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Archaeal ANaerobic MEthanotrophs (ANME) facilitate the anaerobic oxidation of methane (AOM), a process that is believed to proceed via the reversal of the methanogenesis pathway. Carbon isotopic composition studies indicate that ANME are metabolically diverse and able to assimilate metabolites including methane, methanol, acetate, and dissolved inorganic carbon (DIC). Our data support the interpretation that ANME in marine sediments at methane seeps assimilate both methane and DIC, and the carbon isotopic compositions of the tetrapyrrole coenzyme F430 and the membrane lipids archaeol and hydroxy-archaeol reflect their relative proportions of carbon from these substrates. Methane is assimilated via the methyl group of CH 3 -tetrahydromethanopterin (H 4 MPT) and DIC from carboxylation reactions that incorporate free intracellular DIC. F430 was enriched in 13 C (mean δ 13 C = -27‰ for Hydrate Ridge and -80‰ for the Santa Monica Basin) compared to the archaeal lipids (mean δ 13 C = -97‰ for Hydrate Ridge and -122‰ for the Santa Monica Basin). We propose that depending on the side of the tricarboxylic acid (TCA) cycle used to synthesize F430, its carbon was derived from 76% DIC and 24% methane via the reductive side or 57% DIC and 43% methane via the oxidative side. ANME lipids are predicted to contain 42% DIC and 58% methane, reflecting the amount of each assimilated into acetyl-CoA. With isotope models that include variable fractionation during biosynthesis for different carbon substrates, we show the estimated amounts of DIC and methane can result in carbon isotopic compositions of - 73‰ to - 77‰ for F430 and - 105‰ for archaeal lipids, values close to those for Santa Monica Basin. The F430 δ 13 C value for Hydrate Ridge was 13 C-enriched compared with the modeled value, suggesting there is divergence from the predicted two carbon source models., (© 2019 John Wiley & Sons Ltd.)

The carbon and nitrogen isotopic signatures of chloropigments and porphyrins from the sediments of redox-stratified lakes and marine basins reveal details of past biogeochemical nutrient cycling. Such interpretations are strengthened by modern calibration studies, and here, we report on the C and N isotopic composition of pigments and nutrients in the water column and surface sediment of redox-stratified Fayetteville Green Lake (FGL; New York). We also report δ 13 C and δ 15 N values for pyropheophytin a (Pphe a) and bacteriochlorophyll e (Bchl e) deposited in the Black Sea during its transition to a redox-stratified basin ca. 7.8 ka. We propose a model for evolving nutrient cycling in the Black Sea from 7.8 to 6.4 ka, informed by the new pigment data from FGL. The seasonal study of water column nutrients and pigments at FGL revealed population dynamics in surface and deep waters that were also captured in the sediments. Biomass was greatest near the chemocline, where cyanobacteria, purple sulfur bacteria (PSB), and green sulfur bacteria (GSB) had seasonally variable populations. Bulk organic matter in the surface sediment, however, was derived mainly from the oxygenated surface waters. Surface sediment pigment δ 13 C and δ 15 N values indicate intact chlorophyll a (Chl a) was derived from near the chemocline, but its degradation product pheophytin a (Phe a) was derived primarily from surface waters. Bacteriopheophytin a (Bphe a) and Bchl e in the sediments came from chemocline populations of PSB and GSB, respectively. The distinctive δ 13 C and δ 15 N values for Chl a, Phe a, and Bphe a in the surface sediment are inputs to an isotopic mixing model that shows their decomposition to a common porphyrin derivative can produce non-specific sedimentary isotope signatures. This model serves as a caveat for paleobiogeochemical interpretations in basins that had diverse populations near a shallow chemocline., (© 2018 John Wiley & Sons Ltd.)