Search

Long chain alkyl diols are lipids that occur ubiquitously in marine sediments and are used as a proxy for sea surface temperature (SST), using the Long chain Diol Index (LDI), and for upwelling intensity/high nutrient conditions. The distribution of 1,13- and 1,15-diols has been documented in open marine and lacustrine sediments and suspended particulate matter, but rarely in coastal seas receiving a significant riverine, and thus continental organic matter, input. Here we studied the distribution of diols in four shelf seas with major river outflows: the Gulf of Lion, the Kara Sea, the Amazon shelf and the Berau delta, covering a wide range of climate conditions. The relative abundance of the C-32 1,15-diol is consistently higher close to the river mouth and particularly in the suspended particulate matter of the rivers suggesting a terrigenous source. This is supported by statistical analysis which points out a significant positive correlation between the C-32 1,15-diol and the Branched and Isoprenoid Tetraether index, a proxy reflecting soil and riverine input in marine environments. However, the C-32 1,15-diol was not detected in soils and is unlikely to be derived from vegetation, suggesting that the C-32 1,15-diol is mainly produced in rivers. This agrees with the observation that it is a dominant diol in most cultivated freshwater eustigmatophyte algae. We, therefore, suggest that the relative abundance of the C-32 1,15-diol can potentially be used as a proxy for riverine organic matter input in shelf seas. Our results also show that long chain alkyl diols delivered by rivers can substantially affect LDI-reconstructed SSTs in coastal regions close to river mouths. (C) 2016 Elsevier Ltd. All rights reserved.

Marine endosymbiotic heterocystous cyanobacteria make unique heterocyst glycolipids (HGs) containing pentose (C5) moieties. Functionally similar HGs with hexose (C6) moieties found in free-living cyanobacteria occur in the sedimentary record, but C5 HGs have not been documented in the natural environment. Here we developed a high performance liquid chromatography multiple reaction monitoring (MRM) mass spectrometry (HPLC–MS2) method specific for trace analysis of long chain C5 HGs and applied it to cultures of Rhizosolenia clevei Ostenfeld and its symbiont Richelia intracellularis which were found to contain C5 HGs and no C6 HGs. The method was then applied to suspended particulate matter (SPM) and surface sediment from the Amazon plume region known to harbor marine diatoms carrying heterocystous cyanobacteria as endosymbionts. C5 HGs were detected in both marine SPM and surface sediments, but not in SPM or surface sediment from freshwater settings in the Amazon basin. Rather, the latter contained C6 HGs, established biomarkers for free-living heterocystous cyanobacteria. Our results indicate that the C5 HGs may be potential biomarkers for marine endosymbiotic heterocystous cyanobacteria.

The stability and potential vulnerability of soil organic matter (SOM) to global change remains incompletely understood due to the complex processes involved in its formation and turnover. Here we combine compound-specific radiocarbon analysis with fraction-specific and bulk-level radiocarbon measurements in order to further elucidate controls on SOM dynamics in a temperate and sub-alpine forested ecosystem. Radiocarbon contents of individual organic compounds isolated from the same soil interval generally exhibit greater variation than those among corresponding operationally-defined fractions. Notably, markedly older ages of long-chain plant leaf wax lipids (n-alkanoic acids) imply that they reflect a highly stable carbon pool. Furthermore, marked 14C variations among shorter- and longer-chain n-alkanoic acid homologues suggest that they track different SOM pools. Extremes in SOM dynamics thus manifest themselves within a single compound class. This exploratory study highlights the potential of compound-specific radiocarbon analysis for understanding SOM dynamics in ecosystems potentially vulnerable to global change.

Members of the marine Roseobacter clade can degrade dimethylsulfoniopropionate (DMSP) via competing pathways releasing either methanethiol (MeSH) or dimethyl sulfide (DMS). Deuterium-labeled [2H6]DMSP and the synthetic DMSP analogue dimethyltelluriopropionate (DMTeP) were used in feeding experiments with the Roseobacter clade members Phaeobacter gallaeciensis DSM 17395 and Ruegeria pomeroyi DSS-3, and their volatile metabolites were analyzed by closed-loop stripping and solid-phase microextraction coupled to GC–MS. Feeding experiments with [2H6]DMSP resulted in the incorporation of a deuterium label into MeSH and DMS. Knockout of relevant genes from the known DMSP demethylation pathway to MeSH showed in both species a residual production of [2H3]MeSH, suggesting that a second demethylation pathway is active. The role of DMSP degradation pathways for MeSH and DMS formation was further investigated by using the synthetic analogue DMTeP as a probe in feeding experiments with the wild-type strain and knockout mutants. Feeding of DMTeP to the R. pomeroyi knockout mutant resulted in a diminished, but not abolished production of demethylation pathway products. These results further corroborated the proposed second demethylation activity in R. pomeroyi. Isotopically labeled [2H3]methionine and 34SO42−, synthesized from elemental 34S8, were tested to identify alternative sulfur sources besides DMSP for the MeSH production in P. gallaeciensis. Methionine proved to be a viable sulfur source for the MeSH volatiles, whereas incorporation of labeling from sulfate was not observed. Moreover, the utilization of selenite and selenate salts by marine alphaproteobacteria for the production of methylated selenium volatiles was explored and resulted in the production of numerous methaneselenol-derived volatiles via reduction and methylation. The pathway of selenate/selenite reduction, however, proved to be strictly separated from sulfate reduction.

The distributions of branched glycerol dialkyl glycerol tetraethers (brGDGTs), which are transported from the soils where they are predominantly produced to marine sediments via rivers, have been applied in reconstructing mean annual air temperatures (MAT) and pH of soils. However, paleoclimate reconstructions using sedimentary brGDGTs have proven difficult in arid regions including the Iberian Peninsula. Recently, six novel 6-methyl brGDGTs have been described using new analytical methods (in addition to the nine 5-methyl brGDGTs previously used for climate reconstructions), and so new pH and MAT calibrations have been developed that in a set of global soil samples were shown to improve the accuracy of reconstructions, especially in arid regions. Because of this we decided to apply the new method to separate the 5- and 6-methyl isomers along with the novel calibrations to a sample set that runs in a transect from source to sink along the Tagus River and out to the deep ocean off the Portuguese margin and spans the last 6,000 years in order to determine if it improves paleoclimate reconstructions in this area. We found that although pH reconstructions in the soils were improved using the new calibration, MAT reconstructions were not much better even with the separation of the 5- and 6-methyl brGDGTs. This confirmed the conclusion of previous studies that the amount of aquatically produced brGDGTs is overwhelming the soil derived ones in marine sediments and complicating MAT reconstructions in the region. Additionally, the new separation revealed a strong relationship between the new degree of cyclization (DC’) of the brGDGTs and MAT not seen before that could be making temperature reconstructions in this and other arid regions difficult.

In this study, we investigated the seasonal and spatial pattern of sedimentary organic matter (SOM) in five floodplain lakes of the central Amazon basin (Cabaliana, Janauaca, Canaçari, Miratuba, and Curuai) which have different morphologies, hydrodynamics and vegetation coverages. Surface sediments were collected in four hydrological seasons: low water (LW), rising water (RW), high water (HW) and falling water (FW) in 2009 and 2010. We investigated commonly used bulk geochemical tracers such as the C : N ratio and the stable isotopic composition of organic carbon (δ13Corg). These results were compared with lignin-phenol parameters as an indicator of vascular plant detritus and branched glycerol dialkyl glycerol tetraethers (brGDGTs) to trace the input of soil organic matter (OM) from land to the aquatic settings. We also applied the isoprenoid GDGT (iGDGT) crenarchaeol as an indicator of riverine suspended particulate organic matter (SPOM). Our data showed that during the RW and FW seasons, the surface sediments were enriched in lignin and brGDGTs in comparison to other seasons. Our study also indicated that floodplain lake sediments primarily consisted of allochthonous, C3 plant-derived OM. However, a downstream increase in C4 macrophyte derived OM contribution was observed along the gradient of increasing open waters, i.e. from upstream to downstream. Accordingly, we attribute temporal and spatial difference in SOM composition to the hydrological dynamics between the floodplain lakes and the surrounding flooded forests.

Suspended particulate matter (SPM) was collected along the Amazon River in the central Amazon basin and in three tributaries during the rising water (RW), high water (HW), falling water (FW) and low water (LW) season. Changes in the concentration and the distribution of branched glycerol dialkyl glycerol tetraethers (brGDGTs), i.e., the methylation index of branched tetraethers (MBT) and the cyclization of brGDGTs (CBT), were seen in the Amazon main stem. The highest concentration of core lipid (CL) brGDGTs normalized to particulate organic carbon (POC) was found during the HW season. During the HW season the MBT and CBT in the Amazon main stem was also most similar to that of lowland Amazon (terra firme) soils, indicating that the highest input of soil-derived brGDGTs occurred due to increased water runoff. During the other seasons the MBT and CBT indicated an increased influence of in situ production of brGDGTs even though soils remained the main source of brGDGTs. Our results reveal that the influence of seasonal variation is relatively small, but can be clearly detected. Crenarchaeol was mostly produced in the river. Its concentration was lower during the HW season compared to that of the other seasons. Hence, our study shows the complexity of processes that influence the GDGT distribution during the transport from land to ocean. It emphasizes the importance of a detailed study of a river basin to interpret the MBT/CBT and BIT records for paleo reconstructions in adjacent marine setting.

To trace the origin of branched glycerol dialkyl glycerol tetraethers (brGDGTs), their distribution in soils and suspended particulate matter (SPM) of Amazonian rivers and floodplain lakes (varzeas) was studied. Differences in distribution between river SPM and surrounding (lowland) soils suggests an additional brGDGT source to eroded soils in the lowland drainage basin. Erosion of high Andean soils (above 2500 m in altitude) has no major influence because its brGDGT distribution differs substantially from that in river SPM. Furthermore, SPM in the Tapajos River, a tributary that does not derive from the Andes, has a virtually identical brGDGT distribution to that of the Amazon main stem. The higher proportion of phospholipid-derived brGDGTs in river SPM compared to soils indicates that in situ production in the Amazon is an additional source for riverine brGDGTs. This affects the methylation and cyclization index of brGDGTs (MBT-CBT), resulting in slightly lower MBT-CBT-derived temperatures and slightly higher CBT-derived pH values, i.e., between the pH of the basin soil and that of the river. Since the difference between MBT-CBT-derived temperatures of Amazon River SPM and the surrounding soils is relatively small (2 degrees C) compared to other aquatic systems (for lakes a difference of similar to 10 degrees C has been observed), it might still be possible to trace large climate changes in the Amazon basin with the MBT-CBT using river fan cores. However, variations in in situ production of brGDGTs in the Amazon River over time and space have to be evaluated in the future. Likewise, in situ production may affect the application of the MBT-CBT paleothermometer in other river systems. Our results also show that crenarchaeol is primarily produced in the Amazon River and that its varying production influences the branched vs. isoprenoid tetraether (BIT) index. This indicates that the BIT index not only represents the input of soil organic carbon to the river but is also affected by in situ production of brGDGTs and crenarchaeol.

Organisms of the Roseobacter clade are an important component in marine ecosystems, partially due to their metabolic variety. Not much is known, however, about the physiological diversity of different roseobacters present within one habitat. By using serial dilution cultures with low-nutrient media seven roseobacter strains, co-occurring during a phytoplankton bloom in the southern North Sea, were obtained in this study. Physiological characterization exhibited distinct substrate spectra of the isolates. Although no isolate showed growth on algal osmolyte dimethylsulfoniopropionate (DMSP), feeding experiments revealed that all new strains converted [²H₆]DMSP into a variety of volatile compounds. Six strains mainly decomposed DMSP via the demethylation pathway, but four strains were also capable of cleaving DMSP to DMS and acrylate. It is hypothesized that the great physiological diversity of the roseobacters reflects their ability to inhabit different ecological niches and enables the organisms to cope differently with changing substrate supplies during phytoplankton blooms. Denaturing gradient gel electrophoresis and sequencing of excised bands resulted in detection of five additional roseobacters. Three of these sequences showed affiliation with three of the four major clusters of the Roseobacter clade, consisting predominantly of uncultured organisms (i.e. the Roseobacter clade-affiliated (RCA)), the NAC11-7 and the CHAB-I-5 clusters.

In order to trace the transport of soil organic carbon (OC) in the lower Amazon basin, we investigated the distributions of crenarchaeol and branched glycerol dialkyl glycerol tetraethers (GDGTs) by analyzing riverbed sediments and river suspended particulate matter (SPM) collected in the Solimoes-Amazon River mainstem and its tributaries. The Branched and Isoprenoid Tetraether (BIT) index, a proxy for river-transported soil OC into the ocean, was determined from the distributions of these GDGTs. The GDGT-derived parameters were compared with other bulk geochemical data (i.e. C:N ratio and stable carbon isotopic composition). The GDGT-derived and bulk geochemical data indicate that riverine SPM and riverbed sediments in the lower Amazon River and its tributaries are a mixture of C3 plant-derived soil OC and aquatic-derived OC. The branched GDGTs in the SPM and riverbed sediments did not predominantly originate from the high Andes soils (>2500 m in altitude) as was suggested previously. However, further constraint on the soil source area of branched GDGTs was hampered due to the deficiency of soil data from the lower montane forest areas in the Andes. Our study also revealed seasonal and interannual variation in GDGT composition as well as soil OC discharge, which was closely related to the hydrological cycle. By way of a simple binary mixing model using the flux-weighted BIT values at Obidos, the last gauging station in the Amazon River, we estimated that 70–80% of the POC pool in the river was derived of soil OC. However, care should be taken to use the BIT index since it showed a non-conservative behaviour along the river continuum due to the aquatic production of crenarchaeol. Further investigation using a continuous sampling strategy following the full hydrological cycle is required to fully understand how soil-derived GDGT signals are transformed in large tropical river systems through their transport pathway to the ocean.

The volatiles released by Phaeobacter gallaeciensis, Oceanibulbus indolifex and Dinoroseobacter shibae have been investigated by GC-MS, and several MeSH-derived sulfur volatiles have been identified. An important sulfur source in the oceans is the algal metabolite dimethylsulfoniopropionate (DMSP). Labelled [2H6]DMSP was fed to the bacteria to investigate the production of volatiles from this compound through the lysis pathway to [2H6]dimethylsulfide or the demethylation pathway to [2H3]-3-(methylmercapto)propionic acid and lysis to [2H3]MeSH. [2H6]DMSP was efficiently converted to [2H3]MeSH by all three species. Several DMSP derivatives were synthesised and used in feeding experiments. Strong dealkylation activity was observed for the methylated ethyl methyl sulfoniopropionate and dimethylseleniopropionate, as indicated by the formation of EtSH- and MeSeH-derived volatiles, whereas no volatiles were formed from dimethyltelluriopropionate. In contrast, the dealkylation activity for diethylsulfoniopropionate was strongly reduced, resulting in only small amounts of EtSH-derived volatiles accompanied by diethyl sulfide in P. gallaeciensis and O. indolifex, while D. shibae produced the related oxidation product diethyl sulfone. The formation of diethyl sulfide and diethyl sulfone requires the lysis pathway, which is not active for [2H6]DMSP. These observations can be explained by a shifted distribution between the two competing pathways due to a blocked dealkylation of ethylated substrates.