Photosynthetic pigments in sediment core HDP-04 from Lake Hovsgol, Mongolia, and their implication for changes in algal productivity and lake environment for the last 1 Ma

We analyzed photosynthetic pigments of algae and phototrophic bacteria in the 81-m sediment core (HDP-04) of Lake Hovsgol, NW Mongolia, to reconstruct the changes in algal assemblage and lake environment for the last 1 Ma. In HDP-04, pheophytin a, pyropheophytin a, and steryl esters of pyropheophorbide a (steryl chlorin esters: SCEs-a) were detected as major chlorophyll a derivatives. They indicate that algal productivity fluctuated in response to changes in lacustrine environment, presumably in response to climatic changes. Significant concentrations of intact carotenoids, zexanthin, lutein, and β,β-carotene, as well as cis-isomers of zeaxanthin, diatoxanthin, alloxanthin, and β,β-carotene were found to be preserved in the sediments as old as 1 Ma. The relative concentration of these algal carotenoids showed changes in algal assemblage for the last 1 Ma. Besides algal pigments, bacteriochlorophylls e and isorenieratene, characterstic to brown-Chlorobium were detected transiently only at the sediment depth of 19–21 m, providing evidence for the development of anoxia in photic zone because brown-Chlorobium requires hydrogen sulfide as an electron donor in their photosynthesis. The sediment layers between 21 and 52 m where concentrations of algal carotenoids are higher than those of chlorophyll derivatives may correspond to the periods with shallower lake-water level because increased accumulation of algal carotenoids in surface sediments needs short transport pathway in the water column to evade abiotic/biotic oxidation during transportation.