Fuschna Spring in the Swiss Alps (Engadin region) is a bicarbonate iron(II)-rich, pH-neutral mineral water spring that is dominated visually by dark green microbial mats at the side of the flow channel and orange iron(III) (oxyhydr)oxides in the flow channel. Gradients of O2, dissolved iron(II), and bicarbonate establish in the water. Our goals were to identify the dominating biogeochemical processes and to determine to which extent changing geochemical conditions along the flow path and seasonal changes influence mineral identity, crystallinity, and microbial diversity. Geochemical analysis showed microoxic water at the spring outlet which became fully oxygenated within 2.3mdownstream. X-ray diffraction and Mössbauer spectroscopy revealed calcite (CaCO3) and ferrihydrite [Fe(OH)3] to be the dominant minerals which increased in crystallinity with increasing distance from the spring outlet. Denaturing gradient gel electrophoresis banding pattern cluster analysis revealed that the microbial community composition shifted mainly with seasons and to a lesser extent along the flow path. 16S rRNA gene sequence analysis showed that microbial communities differ between the flow channel and the flanking microbial mat. Microbial community analysis in combination with most-probable-number analyses and quantitative PCR (qPCR) showed that the mat was dominated by cyanobacteria and the channel was dominated by microaerophilic Fe(II) oxidizers (1.97×107±4.36×106 16S rRNA gene copies g-1 using Gallionella-specific qPCR primers), while high numbers of Fe(III) reducers (109 cells/g) were identified in both the mat and the flow channel. Phototrophic and nitrate-reducing Fe(II) oxidizers were present as well, although in lower numbers (103 to 104 cells/g). In summary, our data suggest that mainly seasonal changes caused microbial community shifts, while geochemical gradients along the flow path influenced mineral crystallinity.
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