A significant contribution of Methanogenic archaea (methanogens) on methanogenesis in marine subsurface sediments has been suggested on the basis of isotopic compositions of methane from sediments and gas hydrate water lattice. However, habitats and activities of methanogens are still unknown. Lipid biomarkers and their isotope analyses can be useful tools to study distribution of methanogens in marine sediment because their unique metabolism is reflected in carbon isotope compositions of their membrane lipids. The techniques to detect methanogens and their membrane lipid analysis are more recently developed and are applied to gas hydrate bearing marine sediments, and a new approach of function specific compound analysis is suggested.

The Jurassic sediments in two small basins formed on the Precambrian metamorphic terrain of Sri Lanka, which had been located in the eastern Gondwanaland in mid-latitudes of the southern hemisphere, were investigated to make clear the origin of organic matter (OM) and its depositional environment. Total organic carbon (TOC) contents are high (3.05 – 5.10%) in brown to black color mudstones in the Jurassic Andigama Basin, while the grey color sandy sediments in the Jurassic Tabbowa Basin generally have very low TOC (0.04 – 0.17%). The Andigama mudstones are thermally immature, and the Tabbowa sediments and relational Miocene Aruwakkalu limestone are moderately matured in the oil generation stage. The sediment samples from the Jurassic Andigama Basin show a large proportion of middle-chain n-alkanes (nC₂₁-C₂₅), enriched C₂₉ steranes, high C / N ratios (16.3 – 37.8), very low total sulfur (TS < 0.001%) and higher pristane / phytane ratios (Pr / Ph = 2.1 – 3.0). These results suggest that the sediments were deposited in freshwater swamp under oxic condition. Abundant retene, simonellite, perylene, 1-methylphenanthrene, 1, 7-dimethylphenanthrene and 1, 2, 7- / 1, 2, 9-trimethylphenanthrenes in the Andigama mudstones indicate that the OMs were mainly originated from gymnosperm with fungi. On the other hand, the Jurassic sandy sediments in the Tabbowa Basin have predominant nC₁₆-C₂₁ alkanes with a minor peak of waxy n-alkanes (nC₂₉, nC₃₁, nC₃₃), very low TS (< 0.001%), abundant C₂₇ steranes and higher Ts / (Ts + Tm) [Ts: 17 α (H)-22, 29, 30-trisnorhopane, Tm: 18 α (H)-22, 29, 30-trisnorhopane] triterpane ratios, which suggesting the OMs from algal origin with a significant terrestrial higher plants. The Miocene Aruwakkalu limestone shows very low TOC (0.06%) and algal origin. The sediment samples from the relational Pleistocene Aruwakkalu Red Earth show a high unresolved complex mixture (UCM), and high values of carbon preference index (CPI = 4.78) and average chain length (ACL = 29.9), which indicate the OM from oxidized and / or biodegraded ancient terrestrial plant.

Stable nitrogen isotopic composition (δ¹⁵N) of glutamic acid and phenylalanine in a laboratory-cultured calanoid copepod Acartia steueri and in its food items (the diatom Thalassiosira weissflogii), eggs, and fecal pellets was determined to examine the isotopic discrimination of ¹⁵N / ¹⁴N of amino acids among these samples associated with a single grazing process. Glutamic acid has a wide variation in the δ¹⁵N value among the bodies (+ 8.8‰ for 10 days with feeding and + 8.9‰ for the following one day without feeding), eggs (+ 8.8‰), and fecal pellets (+ 3.3‰) of A. steueri and its food items (+ 0.3‰), whereas phenylalanine falls in a narrow range of the values from − 3.7‰ to − 3.1‰, 1σ = 0.3, for all samples. Based on these δ¹⁵N values, the trophic position (TP_{Glu / Phe}) was characterized as 2.1 for bodies and eggs, 1.5 for fecal pellets, and 1.1 for the food items. These TP_{Glu / Phe} values for A. steueri and its food items are consistent with the expected trophic positions (2.0 and 1.0, respectively) of these samples. To our knowledge, this is the first published data for the TP_{Glu / Phe} value of fecal pellets, which is 0.4 units higher than that of the food items. Because fecal pellets supply basal food resources for many organisms living in aphotic zones, our data will provide a better understanding of their trophic positions.