ANALYSIS OF SUBFOSSIL MOLECULAR REMAINS OF PURPLE SULFUR BACTERIA IN A LAKE SEDIMENT

Molecular remains of purple sulfur bacteria (Chromatiaceae) were detec ted in Holocene sediment layers of a meromictic salt lake (Mahoney Lak e, British Columbia, Canada). The carotenoid okenone and bacteriophaeo phytin a were present in sediments up to 11,000 years old. Okenone is specific for only a few species of Chromatiaceae, including Amoebobact er purpureus, which presently predominates in the chemocline bacterial community of the lake. With a primer set specific for Chromatiaceae i n combination with denaturing gradient gel electrophoresis, 16S rRNA g ene sequences of four different Chromatiaceae species were retrieved f rom different depths of the sediment. One of the sequences, which orig inated from a 9,100-year-old sample, was 99.2% identical to the 16S rR NA gene sequence of A. purpureus ML1 isolated from the chemocline. Emp loying primers specific for A. purpureus ML1 and dot blot hybridizatio n of the PCR products, the detection limit for A. purpureus ML1 DNA co uld be lowered to 0.004% of the total community DNA, With this approac h the DNA of the isolate was detected in 7 of 10 sediment layers, indi cating that A. purpureus ML1 constituted at least a part of the ancien t purple sulfur bacterial community. The concentrations of A. purpureu s DNA and okenone in the sediment mere not correlated, and the ratio o f DNA to okenone was much lower in the subfossil sediment layers (2.7. 10(-6)) than in intact cells (1.4), This indicates that degradation ra tes are significantly higher for genomic DNA than for hydrocarbon cell constituents, even under anoxic conditions and at the very high sulfi de concentrations present in Mahoney Lake.