bleasurements of microblal biomass and actlvity were carried out at 6 representative locatlons along a gradient of eutrophlcatlon In the Nordrugensche Bodden (southern Baltic Sea Germany) h4easurements in the water column revealed that turbid~ty, seston content and concentratlons of chlorophyll a and inorganic nutnents (ammonla nitnte, nitrate phosphate) increased from the outer to the inner parts of the Bodden whereas salin~ty decreased Investigations of sedlments conf~rmed this gradlent of eutrophlcatlon Whereas in the outer parts of the Bodden sandy sediments prevailed sandy mud and muddy s e d~m e n t s dominated towards the inner parts Generally, organic carbon and nltrogen, concentratlons of chlorophyll a and phospholip~ds (indicator of microbial biomass) oxygen consumption and hydrolytlc enzyme actlv~tles Increased wlth Increasing level of eutrophlcatlon At the relatively unpolluted locatlon in the outer parts of the Bodden proteolytic enzymes dominated at the sedlment surface In subsurface horlzons carbohydrate-decomposing enzymes galned more importance At the heavily polluted locatlons In the inner parts of the Bodden proteolytlc enzymes were even more Important Wlth increasing sediment depth enzyme actlvlties were greatly reduced h o w ever, and shlfts In the spectrum of hydrolytlc enzymes were less pronounced Among the blologlcal and chemlcal parameters, charactenstlc patterns of lnterrelatlonships became obvious whlch led to the conclus~on that microblal blomass and enzyme actlvlties in sediments of the outer and central parts of the Bodden are limited by organlc carbon The organic-nch sediments of the lnner parts of the Bodden however did not support furthe] increases In microbial biomass and decompos~tlon act~vltles Enzyme actlvltles are discussed In relatlon to the composition and degradablhty of substrates The enzymatic decomposltlon potential was measured by means of fluorogenlc model substrates Methodological investigations showed that the methylumbelllferyl substrates used reacted specifically enough to ]ustlfy the11 use In ecological studies Sedlments can be stored refngerated for over 2 mo without changes in the spectrum of hydrolytic enzymes
A microbial biosensor consisting of an oxygen microelectrode with microbial cells immobilized in polyvinyl alcohol attached to the tip is described. Since the oxygen consumption of the immobilized cells is dependent on the oxidation of assimilable carbon, the biosensor allows the estimation of available dissolved organic carbon (ADOC) in sediment profiles on a microscale. Only that fraction of the ADOC can b e detected which is actually respired by the test organisms. Since the biosensor works only in oxic sediments, oxygen has to be measured separately. Calibration was achieved in mineral salt medium of different oxygen content using different concentrations of carbon (glucose, acetate). The 90'% response time of the blosensor was about 10 S; within 1.5 min a plateau was reached, ~ndlcating that the diffusion of oxygen from the sample was In equ~libnum with the oxygen consumption of the cells. Measurements with immobilized yeast cells revealed that with~n the diffusive boundary layer, concentrations of ADOC Increased, reaching maximum values immediately below the sediment surface. Less than 0.1 "h of the total organic carbon (TOC) was immediately available for microbial metabolism. Based on respiration calculated from measurements by oxygen microelectrodes in the dark, the average turnover time of ADOC within the upper 1 mm of the sediment was in the range of 50 min.
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