With the aim to explore the possible role of mineral phosphate-solubilizing bacteria (PSB) in phosphorus (P) cycling in iron-rich, acidic soils, we conducted a survey of PSB naturally colonizing a limonitic crust in the south-east region of Venezuela (Bolı´var State). A total of 130 heterotrophic bacterial isolates showing different degrees of mineral tri-calcium phosphate (Ca 3 (PO 4 ) 2 )-solubilizing activities were isolated from NBRIP plates. In contrast, no isolates showing iron phosphate (FePO 4 )-or aluminum phosphate (AlPO 4 )-solubilizing activities were detected by this experimental approach. The 10 best Ca 3 (PO 4 ) 2 -solubilizers were selected for further characterization. These isolates were shown to belong to the genera Burkholderia, Serratia, Ralstonia and Pantoea by partial sequencing analysis of their respective 16S rRNA genes. All the PSB isolates were able to mediate almost complete solubilization of Ca 3 (PO 4 ) 2 in liquid cultures; in contrast, the PSB isolates were less effective when solubilizing FePO 4 . Two groups of PSB isolates were clearly differentiated on the basis of their Ca 3 (PO 4 ) 2 solubilization kinetics. Acidification of culture supernatants seemed to be the main mechanism for P solubilization. Indeed, gluconic acid was shown to be present in the supernatant of five isolates. Furthermore, detection of genes involved in the production of this organic acid was possible in three isolates by means of a PCR protocol. r

The mineral phosphate-solubilizing (MPS) activity of a Pantoea agglomerans strain, namely MMB051, isolated from an iron-rich, acidic soil near Ciudad Piar (Bolívar State, Venezuela), was characterized on a chemically defined medium (NBRIP). Various insoluble inorganic phosphates, including tri-calcium phosphate [Ca 3 (PO 4 ) 2 ], iron phosphate (FePO 4 ), aluminum phosphate (AlPO 4 ), and Rock Phosphate (RP) were tested as sole sources of P for bacterial growth. Solubilization of Ca 3 (PO 4 ) 2 was very efficient and depended on acidification of the external milieu when MMB051 cells were grown in the presence of glucose. This was also the case when RP was used as the sole P source. On the other hand, the solubilization efficiency toward more insoluble mineral phosphates (FePO 4 and AlPO 4 ) was shown to be very low. Even though gluconic acid (GA) was detected on culture supernatants of strain MMB051, a consequence of the direct oxidation pathway of glucose, inorganic-P solubilization seemed also to be related to other processes dependent on active cell growth. Among these, proton release by ammonium (NH 4 ? ) fixation appeared to be of paramount importance to explain inorganic-P solubilization mediated by strain MMB051. On the contrary, the presence of nitrate (NO 3 -) salts as the sole N source affected negatively the ability of MMB051 cells to solubilize inorganic P.

Glaciers harbor a wide diversity of microorganisms, metabolically versatile, highly tolerant to multiple environmental stresses and potentially useful for biotechnological purposes. Among these, we hypothesized the presence of bacteria able to exhibit well-known plant growth promoting traits (PGP). These kinds of bacteria have been employed for the development of commercial biofertilizers; unfortunately, these biotechnological products have proven ineffective in colder climates, like the ones prevailing in mountainous ecosystems. In the present work, we prospected glacial ice collected from two small tropical glaciers, located above 4.900 m in the Venezuelan Andes, for cold-active PGP bacteria. The initial screening strategy allowed us to detect the best inorganic-P solubilizers at low temperatures, from a sub-sample of 50 bacterial isolates. Solubilization of tricalcium phosphate, aluminum- and iron-phosphate, occurred in liquid cultures at low temperatures and was dependent on medium acidification by gluconic acid production, when bacteria were supplied with an appropriate source of carbon. Besides, the isolates were psychrophilic and in some cases exhibited a broad range of growth-temperatures, from 4 °C to 30 °C. Additional PGP abilities, including phytohormone- and HCN production, siderophore excretion and inhibition of phytopathogens, were confirmed in vitro. Nucleotidic sequence analysis of 16S rRNA genes allowed us to place the isolates within the Pseudomonas genus. Our results support the possible use of these strains to develop cold-active biofertilizers to be used in mountainous agriculture.

Abstract. Bacterial resistance to mercury (Hg) was investigated in strains isolated from Hg-contaminated tailing ponds located in the gold mining area of El Callao (Bolívar State, Venezuela). High frequencies of resistance were detected to both inorganic-Hg and organomercurials among these strains. A broad range of resistance levels was observed when determining minimal inhibitory concentrations of Hg 2+ . Some strains were able to grow in liquid medium containing 25 lM Hg 2+ , whereas others grew at 300 lM Hg 2+ . Of 190 Hg-resistant strains tested, 58.2% were additionally shown to be resistant to ampicillin (40 mg/L), 33.3% to chloramphenicol (30 mg/L), 24.9% to streptomycin (30 mg/L), 23.3% to tetracycline (30 mg/L), and 1.6% to kanamycin (30 mg/L). Furthermore, we found that 20% of the Hgresistant strains were simultaneously resistant to as many as four of these antibiotics, at the concentrations tested. The presence of large plasmids in 62.9% of 53 Hg-resistant strains screened prompted us to investigate the horizontal transfer of resistance determinants. Mating experiments were performed using Escherichia coli and Pseudomonas aeruginosa as recipient strains. The results obtained confirmed that indigenous Hg-resistant bacteria colonizing the tailing ponds can effectively transfer the phenotype to potentially pathogenic species.

Glacial-ice microorganisms are intensively studied world-wide for a number of reasons, including their psychrophilic lifestyle, their usefulness in biotechnology procedures and their relationship with the search of life outside our planet. However, because of the difficulties for accessing and working at altitudes of >5.000 m above sea level, tropical glaciers have received much less attention than their arctic and antarctic counterparts. In the present work we isolated and characterized a total of forty-five pure isolates originating from direct plating of melted ice collected at the base of a rapidly-retreating, small glacier located at around 4.900 m.a.s.l. in Mount Humboldt (Sierra Nevada National Park, Mérida State, Venezuela). Initial examination of melted ice showed the presence of abundant- (>10⁶ cells ml⁻¹), morphologically diverse- and active bacterial cells, many of which were very small ("dwarf cells"). The majority of the isolates were psychrophilic or psychrotolerant and many produced and excreted cold-active extracellular enzymes (proteases and amylases). The antibiotic tests showed an elevated percentage of isolates resistant to high doses (100 μg/ml) of different antibiotics including ampicillin, penicillin, nalidixic acid, streptomycin, chloramphenicol, kanamycin and tetracycline. Multiresistance was also observed, with 22.22 % of the strains simultaneously resistant up to five of the antibiotics tested. Metal resistance against Ni⁺⁺, Zn⁺⁺ and Cu⁺⁺ was also detected. In accordance with these results, plasmids of low and high molecular weight were detected in 47 % of the isolates. Twenty-two partial 16S rDNA sequences analyzed allowed grouping the isolates within five different phyla/classes: Alpha-, Beta- and Gamma-proteobacteria, Actinobacteria and Flavobacteria. This is the first report concerning South American Andean glacial ice microorganisms.

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Even though tropical glaciers are retreating rapidly and many will disappear in the next few years, their microbial diversity remains to be studied in depth. In this paper we report on the biodiversity of the culturable fraction of bacteria colonizing Pico Bolívar’s glacier ice and subglacial meltwaters, at ∼4950 m in the Venezuelan Andean Mountains. Microbial cells of diverse morphologies and exhibiting uncompromised membranes were present at densities ranging from 1.5 × 104 to 4.7 × 104 cells/mL in glacier ice and from 4.1 × 105 to 9.6 × 105 cells/mL in subglacial meltwater. Of 89 pure isolates recovered from the samples, the majority were eurypsychrophilic or stenopsychrophilic, according to their temperature range of growth. Following analysis of their 16S rDNA nucleotidic sequence, 54 pure isolates were assigned to 23 phylotypes distributed within 4 different phyla or classes: Beta- and Gammaproteobacteria, Actinobacteria, and Bacteroidetes. Actinobacteria dominated the culturable fraction of glacier ice samples, whereas Proteobacteria were dominant in subglacial meltwater samples. Chloramphenicol and ampicillin resistance was exhibited by 73.07% and 65.38%, respectively, of the subglacial isolates, and nearly 35% of them were multiresistant. Considering the fast rate at which tropical glaciers are melting, this study confirms the urgent need to study the microbial communities immured in such environments.

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