A high-molecular-weight c -type cytochrome, Cyc2, and a putative 22-kDa c -type cytochrome were detected in the membrane fraction released during spheroplast formation from Acidithiobacillus ferrooxidans . This fraction was enriched in outer membrane components and devoid of cytoplasmic membrane markers. The genetics, as well as the subcellular localization of Cyc2 at the outer membrane level, therefore make it a prime candidate for the initial electron acceptor in the respiratory pathway between ferrous iron and oxygen.

The regulation of the expression of the rus operon, proposed to encode an electron transfer chain from the outer to the inner membrane in the obligate acidophilic chemolithoautroph Acidithiobacillus ferrooxidans, has been studied at the RNA and protein levels. As observed by Northern hybridization, real-time PCR and reverse transcription analyses, this operon was more highly expressed in ferrous iron- than in sulfur-grown cells. Furthermore, it was shown by immunodetection that components of this respiratory chain are synthesized in ferrous iron- rather than in sulfur-growth conditions. Nonetheless, weak transcription and translation products of the rus operon were detected in sulfur-grown cells at the early exponential phase. The results strongly support the notion that rus-operon expression is induced by ferrous iron, in agreement with the involvement of the rus-operon-encoded products in the oxidation of ferrous iron, and that ferrous iron is used in preference to sulfur.

The development of cold-active biofertilizers and biopesticides could help improve sustainable agriculture in mountainous regions. With this aim, both psychrophilic and psychrotolerant microorganisms have been prospected in cold regions around the world and tested for their plant-growth promoting (PGP) effects. Interestingly, very little is known about the PGP effects of polar microorganisms in commercial crops. This study aimed at isolating cold-active plant-growth promoting Pseudomonas spp. from Antarctic soils and testing their PGP effects, both in vitro and on wheat (Triticum aestivum). Twenty-five Pseudomonas spp. strains isolated from Antarctic soils at Greenwich Island (South Shetland Islands, Antarctic Peninsula) were tested. The isolates grew well at temperatures ranging from 4 to 30 °C and were therefore considered as eury-psychrophiles. The isolates solubilized tri-calcium phosphate at 8 and 16 °C in the presence of different sugars as sole carbon sources. Besides producing indole-acetic acid, siderophores and hydrogen cyanide, several isolates inhibited growth of three plant pathogenic fungi (Fusarium oxysporum, Pythium ultimum and Phytophtora infestans) by means of both soluble- and volatile-secondary metabolites. Bacterization of T. aestivum seeds with selected isolates significantly enhanced root elongation. Moreover, when grown in sterile soil and in a temperature-controlled growth chamber at 14 ± 1 °C, inoculated T. aestivum seedlings showed a significant increase in their root- and shoot-lengths compared to untreated controls. Overall, the results suggest that some of these Antarctic Pseudomonas spp. isolates could act as cold-active biofertilizers.

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 [Ca₃(PO₄)₂], iron phosphate (FePO₄), aluminum phosphate (AlPO₄), and Rock Phosphate (RP) were tested as sole sources of P for bacterial growth. Solubilization of Ca₃(PO₄)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 (FePO₄ and AlPO₄) 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.