SURVIVAL AND ACTIVITY OF BACTERIA IN A DEEP, AGED LAKE SEDIMENT (LAKE-CONSTANCE)

Viable counts and potential activities of different bacteria were dete rmined as a function of depth in the deep profundal sediment of Lake C onstance, Germany. The sediment layer at the bottom of the lake had a total depth of about 7 m and was deposited in the time after the last ice age, i.e., over the past 13,000 years. The high clay content of th e sediment prevents seepage. Below 25 cm all of the viable heterotroph ic bacteria were present as heat-resistant spores. Numbers of viable s pores of both aerobic and anaerobic heterotrophic bacteria decreased e xponentially with sediment depth and were below the detection limit (5 -55 cells ml(-1)) at 4-6 m, i.e., in about 8,900-year-old sediment. Ab sence of viable heterotrophic bacteria in deeper sediment layers demon strated that aseptic sampling conditions were achieved. The decrease o f viable spores with depth may be interpreted as time-dependent death of spores resulting in a death rate of about 0.0013-0.0025 year(-1). V iable units of specific metabolic groups of bacteria were detected onl y in the upper sediment layers (0-50 cm). Nitrifying bacteria could no t be detected below 30 cm. Methane-oxidizing bacteria were present in the sediment down to >30 cm, but were in a dormant state. Nitrate redu ction activity decreased by a factor of 6 within the upper 25 cm of th e sediment, but was still detected at 50 cm. Sulfate reduction, on the other hand, could not be detected at depths of 20 cm and below. By co ntrast, methanogenesis and methanogenic bacteria could be detected dow n to 50 cm. These observations indicate that bacteria eventually becom e nonviable in aged sediments.