SULFUR AND METHANE CYCLING DURING THE HOLOCENE IN ACE LAKE (ANTARCTICA) REVEALED BY LIPID AND DNA STRATIGRAPHY

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Abstract

Postglacial Ace Lake (Vestfold Hills, Antarctica) was initially a freshwater lake, then an open marine system, and finally the present-day saline, stratified basin with anoxic, sulfidic, and methane-saturated bottom waters. Stratigraphic analysis of carotenoids and ancient 16S rDNA in sediment cores revealed that almost immediately after marine waters entered the palaeo freshwater lake as a result of post-glacial sea-level rise, Ace Lake became meromictic with the formation of sul.dic bottom waters and a chemocline colonized by obligate anoxygenic photolithotrophic green sulfur bacteria (Chlorobiaceae). Ancient 16S rDNA stratigraphy revealed that the fossil source of chlorobactene throughout the Holocene as well as in the present-day chemocline of Ace Lake was a species with 99.6% sequence similarity to the 16S rDNA sequence of Chlorobium phaeovibrioides DSMZ 269T. Comparison of the ratio between rDNA and chlorobactene of the latter species in the water column and in Holocene sediment layers revealed that the degradation of DNA was mostly influenced by the preservation conditions of the ancient water column. Within the sulfidic Holocene sediments, the remaining ancient DNA of green sulfur bacteria was more stable than intact carotenoids. We showed the development of anoxygenic photosynthesis with our previous stratigraphic analysis of 16S rDNA and lipid biomarkers indicative for prokaryotes involved in the cycling of methane in order to get a more complete picture of anoxygenic processes in Ace Lake during the Holocene.