Survival and Activity of Bacteria in a Deep, Aged Lake Sediment (Lake Constance)
Viable counts and potential activities of different bacteria were determined as a function of depth in the deep profundal sediment of Lake Constance, 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 the sediment prevents seepage. Below 25 cm all of the viable heterotrophic bacteria were present as heat-resistant spores. Numbers of viable spores of both aerobic and anaerobic heterotrophic bacteria decreased exponentially 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. Absence of viable heterotrophic bacteria in deeper sediment layers demonstrated that aseptic sampling conditions were achieved. The decrease of 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. Viable units of specific metabolic groups of bacteria were detected only in the upper sediment layers (0–50 cm). Nitrifying bacteria could not be detected below 30 cm. Methane-oxidizing bacteria were present in the sediment down to >30 cm, but were in a dormant state. Nitrate reduction activity decreased by a factor of 6 within the upper 25 cm of the 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 contrast, methanogenesis and methanogenic bacteria could be detected down to 50 cm. These observations indicate that bacteria eventually become nonviable in aged sediments.
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