A short surface sediment core (40cm) was collected from a strongly acidic (pH=2.2) freshwater lake, Katanuma, Miyagi Prefecture, Japan. Lake Katanuma is a maar, the water of which is rich in sulfate, sulfides and free sulfur and is located in the active Naruko geothermal field. Lipid compounds in the uppermost sediment were extracted and analyzed for organosulfur compounds (OSCs) by gas chromatography (GC) and gas chromatograph-mass spectrometry (GC-MS). The OSCs identified in the lake sediment were C₁₃-C₂₀ 3-methyl- and 3-ethyl-5-alkyl thiophenes, C₃₅ hopanoid thiophenes and C₁₈ thiophene carboxylic acids (TCAs), in addition to C₂₀ isoprenoid thiophenes. In the present work, production of C₁₈ TCAs was examined by a mild laboratory simulation reaction of reagent grade linoleic acid (cis, cis-9,12-octadecadienoic acid: C_18:2) as well as the solvent extracts from the lake sediments, in H₂S saturated water. We confirmed that the C₁₈ TCAs could be in fact produced under the simple experimental system. However, the abundances of the C₁₈ TCAs detected were too small compared with those of possible precursors, C_18:2 acids, which is ubiquitous in the particulate matter in the Lake Katanuma water, but almost completely disappeared in subsurface sediment. This finding suggests that sulfur plays a role in a very early stage of diagenesis for sequestering chemically reactive lipid molecules by transforming them into complex geopolymers rather than into small OSCs. No apparent variations in concentration of those OSCs were observed downwards the sediment core. The immediate precursors of alkylthiophenes and hopanoid thiophenes remain to be unraveled.

The analysis of stable nitrogen isotope ratios of amino acids (δ¹⁵N_AA) has been employed as a powerful tool to estimate the trophic position of organisms and to illustrate the trophic relationship among them in diverse ecosystems. However, applicability of this analysis is somewhat doubtful in the estimation of the trophic position of marine mammals. Unusual small values of the trophic elevation in the δ¹⁵N_AA value have been apparently expected for marine mammals in previous studies, which may be affected by isotopic alteration during the ammonia recycling through glutamate - glutamine exchange associated with the urea-excretion. In the present study, we developed a chloroformate based method (to form isopropoxy isopropyl ester derivative) for separately measuring the δ¹⁵N values of two amino groups within a single authentic glutamine molecule, as a first step accurately to evaluate the isotopic alteration during the ammonia recycling in wild animals. The isopropyl derivative of glutamine used in the present study can be preserved for longer than 47 days at −20ºC (or for up to 5 days at room temperature), unlike methyl and ethyl derivatives used in previous studies are quickly degraded within several days even at −20ºC. Moreover, together with the hydrolysis of glutamine to form glutamate, the δ¹⁵N values of two amino groups within a single glutamine were measured. Thus, the chloroformate based method developed can be useful to compare the δ¹⁵N values between two amino groups within a single glutamine molecule, and will provide further evaluation of the isotopic alteration on the δ¹⁵N_AA values during the urea-excretion and related processes in marine mammals.

Isotopic compositions in fatty acids have widely been used in the study for tracing sources and delivery of organic materials in the earth surface and for reconstructing environments in the geological timescale. Changes in the isotopic compositions associated with the lipid degradation of organisms, however, have been poorly understood. In this study, we determined changes in the carbon and hydrogen isotopic compositions (i.e., the δ¹³C and δD values, respectively) of palmitate (i.e., C16:0 carboxylic acid esters), a major alkanoate in eukarya and bacteria, during the germination of plant seeds under no photosynthetic activity. During the germination, abundance of palmitate is decreased by 41 and 94 % for 5 and 14 days, which causes a considerable change in the δ¹³C value of palmitate by 3.0 and 17.2‰, respectively. In contrast, non-substantial difference is found in the δD values of palmitate for this germination. These enrichments indicate that isotopic fractionation occurs on a carbon but not hydrogen atom of palmitate in an initial reaction of the degradation, as reaction of carbonyl carbon in palmitate with enzyme (lipase) can be proposed for a candidate mechanism. If such reaction mainly occurs on palmitate components in triacylglycerol for the germination of plant seeds, the enrichment in ¹³C estimated is more than 250‰ for the carbonyl carbon in palmitate based on a mass balance calculation, and the isotope fractionation factor (α) accounts for 0.9086 in this reaction based on the Rayleigh fractionation model.