Aerobic reoxidation in marine sediments This exercise consists of three small experiments\demonstrations that should be run in parallel. Read the whole description and divide the work between the members of your team. The practical work and the subsequent report writing require some coordination thus it is important that everybody knows what the team mates are doing (and why they are doing it). In this exercise you get your data almost instantaneously thus you can already start data treatment during the exercise (see theoretical work below). A full report contains answers to all 16 points listed below and should be delivered no later than the 29 Th of Marts. Most of the 2 Th of Marts is dedicated report-writing at MBL. Microprofile experiment Background In coastal environments, only a minor part of the benthic oxygen consumption is related to direct aerobic respiration. The major part of the oxygen consumption is caused by reoxidation of products from the anaerobic heterotrophic activity in the deeper sediment layers. These products include NH + 4, Fe 2+, Mn 2+ and H 2 S. The latter is the result of sulfate reduction and in sediments with low iron- and manganese- oxide concentrations the H 2 S can diffuse up to the oxic sediment zone. H 2 S is a very potent poison that inactivates many metal carrying enzymes and benthic H 2 S release often leads to fauna death in coastal lagoons and isolated sedimentation basins. H 2 S-oxidation is an exergonic process and many chemoautotrophic bacteria have specialized in gaining energy from aerobic oxidation of H 2 S. In reduced sediments this becomes very apparent as the bacteria form a massive white cover on the sediment surface optimizing their position to the H 2 S - O 2 interphase. In such instances the bacteria mat form the last barrier before H 2 S emerges to the overlying water. At present there is no simple technique to discriminate between the oxygen consumption related to respiration or reoxidation. But from porewater microprofiles of H 2 S and O 2 it is possible to quantitatively evaluate the importance of H 2 S oxidation for the total oxygen consumption rate. Oxygen and H 2 S profiles can be measured by microelectrodes. However, sulfide actually exists in three forms S 2-, HS -, H 2 S and the sensor applied in the present exercise is only sensitive to the H 2 S fraction. The equilibrium between the three forms is ph dependent as outlined by the diagram below:
Thus in order to recalculate H 2 S profiles to total H 2 S (called TH 2 S below), the ph has to be measured in parallel and the ph-effect has to be accounted for. Description of work: We have collected sediment cores from two different locations in Øresund. The two stations mainly differ by the water depth and thereby in the external supply of organic material. The cores are labeled A (shallow water) and B (deep water). Each team has a core from each site and should 1) describe the two cores (color, texture, fauna etc.) and any visual macroscopic irregularities. Both cores are submerged in well-mixed water baths (salinity 30) kept at room temperature (20 o C). Three different calibrated microsensors H 2 S, ph and O 2 are ready for mounting in the micro-manipulator to obtain a number of individual profiles of each chemical specimen (Fig 1). Microsensor Water Air-pump Strip-chart recorder Picoamp meter 214.3 Profiling start 0% 100% Sediment Start profiling with the O 2 microsensor in an area free of shells or hard substrata. When the sensor has been positioned at the selected spot, you estimate the relative position of the sediment surface. This is done by moving the sensor slowly downwards - using the micromanipulator - until a signal change at the strip-chart recorder is observed this position is close to the sediment surface. The sensor is then moved a few hundred microns backwards until the 100%-saturation value (equivalent to 238 µm) is reached, the sensor is now ready for microprofiling. This it is done by moving the sensor downwards in increments of 0.1 mm. Each time the sensor is moved a small mark is made on the recorder paper. The profiling is continued until a low constant value (signal at 0% saturation) is reached. Subsequently the sensor is moved back up in the water phase, moved horizontally to another position and the procedure is repeated at different locations (a total of two profiles are measured in each core).
After this the H 2 S and the ph-sensor is mounted in the micromanipulator. The two sensors are glued together and are vertically aligned so that microprofiles of the two chemical specimens are obtained at the same depth horizon simultaneously. Try to locate the position of the sediment surface visually (for later alignment between the O 2 and the H 2 S/pH profiles) and perform 2-3 microprofiles with a reasonable vertical resolution in the sediment corer from site A (the sediment from the deeper site, B, does not contain H 2 S). Slurry experiment Background Aerobic reoxidation of the reduced products from the anaerobic activity occur spontaneously. However, most processes are catalyzed by chemoautotrophic bacteria that increase the oxidation rates by a factor of 100-1000 and the bacteria harvest the energy. The accumulated reduced products can represent a significant oxygen dept that can be re-paid over short time during storm events. Here sediment is resuspended up into the overlying oxic water column. This can be visualized by incubating reduced sediment in slurries and compare the rates to the results of the microprofile data above. Biological inactivation (by formalin) can indicate to what extent the oxidation during resuspension is biological or chemical mediated and to what extent the O 2 consumption rate is constant with decreasing O 2 concentration provide information on the oxidation kinetics. Description of work Surface sediment (0-5 mm) from a core from each station (A and B) is transferred to each their weighing boat and the sediment is gently homogenized. Subsequently 4x 3.0g (write down the exact weight) of sediment from each weighing-boat is transferred to 4 Winklerbottles that subsequently are filled with 100% air-saturated sea-water (corresponding to 238 µm). Each team now has 4 resuspension-bottles (remember to label them with teamnumber and the respective station index). One ml of formalin (37%) is added to one bottle from station A and B respectively (use gloves). A small magnet is added to each bottle and all bottles are placed on two central stirring plates that are runnig at a speed sufficient to induce resuspension. The O 2 concentration in each bottle is now followed by successively transferring an O 2 microelectrode between the bottles at a reasonable time interval (measurements should preferentially be done in each bottle every 10 min). The sensor current of the calibrated sensor at each time and for each bottle is noted when the signal has stabilized along with the time of measurement. If time allows the O2 concentration is followed until 0 um is reached. After each measurement a small amount of water is added to avoid bubbles inside the bottles. Temperature block experiment Background The aerobic activity of surface sediment is regulated by many environmental controls - on a seasonal basis the bottom-water temperature is one of them. That temperature is
important can be visualized by performing resuspension incubations along a temperature gradient (established by a so-called temperature block) Description of work Oxidized surface sediment (0-5 mm) from the deeper site, B, is transferred to a weighing boat and homogenized. 2.0 gram (note down the exact weight) of the homogenized sediment is transferred to 4 excetainer tubes (for each team) and they are subsequently filled with 100% air saturated sea-water (corresponding to 238 µm O 2 ). Each team adds one glass ball to each tube and 100 µl yeast extract to two of the tubes. Each team then select two temperatures and two tubes (+/- yeast extract) are placed at each temperature. The oxygen consumption rate at each temperature is now followed by measuring the oxygen concentration in the 4 excitainers at roughly 15 min interval (see procedure described above). After each measurements a few drops of air-saturated water is added to the tubes to avoid bubbles. Microelectrode experiment Theoretical work 2) Plot the 2 sets of O 2, H 2 S and ph data versus sediment depth for the shallow site and the 2 O 2 microprofiles from the deeper site - indicate the estimated position of the sediment surface as (Y=0). Use the units µm and mm. 3) Plot the two TH 2 S values versus depth for the shallow site (indicate the estimated position of the sediment surface as (Y=0)). For ph <9 the TH 2 S can be estimated from: TH 2 S = H 2 S (1 + 10-7 /10 -x ), where x equals the ph at the respective sediment depths. Use the unit s µm and mm. 4) Describe the oxygen profiles from each site and calculate the average DBL-thickness, the Diffusive O 2 Uptake (DOU) and the volume specific respiration (R) for each site in the following units mm, mmol m -2 d -1, and µmol cm -3 d -1, respectively DOU (mmol m -2 d -1 ) = β * D * 8640, where β is the slope of the concentration profile within the DBL (unit µm mm -1 ) and D is the molecular diffusion coefficient of oxygen in water (unit 10-5 cm 2 s -1 ). D equals 1.98 *10-5 cm 2 s -1 at salinity 30 and 20 o C. The value 8640 is a conversion factor to get the right units. R (µmolcm -3 d -1 ) = (DOU/OP) * 0.1, where OP is the oxygen penetration depth (unit cm) and DOU the diffusive O 2 uptake (unit mmol m -2 d -1 ). The value 0.1 is a conversion factor to get the right units. This calculation assumes constant activity in the oxic zone and zero-order kinetics. 5) Compare and comment upon the values from the two sites. How close are the values to the theoretical possible DOU at the given conditions?
6) Explain the shape of the TH 2 S profiles and define the zones of sulfide production and sulfide consumption. 7) Calculate the diffusive flux of TH 2 S into the oxic zone using a D-value of 1.4 10-5 cm 2 s -1 and calculate the fraction of O 2 consumption used for TH 2 S oxidation assuming complete aerobic sulfide oxidation to sulfate: H 2 S + 2O 2 -> SO 4 2- + 2H +. Is that a reasonable assumption? Calculate the volume specific consumption rate of TH 2 S in the oxic zone. 8) Calculate the average concentration of O 2 and TH 2 S in the overlap zone. With these concentrations and the rates calculated above what is the turn overtime (in sec), meaning how long time does it take to consume the available O 2 and TH 2 S in the reaction zone? In a pure un-catalyzed chemical oxidation of H 2 S with O 2 it takes approximately 60 min to remove 50% of the sulfide. How much faster is the sulfide oxidation in the sediment of the shallow water station? Slurry experiment 1) Describe how the O 2 concentration in the Winkler bottles decrease. Give an explanation for the respective phases. 2) Calculate the O 2 consumption rate for the two respective stations during the initial and the subsequent oxidation phase. R slurry (µmol cm -3 d -1 ) = γ 0.05 /(y/2.0), where γ is the rate of O 2 decrease in the bottles (um d -1 ), 0.05 the bottle volume, y the number of gram sediment added and 2.0 the estimated density of the sediment. Comment upon potential differences between the incubations how do they compare to your expectations? 3) How does the values compare to the volume specific respiration (R) calculated from the O 2 microprofiles? Comment and give suggestions for any potential differences. 4) How many days of diffusive mediated O 2 consumption does a storm induced resuspension event lasting for 1 day of the upper 5 mm of sediment compare to for each station? Temperature block experiment 1) Calculate the O 2 consumption rate at the respective temperatures R excitainer (umol cm -3 d - 1 ) = γ 0.012 /(y/2.0), where γ is the rate of O 2 decrease in the bottles (um d -1 ), 0.012 the tube volume, y the number of gram sediment added and 2.0 the estimated density of the
sediment. Ignore any potential drop in the O 2 concentration during the initial part of the incubation. 2) Does the addition of yeast extract affect the O 2 consumption rate? give suggestions to why or why not. 3) Plot the respective activities for all teams versus the temperature and describe the curve does it match your expectations? 4) Estimate the Q 10 value for the O 2 consumption rate does it match your expectations?. How relevant do you think this estimate is for describing the seasonal temperature response in central Øresund?