Chapter 12
Downloaded by STANFORD UNIV GREEN LIBR on September 17, 2012 | http://pubs.acs.org Publication Date: February 14, 2002 | doi: 10.1021/bk-2002-0811.ch012
Microsensor Studies of Oxygen, Carbon, and Nitrogen Cycles in Lake Sediments and Microbial Mats Dirk de Beer Max-Planck-Institute for Marine Microbiology, Celsiusstrasse 1, D-28359 Bremen, Germany
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Microsensors for O , pH, CO , NH and NO are described. Measurements with these sensors in sediments and microbial mats were performed to investigate the interrelation of the oxygen-, nitrogen- and carbon cycles. The effect of light was studied in lake sediments, obtained from 1 and 2.5 m depth. In the dark O penetrated ca 1 mm and in the oxic zone NO was formed. However, nitrate transport rates from the waterphase into the sediment was higher than the nitrification rates. Nitrification was restricted to the zone below the photosynthetic layer. In the presence of light, photosynthesis induced a threefold increase in O penetration depth and a proportional increase in nitrification. In the light, NO was consumed by both denitrification and assimilation in the photic zone. NO assimilation was particularly high in the sediments from shallow parts of the lake that were low in NH content. In the deeper parts of the lake NH was used for assimilation. The NH profiles did not reflect the nitrification rates, because NH production and reversible binding to the sediment buffered NH . In microbial mats, CO is depleted in the photosynthetic zone during illumination. The absence of nitrification in the top layer of sediments is explained by competition (between phototrophs and nitrifiers) for NH and CO . 2
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© 2002 American Chemical Society In Environmental Electrochemistry; Taillefert, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.
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Downloaded by STANFORD UNIV GREEN LIBR on September 17, 2012 | http://pubs.acs.org Publication Date: February 14, 2002 | doi: 10.1021/bk-2002-0811.ch012
Introduction. In aquatic systems microbial conversions mainly take place in the sediments, microbial mats and biofilms covering the sediments and solid surfaces. Conversions in the overlaying waterphase are often of less quantitative importance. Due to mass transfer limitations inside these structures, gradients of substrates and products are established, the concentrations of substrates being lower and the concentration of products higher than in the overlying water. The slope of the concentration gradients depends on the conversion rates and mass transfer rates. Significant changes can occur within 10 microns in highly active biofilms while in less active deep-sea sediments significant changes occur typically in millimeters to centimeters. In such systems porewater analysis has limitations. The extraction of porewater may influence the concentration profiles and in case of biofilms its spatial resolution is insufficient. The best technique available nowadays is the use of microsensors, needle shaped devices with a tip size of 1-20 μιη which can measure the concentration of a specific compound. Highly localized measurements are possible, since the spatial resolution is approximately equal to the tip-size of the sensor. There are indications from theoretical and experimental studies that microsensors can influence the concentration profiles. The evidence is conflicting: while the theory predicts underestimation especially by sensors larger than 10 μηι(/), experiments with 0 microsensors in a biofilm showed an overestimation of local concentrations by sensors larger than 16 μπι(2). Microsensors may change the local concentrations by the consumption of substrate (only amperometric sensors), by compression of the local matrix (5), by changing the diffusion field (blocking diffusion by the sensor body) (/), or by compressing the boundary layer (4). Although microelectrodes have a small influence on structures and processes, it is the best choice for direct concentration measurements inside mats and sediments. Microsensors were introduced in microbial ecology by Bungay et al (1969) (5) who measured 0 profiles in biofilms. The technique was improved by N.P. Revsbech who constructed reliable 0 microsensors for profiling sediments and biofilms.(5, 7) More microsensors relevant for microbial ecology were developed and used, such as for N 0 (
