Light-enhanced bioaccumulation of molybdenum by nitrogen-deprived recombinant anoxygenic photosynthetic bacterium Rhodopseudomonas palustris

Trace metal interaction with thermophilic phototrophic anaerobic bacterium Chloroflexus aurantiacus

Towards improving the knowledge of possible paleo-microorganisms interaction with trace metals (micro-nutrients and toxicants), we studied adsorption of Mn, Zn, Sr, Cd, and Pb onto modern Chloroflexus aurantiacus, thermophilic anoxygenic phototrophic bacterium which could be highly abundant in the Precambiran aquatic environments. Acid-base surface titrations allowed quantifying the number of proton-active surface groups, whereas non-electrostatic linear programming method (LPM) was used to assess the surface site concentrations and adsorption reaction constants between divalent cations (Zn, Mb, Sr, Cd, Pb) and bacterial surface, based on results of pH-dependent adsorption edge and constant-pH 'langmuirian' adsorption experiments. The total proton/hydroxyl binding site number of Chl. aurantiacus surfaces was sizably lower than that of other phototrophic anaerobic bacteria studied previously using similar experimental and modeling approach. Divalent metals exhibited a decreasing order of adsorption affinity (Pb > Cd ≥ Zn ≥ Mn > Sr), which reflected the order of cation hydrolysis and was similar to adsorption order on other phototrophic bacteria. At the same time, adsorption of Zn increased with increasing of temperature, from 4 °C to 60 °C and was stronger under light compared to the darkness. This suggested some active metabolic control involved in this metal interaction with bacterial surfaces. Overall, Chl. aurantiacus exhibited trace metal adsorption parameters (site number and binding constants) which were lower compared to other anoxygenic phototrophic bacteria (Rhodopseudomonas palustris; Rhodobacter blasticus) and cyanobacteria. This may reflect different bioavailability of trace metals in the paleo-ocean, given that thermophilic Chl. aurantiacus are among the oldest phototrophs on the planet.

Influence of sulfide on diazotrophic growth of the methanogen Methanococcus maripaludis and its implications for the origin of nitrogenase

Methanogens inhabit euxinic (sulfide-rich) or ferruginous (iron-rich) environments that promote the precipitation of transition metals as metal sulfides, such as pyrite, reducing metal or sulfur availability. Such environments have been common throughout Earth's history raising the question as to how anaerobes obtain(ed) these elements for the synthesis of enzyme cofactors. Here, we show a methanogen can synthesize molybdenum nitrogenase metallocofactors from pyrite as the source of iron and sulfur, enabling nitrogen fixation. Pyrite-grown, nitrogen-fixing cells grow faster and require 25-fold less molybdenum than cells grown under euxinic conditions. Growth yields are 3 to 8 times higher in cultures grown under ferruginous relative to euxinic conditions. Physiological, transcriptomic, and geochemical data indicate these observations are due to sulfide-promoted metal limitation, in particular molybdenum. These findings suggest that molybdenum nitrogenase may have originated in a ferruginous environment that titrated sulfide to form pyrite, facilitating the availability of sufficient iron, sulfur, and molybdenum for cofactor biosynthesis.

Nutrient limitation regulates the properties of extracellular electron transfer and hydraulic shear resistance of electroactive biofilm

Understanding the roles of nutrient restriction in extracellular electron transfer (EET) and stability of mixed electroactive biofilm is essential in pollutant degradation and bioenergy production. However, the relevant studies are still limited so far. Herein, the effect of nutrient restriction on the EET pathways and stability of mixed electroactive biofilm was explored. It was found that the electroactive Pseudomonas and Geobacter genera were selectively enriched in the biofilms cultured under total nutrient and P-constrained conditions, and two EET pathways including direct and indirect were found, while Rhodopseudomonas genus was enriched in the N-constrained biofilm, which only had the direct EET pathway. Moreover, multiple analyses including 2D confocal Raman spectra revealed that P-constrained biofilm was rich in extracellular polymeric substances (EPS) especially for polysaccharide, presented a dense and uniform layered distribution, and had better stability than N-constrained biofilm with lower EPS and biofilm with heterostructures cultured under total nutrient conditions.

Potential of Phototrophic Purple Nonsulfur Bacteria to Fix Nitrogen in Rice Fields

Biological nitrogen fixation catalyzed by Mo-nitrogenase of symbiotic diazotrophs has attracted interest because its potential to supply plant-available nitrogen offers an alternative way of using chemical fertilizers for sustainable agriculture. Phototrophic purple nonsulfur bacteria (PNSB) diazotrophically grow under light anaerobic conditions and can be isolated from photic and microaerobic zones of rice fields. Therefore, PNSB as asymbiotic diazotrophs contribute to nitrogen fixation in rice fields. An attempt to measure nitrogen in the oxidized surface layer of paddy soil estimates that approximately 6–8 kg N/ha/year might be accumulated by phototrophic microorganisms. Species of PNSB possess one of or both alternative nitrogenases, V-nitrogenase and Fe-nitrogenase, which are found in asymbiotic diazotrophs, in addition to Mo-nitrogenase. The regulatory networks control nitrogenase activity in response to ammonium, molecular oxygen, and light irradiation. Laboratory and field studies have revealed effectiveness of PNSB inoculation to rice cultures on increases of nitrogen gain, plant growth, and/or grain yield. In this review, properties of the nitrogenase isozymes and regulation of nitrogenase activities in PNSB are described, and research challenges and potential of PNSB inoculation to rice cultures are discussed from a viewpoint of their applications as nitrogen biofertilizer.

Nitrogen fixation in Rhodopseudomonas palustris co-cultured with Bacillus subtilis in the presence of air

Nitrogen fixation in purple non-sulfur bacteria (PNSB) does not take place even in N-free medium when they are cultured under aerobic conditions. It is assumed that PNSB might possess inadequate capability to protect their cellular components from exposure to air (20.95 vol.% oxygen). In this study, therefore, Bacillus subtilis was inoculated together with a purple non-sulfur bacterium Rhodopseudomonas palustris in N-free medium in order to examine whether nitrogen fixation in Rps. palustris takes place when the co-culture is exposed to 20.95 vol.% oxygen. Rps. palustris grew and formed biofilm only when it was inoculated together with B. subtilis. When the biofilm formed in the co-culture was inoculated in N-free medium, diazotrophic growth was observed in the sequential subcultures. Expression of nifH gene, derepression of nitrogenase activity, an increase of total nitrogen, and a decrease of C/N in the co-culture of Rps. palustris and B. subtilis demonstrated the occurrence of nitrogen fixation under aerobic conditions. The diazotrophic growth was suppressed at a lower medium-to-air ratio in a sealed culture vessel, and growth of B. subtilis preceded growth of Rps. palustris in the co-culture. These results suggest that growth of B. subtilis, which is usually accompanied with oxygen consumption, might cause a decrease of dissolve oxygen concentration in medium and contribute to the occurrence of nitrogenase activity in Rps. palustris.