ABSTRACT

Lipid compounds in sediments from an inorganically acid freshwater Lake Tazawa-ko in Japan were analyzed. Of first remarked was the occurrence of series of long-chain (C₂₁–C₃₂) branched alkanes, alkenes, alcohols and alkanoic acids, which showed quite small odd-even carbon number preferences with the maximum abundance in the range of C₂₈–C₃₀. These long-chain branched compound series have been observed almost ubiquitously in moderately acidic (pH = 3–6) but neither in circumneutral nor strongly acidic (pH < 2) lake sediments in Japan. Mass spectra of the branched alkanes and alkenes with strong fragment ions of (M-29)⁺ and (M-30)⁺, respectively, showed that both series possess an anteiso (3-methyl or (ω-2)-methyl) structure. A relatively large intensity of 57⁺ ion in the mass spectra of the branched alcohols (as TMS ethers) and alkanoic acids (as methyl esters) indicated that they also have the same anteiso, i.e., (ω-2)-methyl structure. Occurrence of these anteiso compounds was accompanied by a significant decrease of the odd or even carbon-number preference of concomitant straight-chain compounds, which suggested the source of these anteiso compounds might be bacteria. Intimate relationship between these characteristic lipid compositions and the acidic nature of the lake water was substantiated by the observation of core sediments, since the acidification of the Lake Tazawa-ko was onset in 1940 due to artificial introduction of the River Tamagawa water charged with a strongly acidic hot spring water.