Functional exoenzymes as indicators of metabolically active bacteria in 124,000-year-old sapropel layers of the eastern Mediterranean Sea

Hydrolytic exoenzymes as indicators of metabolically active bacteria were i nvestigated in four consecutive sapropel layers collected from bathyal sedi ments of the eastern Mediterranean Sea. For comparison, the organic carbon- poor layers between the sapropels, sediment from the anoxic Urania basin, a nd sediments of intertidal mud flats of the German Wadden Sea were also ana lyzed. The sapropel layers contained up to 1.5 . 10(8) bacterial cells cm(- 3), whereas cell numbers in the intermediate layers were lower by a factor of 10, In sapropels, the determination of exoenzyme activity with fluoresce ntly labeled substrate analogues was impaired by the strong adsorption of u p to 97% of the enzymatically liberated fluorophores (4-methylumbelliferone [MUF] and 7-amino-4-methylcoumarin [MCA]) to the sediment particles. Becau se all established methods for the extraction of adsorbed fluorophores prov ed to be inadequate for sapropel sediments, we introduce a correction metho d which is based on the measurement of equilibrium adsorption isotherms for both compounds. Using this new approach, high activities of aminopeptidase and alkaline phosphatase were detected even in a 124,000-year-old sapropel layer, whereas the activity of beta-glucosidase was low in all layers. So far, it had been assumed that the organic matter which constitutes the sapr opels is highly refractory. The high potential activities of bacterial exoe nzymes indicate that bacteria in Mediterranean sapropels are metabolically active and utilize part of the subfossil kerogen. Since a high adsorption c apacity was determined not only for the low-molecular-weight compounds MUF and MCA but also for DNA, the extraordinarily strong adsorption of structur ally different substrates to the sapropel matrix appears to be the major re ason for the long-term preservation of biodegradable carbon in this environ ment.