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 antioxidant effect of several polyphenolic compounds is well known. However, little is known about the antioxidant capacity of Venezuelan honey, which has a high content of polyphenolic compounds. In this work, the antioxidant capacity of a genuine honey produced in Mérida, Venezuela was studied using the ferrous iron oxidation with xylenol orange method, the thiobarbituric acid method, and the determination of antioxidant activity. We found that this honey has the capacity to decrease significantly the concentration of lipid hydroperoxides and malondialdehyde, produced during the lipid peroxidation process, in a comparable way with other widely studied antioxidants such as melatonin and vitamin E. It was found that the antioxidant activity in the 50% honey dilution, the highest concentration we tested, was equivalent to a concentration of uric acid of 0.62 mM.

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.

No abstract