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Larsen M.,University of Southern Denmark | Larsen M.,Scottish Association for Marine Science | Larsen M.,Greenland Climate Research Center Greenland Institute of National resources | Santner J.,University of Natural Resources and Life Sciences, Vienna | And 6 more authors.
Plant and Soil | Year: 2015

Results: O2 leakage was heterogeneously distributed with zones of intense leakage around roots tips and young developing roots. While the majority of roots exhibited high ROL others remained surrounded by anoxic soil. ROL was affected by ambient O2 levels around the plant, as well as irradiance, indicating a direct influence of photosynthetic activity on ROL. At onset of darkness, oxia in the rhizosphere was drastically reduced, but subsequently oxia gradually increased, presumably as root and/or soil respiration declined.Background and aims: Radial O2 loss (ROL) strongly affect the O2 availability in the rhizosphere of rice. The ROL create an oxic zone around the roots, protecting the plant from toxic reduced chemical species and regulates the redox chemistry in the soil. This study investigates the spatio-temporal variability in O2 dynamics in the rice rhizosphere.Method: Applying high-resolution planar optode imaging, we investigated the O2 dynamics of plants grown in water saturated soil, as a function of ambient O2 level, irradiance and plant development, for submerged and emerged plants.Conclusion: The study demonstrates a high spatio-temporal heterogeneity in rhizosphere O2 dynamics and difference in ROL between different parts of the rhizosphere. The work documents that spatio-temporal measurements are important to fully understand and account for the highly variable O2 dynamics and associated biogeochemical processes and pathways in the rice rhizosphere. © 2015, The Author(s). Source

Jovanovic Z.,University of Southern Denmark | Pedersen M.O.,University of Southern Denmark | Larsen M.,University of Southern Denmark | Larsen M.,Scottish Association for Marine Science | And 6 more authors.
Marine Ecology Progress Series | Year: 2015

Zostera marina and Ruppia maritima often share the same habitat, but R. maritima appears more resistant to environmental stress. We investigated the impact of light intensity and water column O2 concentrations on radial oxygen loss (ROL), in young specimens of Z. marina and R. maritima. Planar optode imaging revealed that ROL of Z. marina was localized to the root tip, while R. maritima showed ROL along extensive root sections. The total root biomass of the 2 species was similar, but, while R. maritima had only 1 root, of which 33% of its length showed ROL, Z. marina had 2 to 5 individual roots, where only 2 to 3 exhibited O2 leakage, but then only at root tips. ROL resulted in an oxic volume of 4.26 ± 0.51 mm3 plant-1 for Z. marina and 5.39 ± 0.47 mm3 plant-1 for R. maritima (n = 3). ROL per plant at light saturation was 2.32 ± 0.30 and 2.89 ± 0.38 nmol h-1 for Z. marina and R. maritima, respectively. These values declined by 71 and 60% in darkness. However, both species were able to maintain ROL as long as ambient O2 levels remained >50% air saturation. The calculated ROL integrated over a 24 h cycle was 48.8 ± 10.6 nmol O2 plant-1 d-1 (n = 3) for R. maritima and 30% less for Z. marina. The ability of R. maritima to maintain higher ROL than Z. marina could be an important feature defining its potential for colonizing and maintaining growth in eutrophic sediments. © Inter-Research 2015. Source

Jovanovic Z.,University of Southern Denmark | Larsen M.,University of Southern Denmark | Larsen M.,Scottish Association for Marine Science | Larsen M.,Greenland Climate Research Center Greenland Institute of National resources | And 6 more authors.
Marine Ecology Progress Series | Year: 2014

The polychaete Marenzelleria viridis is an invasive species and often replaces the native Nereis diversicolor. This shift leads to more reduced conditions and changes in the biogeochemical function of the sediments. By combining imaging techniques for O2 (planar optodes) and irrigation patterns (rhodamine WT and brilliant blue), we investigated the relationship between irrigation and O2 dynamics in burrows of M. viridis. The investigated animals shifted between 2 modes of ventilation: ciliary pumping for 77% of the time and muscular pumping for 23% of the time. On average, muscular pumping was induced every 0.4 h. During ciliary pumping, oxic water was pumped into blind-ended burrows and into the surrounding sediment, inducing an upward porewater transport of O2-depleted water. This pattern was reversed during muscular pumping. The 2 pumping modes induced oscillating O2 penetration along the burrow wall and along the primary sediment-water interface. The average net downward irrigation rate, including both pumping modes, amounted to 11.1 × 10-3 ± 2.4 × 10 -3 ml min-1. The estimated average oxic sediment volume was 2.1 ± 0.5 cm3 per burrow, and the burrow-specific O 2 consumption was 45.6 ± 18.1 nmol min-1. M. viridis burrows and the ambient sediment are relatively O2 depleted, with intensified rates of O2 consumption, compared to similar-sized native N. diversicolor. The complex O2 dynamics induced a unique microenvironment that must favor meiofauna and microbial communities that are tolerant to oxic-anoxic oscillations or that have the ability to migrate along with the pulsing oxic-anoxic interface. © Inter-Research 2014. Source

Larsen M.,Scottish Association for Marine Science | Larsen M.,Greenland Climate Research Center Greenland Institute of National resources | Larsen M.,University of Southern Denmark | Borisov S.M.,University of Graz | And 4 more authors.
Limnology and Oceanography: Methods | Year: 2011

A simple, high resolution colormetric planar optode imaging approach is presented. The approach is simple and inexpensive yet versatile, and can be used to study the two-dimensional distribution and dynamics of a range of analytes. The imaging approach utilizes the inbuilt color filter of standard commercial digital single lens reflex cameras to simultaneously record different colors (red, green, and blue) of luminophore emission light using only one excitation light source. Using the ratio between the intensity of the different colors recorded in a single image analyte concentrations can be calculated. The robustness of the approach is documented by obtaining high resolution data of O 2 and pH distributions in marine sediments using easy synthesizable sensors. The sensors rely on the platinum(II)octaethylporphyrin (PtOEP) and lipophilic 8-Hydroxy-1,3,6-pyrenetrisulfonic acid trisodium (HPTS) salt derivate for O 2 and pH measurements, respectively. The brightness of both indicators is dramatically enhanced by making use of energy transfer from a donor molecule (Macrolex yellow coumarin). Furthermore, the emission from the donor serves as an internal reference for the O 2 sensor. The approach relies on semitransparent sensors, facilitating visual inspection of the sediment behind the sensors during measurements. Software for data acquisition and calibration will be available from the authors, whereas all hardware is available from a range of commercial sources. The total cost of the complete measuring system is approximately $3000 US. © 2011, by the American Society of Limnology and Oceanography, Inc. Source

Larsen M.,Scottish Association for Marine Science | Larsen M.,Greenland Climate Research Center Greenland Institute of National resources | Larsen M.,University of Southern Denmark | Thamdrup B.,University of Southern Denmark | And 4 more authors.
Progress in Oceanography | Year: 2013

Benthic carbon mineralization and solute exchange was investigated on a Celtic Sea sandbank during July 2008. The sediment on the top of the bank consisted of consolidated sand, characterized by advective porewater transport and was difficult to sample, this to some extent compromised the investigations at this site. However, intact sediment cores were sampled at 4 stations at the slopes, base of the bank and off the bank (reference site). This sediment was used to assess rates and pathways for benthic diagenesis. Total sediment O2 uptake (TOU) ranged from 5.8 to 9.0mmolm-2d-1 and total sediment release rates of dissolved inorganic carbon (DIC) ranged between 8.60 and 13.8mmolm-2d-1. Microbial denitrification and sulfate reduction accounted for <2% and 12-28% of the total benthic carbon mineralization, respectively. The remaining mineralization is ascribed to O2 and Fe/Mn respiration, respectively. Activity profiles of unsupported 210Pb from all stations indicated deep mixing, presumably caused by intense trawling activity in the area. Calculation based on satellite tracking of fishing vessels suggest that on average 33% of the sediment is affected by trawling activity every year. This is presumed to facilitate high metal respiration rates by continuously oxidizing reduced Fe that would otherwise accumulate in the sediment. Extracted porewater profiles reflected elevated NO3- levels as compared to microsensor determined NO3- profiles that were measured in parallel. We suggest that this reflects NO3- leakage from meiofauna - and that the intracellular NO3- pool in the top 5cm of the sediment exceeds the porewater pool up to a factor of 45. However, this intracellular pool is presumably turned over at very slow rates, compared to the porewater pool subject to microbial denitrification. © 2013 Elsevier Ltd. Source

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