Microbial processes associated with roots of bulbous rush coated with iron plaques.

Bulbous rush (Juncus bulbosus) is a pioneer species in acidic, iron-rich, coal mining lakes in the eastern part of Germany. Juncus roots are coated with iron plaques, and it has been suggested that microbial processes under the iron plaques might be supportive for Juncus plant growth. The objectives of this work were to enumerate the microbes involved in the turnover of iron and organic root exudates in the rhizoplane, to investigate the effect of oxygen and pH on the utilization of these exudates by the rhizobacteria, and to study the ability of the root-colonizing microbiota to reduce sulfate. Enumeration studies done at pH 3 demonstrated that 10(6) Fe(III) reducers and 10(7) Fe(II) oxidizers g (fresh wt root)(-1) were associated with Juncus roots. When roots were incubated in goethite-containing medium without and with supplemental glucose, Fe(II) was formed at rates approximating 1.1 mmol g (fresh wt root) (-1) d(-1) and 3.6 mmol g (fresh wt root)(-1) d(-1) under anoxic conditions, respectively. These results suggest that a rapid microbially mediated cycling of iron occurs in the rhizosphere of Juncus roots under changing redox conditions. Most-probable-number estimates of aerobes and anaerobes capable of consuming root exudates at pH 3 were similar in the rhizosphere sediment and in Juncus roots, but numbers of aerobes were significantly higher than those of anaerobes. At pH 3, supplemental organic exudates were primarily subject to aerobic oxidation to CO2 and not subject to fermentation. However, at pH 4.5, root exudates were also rapidly utilized under anoxic conditions. Root-associated sulfate reduction was not observed at pH 3 to 4.5 but was observed at pH 4.9. The pH increased during all root-incubation studies both under oxic and anoxic conditions. Thus, as result of the microbial turnover of organic root exudates, pH and CO2 levels might be elevated at the root surface and favor Juncus plants to colonize acidic habitats.

Physiological ecology of Clostridium glycolicum RD-1, an aerotolerant acetogen isolated from sea grass roots.

An anaerobic, H(2)-utilizing bacterium, strain RD-1, was isolated from the highest growth-positive dilution series of a root homogenate prepared from the sea grass Halodule wrightii. Cells of RD-1 were gram-positive, spore-forming, motile rods that were linked by connecting filaments. Acetate was produced in stoichiometries indicative of an acetyl coenzyme A (acetyl-CoA) pathway-dependent metabolism when RD-1 utilized H(2)-CO(2), formate, lactate, or pyruvate. Growth on sugars or ethylene glycol yielded acetate and ethanol as end products. RD-1 grew at the expense of glucose in the presence of low initial concentrations (up to 6% [vol/vol]) of O(2) in the headspace of static, horizontally incubated culture tubes; the concentration of O(2) decreased during growth in such cultures. Peroxidase, NADH oxidase, and superoxide dismutase activities were detected in the cytoplasmic fraction of cells grown in the presence of O(2). In comparison to cultures incubated under strictly anoxic conditions, acetate production decreased, higher amounts of ethanol were produced, and lactate and H(2) became significant end products when RD-1 was grown on glucose in the presence of O(2). Similarly, when RD-1 was grown on fructose in the presence of elevated salt concentrations, lower amounts of acetate and higher amounts of ethanol and H(2) were produced. When the concentration of O(2) in the headspace exceeded 1% (vol/vol), supplemental H(2) was not utilized. The 16S rRNA gene of RD-1 had a 99.7% sequence similarity to that of Clostridium glycolicum DSM 1288(T), an organism characterized as a fermentative anaerobe. Comparative experiments with C. glycolicum DSM 1288(T) demonstrated that it had negligible H(2)- and formate-utilizing capacities. However, carbon monoxide dehydrogenase was detected in both RD-1 and C. glycolicum DSM 1288(T). A 91.4% DNA-DNA hybridization between the genomic DNA of RD-1 and that of C. glycolicum DSM 1288(T) confirmed that RD-1 was a strain of C. glycolicum. These results indicate that (i) RD-1 metabolizes certain substrates via the acetyl-CoA pathway, (ii) RD-1 can tolerate and consume limited amounts of O(2), (iii) oxic conditions favor the production of ethanol, lactate, and H(2) by RD-1, and (iv) the ability of RD-1 to cope with limited amounts of O(2) might contribute to its survival in a habitat subject to daily gradients of photosynthesis-derived O(2).