Although water-logged rice paddies are characterized by anoxic conditions, radial oxygen loss (ROL) from rice roots temporarily oxygenates the soil rhizosphere. ROL not only triggers the abiotic oxidation of ferrous iron (Fe(II)) but also provides the electron acceptor for microaerophilic Fe(II)-oxidizing bacteria (microFeOx). Both processes contribute to the formation of ferric (Fe(III)) iron plaque on root surfaces. Redox interactions at single roots have been studied intensively. However, temporally resolved spatial changes of ROL in the entire rhizosphere and the impact on redoximorphic biogeochemistry are currently poorly understood. Here, we show how ROL spatiotemporally evolves and correlates with Fe-redox transformations. Applying noninvasive measurements in a transparent artificial soil, we were able to visualize opposing O 2 and Fe(II) gradients that extend from the root surface 10−25 mm into the rhizosphere. The microoxic zone expanded exponentially in size throughout the entire rhizosphere creating niches for microFeOx. Following iron mineral formation and pH, we show that rootrelated ROL induces iron redox transformations on and around roots and correlates with rhizosphere acidification. These findings highlight the dynamic nature of roots in the rice plant rhizosphere, and our approach spatiotemporally resolved their impact on iron redox chemistry and microbial niche formation in the rice plant rhizosphere.

Neutrophilic microbial aerobic oxidation of ferrous iron (Fe­(II)) is restricted to pH-circumneutral environments characterized by low oxygen where microaerophilic Fe­(II)-oxidizing microorganisms successfully compete with abiotic Fe­(II) oxidation. However, accumulation of ferric (bio)­minerals increases competition by stimulating abiotic surface-catalyzed heterogeneous Fe­(II) oxidation. Here, we present an experimental approach that allows quantification of microbial and abiotic contribution to Fe­(II) oxidation in the presence or initial absence of ferric (bio)­minerals. We found that at 20 μM O2 and the initial absence of Fe­(III) minerals, an iron­(II)-oxidizing enrichment culture (99.6% similarity to Sideroxydans spp.) contributed 40% to the overall Fe­(II) oxidation within approximately 26 h and oxidized up to 3.6 × 10–15 mol Fe­(II) cell–1 h–1. Optimum O2 concentrations at which enzymatic Fe­(II) oxidation can compete with abiotic Fe­(II) oxidation ranged from 5 to 20 μM. Lower O2 levels limited biotic Fe­(II) oxidation, while at higher O2 levels abiotic Fe­(II) oxidation dominated. The presence of ferric (bio)­minerals induced surface-catalytic heterogeneous abiotic Fe­(II) oxidation and reduced the microbial contribution to Fe­(II) oxidation from 40% to 10% at 10 μM O2. The obtained results will help to better assess the impact of microaerophilic Fe­(II) oxidation on the biogeochemical iron cycle in a variety of environmental natural and anthropogenic settings.

 The tectonic migration of ocean floor led to a transition from an organic carbon-starved to an elevated organic carbon burial environment  Diagenetic transformation of iron oxides into pyrite within the carbon-lean sediments occurred several millions of years after deposition Accepted ArticleThis article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as

The Arctic has the highest warming rates on Earth. Glaciated fjord ecosystems, which are hotspots of carbon cycling and burial, are extremely sensitive to this warming. Glaciers are important for the transport of iron from land to sea and supply this essential nutrient to phytoplankton in high-latitude marine ecosystems. However, up to 95% of the glacially-sourced iron settles to sediments close to the glacial source. Our data show that while 0.6–12% of the total glacially-sourced iron is potentially bioavailable, biogeochemical cycling in Arctic fjord sediments converts the glacially-derived iron into more labile phases, generating up to a 9-fold increase in the amount of potentially bioavailable iron. Arctic fjord sediments are thus an important source of potentially bioavailable iron. However, our data suggests that as glaciers retreat onto land the flux of iron to the sediment-water interface may be reduced. Glacial retreat therefore likely impacts iron cycling in coastal marine ecosystems.

To distinguish between biotic and abiotic processes in laboratory experiments with environmental samples, an effective sterilization method is required that prevents biological activity but does not change physico-geochemical properties of samples. We compared standard sterilization methods with respect to their impact on microbial abundance and activity. We exposed marine sediment to (i) autoclaving, (ii) gamma-radiation or (iii) sodium azide (NaN3) and determined how nucleic acids, microbial productivity, colony forming units (CFUs) and community composition of microorganisms, fungi, unicellular protists and protozoa were affected. In autoclaved and gamma-sterilized sediments, only few colonies formed within 16 days. After addition of NaN3 to the sediment, numerous CFUs (>50) but lower 3H-leucine incorporation rates, i.e. lower protein biosynthesis rates, were found compared to the other two sterilization techniques. Extractable RNA was detected immediately after all sterilization treatments (0.2-17.9 ng/g dry sediment) but decreased substantially by 84%-98% after 16 days of incubation. The total organic carbon content increased from 18 mg L-1 to 220 mg L-1 (autoclaving) and 150 mg L-1 (gamma-radiation) after sterilization. We compare advantages and disadvantages for each tested sterilization method and provide a helpful decision-making resource for choosing the appropriate sterilization technique for environmental studies, particularly for marine sediments.