Vertical Distribution of Biomolecules in the Carbonate Sediments and Bacterial Mat of Steppe Alkaline Lake, Eastern Transbaikalia

A wide range of lipid biomarkers was identified in the bacterial mat (BM) and carbonate sediment core of Lake Doroninskoe (0–90 cm). The main bioproducers and ecological conditions providing the predominance of definite biota and peculiarities of biochemical processes were determined. The revealed peculiar composition of biomolecules within separate intervals of the bottom sediment (BS) core can be caused by climatic change in Eastern Transbaikalia for the last centuries. Biomolecules produced by bacteria, microalgae, and terrestrial plants were found in variable proportions throughout the studied core. The composition and distribution of steroids, triterpenoids, alkanes, alkanols, alkanones, α-tocopherol, perylene, dihydroactinidiolide, methyl esters of saturated normal and branched, as well as unsaturated fatty acids in BM and BS in combination with mineral and element composition of BS allowed us to divide the studied core section into three intervals. These intervals differ in the content and proportions of some chemical elements and biomolecules, the presence of peculiar organic compounds in the lipophilic extractives, as well as variation trends of some parameters or their groups. The lower oldest part of the sediment contains organic matter (OM) produced mainly by terrestrial plants. Upsection, the fraction of stigmasterol and sitosterol in steroids increases, while that of brassicasterol and cholesterol, steroid ketones, and sigmasterol/sitosterol ratio decreases. At a depth corresponding approximately to the mid-19th century, the OM composition changes. Steroids show an increase of 3-ketone-substituted structures and cholest-5-en-3-ol, with appearance of ergosta-5,7,22-trien-3-ol, stigmasta-5,7,22-trien-3-ol, ergost-7-en-3-ol, and stigmast-7-en-3-ol, in combination with increase of α-tocopherol and dihydroacinidiolide. The content of high molecular weight homologues in n-alkanes and methyl esters of fatty acids decreases. In BS precipitated during 20th century, OM composition and its variation trends are opposite to those observed in the lower part of the section. Obtained data indicate a sequential environmental changes in Transbaikalia for the last ~250 years. Climatic warming during accumulation of the lower part of sediment core of Lake Doroninskoe was replaced by the hot dry period in the second half of 19th century and then by cooling, which lasted up to the last decade. An increase of radiation background by the mid-20th century led to the accumulation of 4-methyl substituted steroids, the intermediate products of sterol biosynthesis, in BS. Our study demonstrates additional approaches to study of the past ecological and climatic changes.