Microbial abundance in lacustrine sediments: a case study from Lake Van, Turkey

Abstract:

The ICDP “PaleoVan” drilling campaign at Lake Van, Turkey, provided a long (>100 m) record of lacustrine subsurface sedimentary microbial cell abundance. After the ICDP campaign at Potrok Aike, Argentina, this is only the second time that deep lacustrine cell counts have been documented.

The lacustrine cell count data are significantly different from published marine records, the most probable cause being differences in sedimentary organic matter composition, with marine sediments containing a higher fraction of labile organic matter.

Previous studies showed that microbial activity and abundance increase within centimetres to meters around geologic interfaces. The finely laminated Lake Van sediment allowed studying this phenomenon on the microscale. We sampled at the scale of individual laminae (0.5 to 1 mm thick) and in some depth intervals we found differences in microbial abundance of up to two orders of magnitude between the different laminae. This small-scale heterogeneity is normally overlooked due to much larger sampling intervals that integrate over several centimetres. However, not all laminated intervals exhibit such large differences in microbial abundance, and some non-laminated horizons show large variability on the millimetre-scale as well. The reasons for such contrasting observations remain elusive, but indicate that heterogeneity of microbial abundance in subsurface sediments has not been taken into account sufficiently. These findings have implications not just for microbiological studies but for geochemistry as well, as the large differences in microbial abundance clearly show that there are distinct microhabitats that deviate considerably from the surrounding layers.