Occurrence and distribution of ladderane oxidation products in different oceanic regimes

Ladderane fatty acids are commonly used as biomarkers for bacteria involved in anaerobic ammonium oxidation (anammox). These lipids have been experimentally shown to undergo aerobic microbial degradation to form short chain ladderane fatty acids. However, nothing is known of the production or the distribution of these oxic biodegradation products in the natural environment. In this study, we analysed marine water column particulate matter and sediment from three different oceanic regimes for the presence of ladderane oxidation products (C-14 ladderane fatty acids) and of original ladderane fatty acids (C-18 and C-20 ladderane fatty acids). We found that ladderane oxidation products, i.e. C-14 ladderane fatty acids, are already produced within the water column of the Arabian Sea oxygen minimum zone (OMZ) and thus only low amounts of oxygen (< 3 mu M) are needed for the beta-oxidation of original ladderane fatty acids to proceed. However, no short chain ladderane fatty acids were detected in the Cariaco Basin water column, where oxygen concentrations were below detection limit, suggesting that the beta-oxidation pathway is inhibited by the absence of molecular oxygen, or that the microbes performing the degradation are not proliferating under these conditions. Comparison of distributions of ladderane fatty acids indicates that short chain ladderane fatty acids are mostly produced in the water column and at the sediment surface, before being preserved deeper in the sediments. Short chain ladderane fatty acids were abundant in Arabian Sea and Peru Margin sediments (ODP Leg 201), often in higher concentrations than the original ladderane fatty acids. In a sediment core taken from within the Arabian Sea OMZ, short chain ladderanes made up more than 90% of the total ladderanes at depths greater than 5 cm below sea floor. We also found short chain ladderanes in higher concentrations in hydrolysed sediment residues compared to those freely occurring in lipid extracts, suggesting that they had become bound to the sediment matrix. Furthermore, these matrix-bound short chain ladderanes were found at greater sediment depths than short chain ladderanes in the lipid extract, suggesting that binding to the sediment matrix aids the preservation of these lipids. Though sedimentary degradation of short chain ladderane fatty acids did occur, it appeared to be at a slower rate than that of the original ladderane fatty acids, and short chain ladderane fatty acids were found in sediments from the Late Pleistocene (similar to 100 kyr). Together these results suggest that the oxic degradation products of ladderane fatty acids may be suitable biomarkers for past anammox activity in OMZs

Editorial: Current Topics in Marine Organic Biogeochemical Research

Complex and incompletely understood chemical, biological and physical processes affect the delivery, transport, and storage of organic matter (OM) in the ocean. Atmospheric and climate-relevant gases, primarily carbon dioxide, are closely linked to the production and flux of organic matter within the ocean via the biological pump. Marine and continental OM is transformed in the water column, some is transported to the sediments where a fraction is stored over geological time, and some marine OM is released to the atmosphere. These functions are intimately linked to global nutrient cycles and ecosystem processes. Perhaps indicative of how the field has matured, the multidisciplinary nature of ocean carbon studies and the importance of biological processes in organic carbon cycling has resulted in a morphing of “marine organic geochemistry” into “marine organic biogeochemistry.” Marine organic biogeochemistry now provides a molecular-level window onto the functioning and scale of processes that control the behavior of OM in the ocean. New sampling tools, analytical methods, and data handling capabilities have been applied to marine chemistry since the 1970s; and in tandem with coordinated, international and interdisciplinary research programs, this has led to explosive growth of marine organic biogeochemistry

Evidence for anaerobic methane oxidation by archaea in euxinic waters of the Black Sea

13C-depleted archaeal-derived ether-bound cyclic biphytanes occur in particulate organic matter from the euxinic waters of the Black Sea. These strongly 13C-depleted biomarkers (up to −58 ) provide evidence that archaea are consuming methane in euxinic waters. 13C-depleted archaeal compounds were absent from surface sediments, suggesting that the responsible organisms are in low abundance and leave no characteristic fingerprint in the sedimentary recor

Temporal and spatial variation in tetraether membrane lipids of marine Crenarchaeota in particulate organic matter: implications for TEX86 paleothermometry

The TEX86 is a new temperature proxy which is based on the number of cyclopentane moieties in the glycerol dialkyl glycerol tetraether (GDGT) lipids of the membranes of Crenarchaeota that occur ubiquitously in oceans and shelf seas. This proxy was calibrated by core top sediments, but it is as yet not clear during which season and at which depth in the water column the GDGT signal used for TEX86 paleothermometry is biosynthesized. Here we analyzed >200 particulate organic matter (POM) samples from 11 different marine settings for TEX86. This revealed that the GDGTs occur seasonally in surface waters and occur in higher abundances during the winter and spring months. The depth distribution showed that GDGTs generally appeared in higher amounts below 100 m depth in the water column. However, the TEX86 values for waters below the photic zone (150–1500 m) did not correlate with the in situ temperature but rather correlated linearly with surface temperature. The TEX86 for POM from the upper 100 m showed a linear correlation with in situ temperature, which was nearly identical to the previously reported core top equation. The correlation of all POM samples with surface temperature was also strikingly similar to the core top correlation. These findings demonstrate that the GDGT signal which reaches the sediment is mainly derived from the upper 100 m of the water column. This may be caused by the fact that GDGTs from the photic zone are much more effectively transported to the sediment by grazing and repackaging in large particles than GDGTs from deeper waters

Archaeal lipids and anaerobic oxidation of methane : A comparative study of the euxinic Black Sea and Cariaco Basin

The Black Sea and the Cariaco Basin are both large, euxinic marine basins in which methane concentrations are high and where anaerobic oxidation of methane (AOM) is an important part of the carbon cycle. AOM can be recognized by lipid biomarkers that are specific to methanotrophic archaea involved and by strongly 13C-depleted isotope compositions consistent with uptake of 13C-depleted methane-derived carbon. The working hypothesis for our investigation was that AOM in both the Black Sea and Cariaco Basin would generate measurable diagnostic biomarkers and isotope depletions. To test this hypothesis, we analyzed particulate matter and surface sediments for intact glycerol dialkyl glycerol tetreaethers (GDGTs), components of archaeal membrane lipids, and measured stable carbon isotope compositions of their constituent biphytanes. Several GDGTs and strongly 13C-depleted biphytanes indicative of AOM were present in the deep anoxic region of the Black Sea (>700 m). Unexpectedly, this biomarker signal was not detected in the upper anoxic zone of the Black Sea or in the entire water column of the Cariaco Basin, even though previous studies had shown high rates of AOM to occur in both basins. It is possible that the AOM-derived biomarker signal is masked by archaeal lipids derived from non-methanotrophic archaea which utilize 13C-enriched carbon substrates. Alternately we speculate that the methanotrophic community may be highly diverse in euxinic basins, possibly producing another suite of biomarkers that we did not measure. This conclusion will require further testing by coordinated organic geochemicalmicrobial ecology studies

Archaeal tetraether membrane lipid fluxes in the northeastern Pacific and the Arabian Sea: implications for TEX86 paleothermometry

The newly introduced temperature proxy, the tetraether index of archaeal lipids with 86 carbon atoms (TEX86), is based on the number of cyclopentane moieties in the glycerol dialkyl glycerol tetraether (GDGT) lipids of marine Crenarchaeota. The composition of sedimentary GDGTs used for TEX86 paleothermometry is thought to reflect sea surface temperature (SST). However, marine Crenarchaeota occur ubiquitously in the world oceans over the entire depth range and not just in surface waters. We analyzed the GDGT distribution in settling particulate organic matter collected in sediment traps from the northeastern Pacific Ocean and the Arabian Sea to investigate the seasonal and spatial distribution of the fluxes of crenarchaeotal GDGTs and the origin of the TEX86 signal transported to the sediment. In both settings the TEX86 measured at all trap deployment depths reflects SST. In the Arabian Sea, analysis of an annual time series showed that the SST estimate based on TEX86 in the shallowest trap at 500 m followed the in situ SST with a 1 to 3 week time delay, likely caused by the relatively low settling speed of sinking particles. This revealed that the GDGT signal that reaches deeper water is derived from the upper water column rather than in situ production of GDGTs. The GDGT temperature signal in deeper traps at 1500 m and 3000 m did not show a seasonal cyclicity observed in the 500 m trap but rather reflected the annual mean SST. This is probably due to a homogenization of the TEX86 SST signal carried by particles as they ultimately reach the interior of the ocean. Our data confirm the use of TEX86 as a temperature proxy of surface ocean waters

Organic sulphur compounds formed during early diagenesis in the Black Sea

Sediments from Units I and II of the Black Sea were analyzed to assess the early diagenetic formation of organic sulfur compounds (OSC). A series of isomeric C28-2,4-dialkylthiophenes was found at low concentrations in surface sediments. OSC with C25-highly branched isoprenoid (RBI) skeletons were not found in surface sediments even though there was a rapid decrease in the concentrations of C25-HBI alkenes in the same sediments, nor were OSC found which contained carbon skeletons derived from C37C39 alkenes which are also abundant in the sediment. As depth increased in Unit I, concentrations of the C28-2,4-dialkylthiophenes decreased while C25- and C30-HBI thiolanes appeared at the bottom of Unit I and increased in concentration into Unit II. Treatment of macromolecular material from sediment extracts with Raney nickel yielded phytane as the dominant hydrocarbon as well as series of C25- and C30-HBI hydrocarbons, -carotane, and isorenieratane. These latter two components were generally absent from the free hydrocarbon fractions. These results indicate that sulfur incorporation into functionalized lipids can occur during the very early stages of sedimentary diagenesis, even at the sediment-water interface. On the other hand, the rapid loss of C25-HBI alkenes in surface sediments could not be accounted for by sulfur quenching, and other potential OSC-precursors, such as C37-C39 alkenes, also apparently did not incorporate sulfur into readily-analyzable material

Archaea mediate anaerobic oxidation of methane in deep euxinic waters of the Black Sea

We evaluate anaerobic oxidation of methane (AOM) in the Black Sea water column by determining distributions of archaea-specific glyceryl dialkyl glyceryl tetraethers (GDGTs) and 13C isotopic compositions of their constituent biphytanes in suspended particulate matter (SPM), sinking particulate matter collected in sediment traps, and surface sediments. We also determined isotopic compositions of fatty acids specific to sulfate-reducing bacteria to test for biomarker and isotopic evidence of a syntrophic relationship between archaea and sulfate-reducing bacteria in carrying out AOM. Bicyclic and tricyclic GDGTs and their constituent 13C-depleted monocyclic and bicyclic biphytanes (down to −67) indicative of archaea involved in AOM were present in SPM in the anoxic zone below 700 m depth. In contrast, GDGT-0 and crenarchaeol derived from planktonic crenarchaeota dominated the GDGT distributions in the oxic surface and shallow anoxic waters. Fatty acids indicative of sulfate-reducing bacteria (i.e., iso- and anteiso-C15) were not strongly isotopically depleted (e.g., −32 to −25), although anteiso-C15 was 5 more depleted in 13C than iso-C15. Our results suggest that either AOM is carried out by archaea independent of sulfate-reducing bacteria or those sulfate-reducing bacteria involved in a syntrophy with methane-oxidizing archaea constitute a small enough fraction of the total sulfate-reducing bacterial community that an isotope depletion in their fatty acids is not readily detected. Sinking particulate material collected in sediment traps and the underlying sediments in the anoxic zone contained the biomarker and isotope signature of upperwater column archaea. AOM-specific GDGTs and 13C-depleted biphytanes characteristic of the SPM in the deep anoxic zone are not incorporated into sinking particles and are not efficiently transported to the sediments. This observation suggests that sediments may not always record AOM in overlying euxinic water columns and helps explain the absence of AOM-derived biomarkers in sediments deposited during past periods of elevated levels of methane in the ocean

Chemocline of the Black Sea - Reply

Our paper is about sulphur-bound biomarkers and their isotope compositions in the Black Sea. Anthropogenic effects are not a prerequisite for a shallow chemocline

C32-C36 polymethyl alkenes in Black Sea sediments

A series of novel C32-C36 polymethylhentriacontenes has been identified in near-surface sediments of the Black Sea. Hydrogenation of these components indicated that they possess the 5,8,14,24,27-pentamethyl-, 8,14,24,27-tetramethyl-, 8,14,24-trimethyl-, 8,14-dimethyl-, and 14- and 15-methylhentriacontane C skeletons, which was confirmed by synthesis of an authentic standard of the C34 member. The dominant C36 member of this series contains eight double bonds. The 13C content of these polymethylhentricontenes indicated that they are probably biosynthesized by photoautotrophs and that blooming or bicarbonate pumping affected the 13C content of the fixed C. Their structures suggest that they were biosynthesized by methylation of an unsaturated n-C31 precursor at specific positions in a well-defined sequence. This biosynthetic pathway represents an alternative for the biosynthesis of isoprenoid-like components