At each time point, profiles were made of 1–3 randomly chosen core(s). Microprofiles were measured, with the aeration turned off, by moving in-house made O2, pH and EP microsensors (Revsbech and Jørgensen, 1986 ; Revsbech, 1989 (link); Damgaard et al., 2014 (link)) stepwise (100–500 μm) downwards from ~3,000 μm above the sediment–water interface (SWI) to a depth of ~35,000 μm using a motorized micromanipulator (Unisense A/S, Denmark). At each step there was a 2 s waiting time before measuring a value for 2 s. All profiles were acquired using a four-channel multimeter with built-in A/D converter (Unisense). For the EP and pH sensors an in-house made millivoltmeter (resistance >1014 Ω,), and for the O2 sensor a picoamperemeter (Unisense) that was digitized with a 16-bit A/D converter (ADC-216, Unisense), was used. The software SensorTrace PRO (Unisense) was used to control the micromanipulator and acquire measurements.
EP was used as a proxy for cable bacteria activity (Damgaard et al., 2014 (link)) and was measured downwards combined with an upwards profile. EP and pH profiles were measured against a general-purpose reference electrode (REF201 Radiometer Analytical, Denmark).
O2 profiles were calibrated using Na-ascorbate in alkaline anoxic tap water and air-saturated (15°C) overlying water. The pH sensor was calibrated by a 3-point calibration (pH 4.0, 7.0, 10.0) with AVS TITRINORM buffers (VWR Chemicals, Denmark), and pH profiles were always measured in the dark. The reach of the anoxic zones was derived by determining the oxygen penetration depth from the O2 measurements after the profiles were adjusted to the SWI. Two EP profiles (downwards and upwards) were made for each core, and were drift corrected, using the measurements from the overlying water and the time between the up and down profiles. Current densities were calculated per core and averaged per time point (1–3 profile pairs/cores), as showcased in Risgaard-Petersen et al. (2015) (link), using a sediment conductivity of 0.04 S m−1 (water conductivity of 0.05 S m−1 and sediment porosity of 0.87).
The cathodic oxygen consumption (COC) of cable bacteria, which is the contribution of the cable bacterial cathodic cells to the total oxygen consumption performed in the sediment, and the (electric) current density (mA m−2) in the sediment generated by the cable bacteria, used for this purpose were calculated as described in Risgaard-Petersen et al. (2015) (link) from the EP and O2 microsensor measurements of the exact cores that were subsequently sampled for flocking observations.
EP was used as a proxy for cable bacteria activity (Damgaard et al., 2014 (link)) and was measured downwards combined with an upwards profile. EP and pH profiles were measured against a general-purpose reference electrode (REF201 Radiometer Analytical, Denmark).
O2 profiles were calibrated using Na-ascorbate in alkaline anoxic tap water and air-saturated (15°C) overlying water. The pH sensor was calibrated by a 3-point calibration (pH 4.0, 7.0, 10.0) with AVS TITRINORM buffers (VWR Chemicals, Denmark), and pH profiles were always measured in the dark. The reach of the anoxic zones was derived by determining the oxygen penetration depth from the O2 measurements after the profiles were adjusted to the SWI. Two EP profiles (downwards and upwards) were made for each core, and were drift corrected, using the measurements from the overlying water and the time between the up and down profiles. Current densities were calculated per core and averaged per time point (1–3 profile pairs/cores), as showcased in Risgaard-Petersen et al. (2015) (link), using a sediment conductivity of 0.04 S m−1 (water conductivity of 0.05 S m−1 and sediment porosity of 0.87).
The cathodic oxygen consumption (COC) of cable bacteria, which is the contribution of the cable bacterial cathodic cells to the total oxygen consumption performed in the sediment, and the (electric) current density (mA m−2) in the sediment generated by the cable bacteria, used for this purpose were calculated as described in Risgaard-Petersen et al. (2015) (link) from the EP and O2 microsensor measurements of the exact cores that were subsequently sampled for flocking observations.
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