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

In laboratory experiments, the consumption of terminal electron accept ors was studied under defined redox conditions during different season s. Under oxic conditions oxygen together with nitrate and Mn was reduc ed. Under suboxic conditions nitrate reduction occurred simultaneously with Mn and ferric Fe reduction and the fermentation of organic matte r. Over an annual cycle, maximum reduction rates were found in early s ummer. 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 Nove mber at about 5.62 mumol g-1 d-1 and decreased 2.8-fold by February. T he production of CO2 due to fermentation of organic substances was the same magnitude as that for oxygen respiration in summer. When ferment ation processes stopped at the end of summer, nitrate respiration out- competed Fe and Mn reduction for organic substrate. These results indi cate that even great differences in free energy do not prevent coexist ence of different respiration processes in the same sediment layer. Mi crobial reduction rates in the sediment appear to depend on organic ma tter of low molecular weight which is produced during fermentation.