed a surprisingly diverse metabolism comprising all known types of As resistance and energy generating pathways. While the ars operon was the main mechanism, an important abundance of arsM genes was observed in selected phyla. The data resulting from this work will prove a cornerstone for further studies on this rare microbial community.

The genomic sequence of Exiguobacterium chiriqhucha str. N139 reveals a species that thrives in cold waters and extreme environmental conditions

We report the genome sequence of Exiguobacterium chiriqhucha str. N139, isolated from a high-altitude Andean lake. Comparative genomic analyses of the Exiguobacterium genomes available suggest that our strain belongs to the same species as the previously reported E. pavilionensis str. RW-2 and Exiguobacterium str. GIC 31. We describe this species and propose the chiriqhucha name to group them. 'Chiri qhucha' in Quechua means 'cold lake', which is a common origin of these three cosmopolitan Exiguobacteria. The 2,952,588-bp E. chiriqhucha str. N139 genome contains one chromosome and three megaplasmids. The genome analysis of the Andean strain suggests the presence of enzymes that confer E. chiriqhucha str. N139 the ability to grow under multiple environmental extreme conditions, including high concentrations of different metals, high ultraviolet B radiation, scavenging for phosphorous and coping with high salinity. Moreover, the regulation of its tryptophan biosynthesis suggests that novel pathways remain to be discovered, and that these pathways might be fundamental in the amino acid metabolism of the microbial community from Laguna Negra, Argentina.

High-Up: A Remote Reservoir of Microbial Extremophiles in Central Andean Wetlands

The Central Andes region displays unexplored ecosystems of shallow lakes and salt flats at mean altitudes of 3700 m. Being isolated and hostile, these so-called "High-Altitude Andean Lakes" (HAAL) are pristine and have been exposed to little human influence. HAAL proved to be a rich source of microbes showing interesting adaptations to life in extreme settings (poly-extremophiles) such as alkalinity, high concentrations of arsenic and dissolved salts, intense dryness, large daily ambient thermal amplitude, and extreme solar radiation levels. This work reviews HAAL microbiodiversity, taking into account different microbial niches, such as plankton, benthos, microbial mats and microbialites. The modern stromatolites and other microbialites discovered recently at HAAL are highlighted, as they provide unique modern-though quite imperfect-analogs of environments proxy for an earlier time in Earth's history (volcanic setting and profuse hydrothermal activity, low atmospheric O2 pressure, thin ozone layer and high UV exposure). Likewise, we stress the importance of HAAL microbes as model poly-extremophiles in the study of the molecular mechanisms underlying their resistance ability against UV and toxic or deleterious chemicals using genome mining and functional genomics. In future research directions, it will be necessary to exploit the full potential of HAAL poly-extremophiles in terms of their biotechnological applications. Current projects heading this way have yielded detailed molecular information and functional proof on novel extremoenzymes: i.e., DNA repair enzymes and arsenic efflux pumps for which medical and bioremediation applications, respectively, are envisaged. But still, much effort is required to unravel novel functions for this and other molecules that dwell in a unique biological treasure despite its being hidden high up, in the remote Andes.

Proteomic approach of adaptive response to arsenic stress in Exiguobacterium sp. S17, an extremophile strain isolated from a high-altitude Andean Lake stromatolite

The North-Western part of Argentina is particularly rich in wetlands located in the Puna in an altitude between 3,600 and 4,600 m above sea level. Most of these high-altitude Andean lakes are inhospitable areas due to extreme habitat conditions such as high contents of toxic elements, particularly arsenic. Exiguobacterium sp. S17, isolated from stromatolites in Laguna Socompa, exhibited remarkable tolerance to high arsenic concentration, i.e., it tolerated arsenic concentration such as 10 mM of As(III) and 150 mM of As(V). A proteomics approach was conducted to reveal the mechanisms that provide the observed outstanding resistance of Exiguobacterium sp. S17 against arsenic. A comparative analysis of S17, exposed and unexposed to arsenic revealed 25 differentially expressed proteins. Identification of these proteins was performed by MALDI-TOF/MS revealing upregulation of proteins involved in energy metabolism, stress, transport, and in protein synthesis being expressed under arsenic stress. To our knowledge, this work represents the first proteomic study of arsenic tolerance in an Exiguobacterium strain.

Extremophile culture collection from Andean lakes: extreme pristine environments that host a wide diversity of microorganisms with tolerance to UV radiation

A total of 88 bacterial strains were isolated from six Andean lakes situated at altitudes ranging from 3,400 to 4,600 m above sea level: L. Aparejos (4,200 m), L. Negra (4,400 m), L. Verde (4,460 m), L. Azul (4,400 m), L. Vilama (4,600 m), and Salina Grande (3,400 m). Salinity ranged from 0.4 to 117 ppm. General diversity was determined by denaturing gradient gel electrophoresis (DGGE) analysis. From the excised DGGE bands, 182 bacterial sequences of good quality were obtained. Gammaproteobacteria and Cytophaga/Flavobacterium/Bacteroides (CFB) were the most abundant phylogenetic groups with 42% and 18% of identified bands, respectively. The isolated strains were identified by sequence analysis. Isolated bacteria were subjected to five different UV-B exposure times: 0.5, 3, 6, 12, and 24 h. Afterwards, growth of each isolate was monitored and resistance was classified according to the growth pattern. A wide interspecific variation among the 88 isolates was observed. Medium and highly resistant strains accounted for 43.2% and 28.4% of the isolates, respectively, and only 28.4% was sensitive. Resistance to solar radiation was equally distributed among the isolates from the different lakes regardless of the salinity of the lakes and pigmentation of isolates. Of the highly resistant isolates, 44.5% belonged to gammaproteobacteria, 33.3% to betaproteobacteria, 40% to alphaproteobacteria, 50% to CFB, and among gram-positive organisms, 33.3% were HGC and 44.5% were Firmicutes. Most resistant strains belonged to genera like Exiguobaceterium sp., Acinetobacter sp., Bacillus sp., Micrococcus sp., Pseudomonas sp., Sphyngomonas sp., Staphylococcus sp., and Stenotrophomonas sp. The current study provides further evidence that gammaproteobacteria are the most abundant and the most UV-B-resistant phylogenetic group in Andean lakes and that UV resistance in bacteria isolated from these environments do not depend on pigmentation and tolerance to salinity.