COUPLING OF MICROBIAL FERMENTATION AND RESPIRATION PROCESSES IN AN INTERTIDAL MUDFLAT OF THE ELBE ESTUARY

In laboratory experiments, the consumption of terminal electron acceptors was studied under defined redox conditions during different seasons. Under oxic conditions oxygen together with nitrate and Mn was reduced. Under suboxic conditions nitrate reduction occurred simultaneously with Mn and ferric Fe reduction and the fermentation of organic matter. Over an annual cycle, maximum reduction rates were found in early summer. These were 24.71 for oxygen, 1. 13 for Mn(IV) and 3.33 mumol g-1 d-1 for Fe(III). In winter, the respective rates decreased 3.4-, 4.7-, and 9.2-fold. Nitrate reduction remained constant from July to November at about 5.62 mumol g-1 d-1 and decreased 2.8-fold by February. The production of CO2 due to fermentation of organic substances was the same magnitude as that for oxygen respiration in summer. When fermentation processes stopped at the end of summer, nitrate respiration out-competed Fe and Mn reduction for organic substrate. These results indicate that even great differences in free energy do not prevent coexistence of different respiration processes in the same sediment layer. Microbial reduction rates in the sediment appear to depend on organic matter of low molecular weight which is produced during fermentation.