『Abstract
Iron(II) oxidation in natural waters at circumneutral pH, often
regarded as an abiotic process, is frequently biologically mediated
at iron-rich redox gradients. West Berry Creek, a small circumneutral
tributary that flows through a mixed coniferous forest in Big
Basin State Park, California, contains localized iron (hydr)oxide
precipitates at points along its course where anoxic groundwater
meets oxygenerated creek water. These mixing zones establish redox
gradients that may be exploited by microbes forming microbial
mats that are intimately associated with iron (hydr)oxide precipitates.
Water sampling revealed strong correlations between the concentrations
of aqueous inorganic species, suggesting a rock-weathering source
for most of these solutes. Liquid chromatography-electrospray
ionization-mass spectrometry techniques detected significant concentrations
of organic exudates, including low molecular mass organic acids
and siderophores, indicating that active biogeochemical cycling
of irons is occurring in the creek. X-ray diffraction and elemental
analysis showed the precipitates to be amorphous, or possibly
poorly crystalline, iron-rich minerals. Clone libraries developed
from 16S rDNA sequences extracted from microbial mat communities
associated with the precipitates revealed the presence of microorganisms
related to the neutrophilic iron oxidizing bacteria Gallionella
and Sideroxydans. Sequences from these libraries also indicated
the presence of significant populations of organisms related to
bacteria in the genera Aquaspirillum, Pseudomonas,
Sphingomonas, and Nitrospira. These geosymbiotic
systems appears to be significant not only for the biogeochemical
cycling of iron in the creek, but also for the cycling of organic
species, inorganic nutrients, and trace metals.
Keywords: Iron oxide; Biomineral; Siderophores; Organic acids;
Neutrophilic iron oxidizing bacteria; Circumneutral iron oxidation』
1. Introduction
2. Materials and methods
2.1. Site description and sampling procedure
2.2. Inorganic aqueous chemistry
2.3. Organic aqueous chemistry
2.4. Geochemical modeling
2.5. Characterization of sediments
2.6. Optical microscopy
2.7. Electron microscopy
2.8. Microbial community analysis
3. Results and discussion
3.1. Aqueous chemistry
3.2. Organic species
3.3. Microscopy of sediments and microbial mats
3.4. Microbial ecology of creek microbial mats
3.5. Microbial populations, iron oxidation, and siderophore production
in sample 3BR
3.6. Microbial populations, iron oxidation, and siderophore production
in sample 3BH
4. Conclusions
Acknowledgements
Appendix A. Supplementary data
References