Peatland warming influences the abundance and distribution of branched tetraether lipids: Implications for temperature reconstruction.

Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are bacterial membrane lipids whose distribution in peatland soils serves as an important proxy for past climate changes due to strong linear correlations with temperature in modern environments. However, commonly used brGDGT-based temperature models are characterized by high uncertainty (ca. 4 °C) and these calibrations can show implausible correlations when applied at an ecosystem level. This lack of accuracy is often attributed to our limited understanding of the exact mechanisms behind the relationship between brGDGTs and temperature and the potential effect of temperature-independent factors on brGDGT distribution. Here, we examine the abundance and distribution of brGDGTs in a boreal peatland after four years of in-situ warming (+0, +2.25, +4.5, +6.75 and +9 °C). We observed that with warming, concentrations of total brGDGTs increased. Furthermore, we determined a shift in brGDGT distribution in the surface aerobic layers of the acrotelm (0-30 cm depth), whereas no detectable change was observed at deeper anaerobic depths (>40 cm), possibly due to limited microbial activity. The response of brGDGTs to warming was also reflected by a strong increase in the methylation index of 5-methyl brGDGTs (MBT'5Me), classically used as a temperature proxy. Further, the relationship between the MBT'5Me index and soil temperature differed between 0-10, 10-20 and 20-30 cm depth, highlighting depth-specific response of brGDGTs to warming, which should be considered in paleoenvironmental and paleoecological studies. As the bacterial community composition was generally unaltered, the rapid changes in brGDGT distribution argue for a physiological adaptation of the microorganisms producing these lipids. Finally, soil temperature and water table depth were better predictors of brGDGT concentration and distribution, highlighting the potential for these drivers to impact brGDGT-based proxies. To summarize, our results provide insights on the response of brGDGT source microorganisms to soil warming and underscore brGDGTs as viable temperature proxies for better understanding of climatic perturbation in peatlands.

New insights into the mechanism of phosphate release during particulate organic matter photodegradation based on optical and molecular signatures.

Phosphate release from particulate organic matter (POM) dominates phosphorus (P) cycling in aquatic ecosystems. However, the mechanisms underlying P release from POM remain poorly understood because of complex fractionation and analytical challenges. In this study, the release of dissolved inorganic phosphate (DIP) during POM photodegradation was assessed using excitation-emission matrix (EEM) fluorescence spectroscopy and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). POM in suspension was significantly photodegraded under light irradiation, concomitantly with the production and release of DIP in the aqueous solution. Chemical sequential extraction revealed that organic phosphorus (OP) in POM participated in photochemical reactions. Moreover, FT-ICR MS analysis revealed that the average molecular weight of P-containing formulas decreased from 374.2 to 340.1 Da. Formulas containing P with a lower oxidation degree and unsaturation were preferentially photodegraded, generating oxygen-enriched and saturated formula compounds, such as protein- and carbohydrate-like P-containing formulas, benefiting further utilization of P by organisms. Reactive oxygen species played an important role in the photodegradation of POM, and excited triplet state chromophoric dissolved organic matter (3CDOM*) was mainly responsible for POM photodegradation. These results provide new insights into the P biogeochemical cycle and POM photodegradation in aquatic ecosystems.

Membrane lipid adaptation of soil Bacteroidetes isolates to temperature and pH.

3-hydroxy fatty acids (3-OH FAs) are characteristic components of the Gram-negative bacterial membrane, recently proposed as promising temperature and pH (paleo) proxies in soil. Nevertheless, to date, the relationships between the 3-OH FA distribution and temperature/pH are only based on empirical studies, with no ground truthing work at the microbial level. This work investigated the influence of growth temperature and pH on the lipid composition of three strains of soil Gram-negative bacteria belonging to the Bacteroidetes phylum. Even though non-hydroxy FAs were more abundant than 3-OH FAs in the investigated strains, our results suggest that 3-OH FAs are involved in the membrane adaptation of these bacteria to temperature. The strains shared a common adaptation mechanism to temperature, with a significant increase in the ratio of anteiso vs. iso or normal 3-OH FAs at lower temperature. In contrast with temperature, no common adaptation mechanism to pH was observed, as the variations in the FA lipid profiles differed from one strain to another. We suggest that models reconstructing environmental changes in soils should include the whole suite of 3-OH FAs present in the membrane of Gram-negative bacteria, as all of them could be influenced by temperature or pH at the microbial level.

Improving the design and implementation of sediment fingerprinting studies: summary and outcomes of the TRACING 2021 Scientific School.

Purpose: Identifying best practices for sediment fingerprinting or tracing is important to allow the quantification of sediment contributions from catchment sources. Although sediment fingerprinting has been applied with reasonable success, the deployment of this method remains associated with many issues and limitations. Methods: Seminars and debates were organised during a 4-day Thematic School in October 2021 to come up with concrete suggestions to improve the design and implementation of tracing methods. Results: First, we suggest a better use of geomorphological information to improve study design. Researchers are invited to scrutinise all the knowledge available on the catchment of interest, and to obtain multiple lines of evidence regarding sediment source contributions. Second, we think that scientific knowledge could be improved with local knowledge and we propose a scale of participation describing different levels of involvement of locals in research. Third, we recommend the use of state-of-the-art sediment tracing protocols to conduct sampling, deal with particle size, and examine data before modelling and accounting for the hydro-meteorological context under investigation. Fourth, we promote best practices in modelling, including the importance of running multiple models, selecting appropriate tracers, and reporting on model errors and uncertainty. Fifth, we suggest best practices to share tracing data and samples, which will increase the visibility of the fingerprinting technique in geoscience. Sixth, we suggest that a better formulation of hypotheses could improve our knowledge about erosion and sediment transport processes in a more unified way. Conclusion: With the suggested improvements, sediment fingerprinting, which is interdisciplinary in nature, could play a major role to meet the current and future challenges associated with global change. Supplementary information: The online version contains supplementary material available at 10.1007/s11368-022-03203-1.

Molecular acclimation of Halobacterium salinarum to halite brine inclusions.

Halophilic microorganisms have long been known to survive within the brine inclusions of salt crystals, as evidenced by the change in color for salt crystals containing pigmented halophiles. However, the molecular mechanisms allowing this survival has remained an open question for decades. While protocols for the surface sterilization of halite (NaCl) have enabled isolation of cells and DNA from within halite brine inclusions, "-omics" based approaches have faced two main technical challenges: (1) removal of all contaminating organic biomolecules (including proteins) from halite surfaces, and (2) performing selective biomolecule extractions directly from cells contained within halite brine inclusions with sufficient speed to avoid modifications in gene expression during extraction. In this study, we tested different methods to resolve these two technical challenges. Following this method development, we then applied the optimized methods to perform the first examination of the early acclimation of a model haloarchaeon (Halobacterium salinarum NRC-1) to halite brine inclusions. Examinations of the proteome of Halobacterium cells two months post-evaporation revealed a high degree of similarity with stationary phase liquid cultures, but with a sharp down-regulation of ribosomal proteins. While proteins for central metabolism were part of the shared proteome between liquid cultures and halite brine inclusions, proteins involved in cell mobility (archaellum, gas vesicles) were either absent or less abundant in halite samples. Proteins unique to cells within brine inclusions included transporters, suggesting modified interactions between cells and the surrounding brine inclusion microenvironment. The methods and hypotheses presented here enable future studies of the survival of halophiles in both culture model and natural halite systems.

Characterization of Functional Groups in Estuarine Dissolved Organic Matter by DNP-enhanced 15 N and 13 C Solid-State NMR.

Estuaries are key ecosystems with unique biodiversity and are of high economic importance. Along the estuaries, variations in environmental parameters, such as salinity and light penetration, can modify the characteristics of dissolved organic matter (DOM). Nevertheless, there is still limited information about the atomic-level transformations of DOM in this ecosystem. Solid-state NMR spectroscopy provides unique insights into the nature of functional groups in DOM. A major limitation of this technique is its lack of sensivity, which results in experimental time of tens of hours for the acquisition of 13 C NMR spectra and generally precludes the observation of 15 N nuclei for DOM. We show here how the sensitivity of solid-state NMR experiments on DOM of Seine estuary can be enhanced using dynamic nuclear polarization (DNP) under magic-angle spinning. This technique allows the acquisition of 13 C NMR spectra of these samples in few minutes, instead of hours for conventional solid-state NMR. Both conventional and DNP-enhanced 13 C NMR spectra indicate that the 13 C local environments in DOM are not strongly modified along the Seine estuary. Furthermore, the sensitivity gain provided by the DNP allows the detection of 15 N NMR signal of DOM, in spite of the low nitrogen content. These spectra reveal that the majority of nitrogen is in the amide form in these DOM samples and show an increased disorder around these amide groups near the mouth of the Seine.

Multiple stages of plant root calcification deciphered by chemical and micromorphological analyses.

Rhizoliths, that is, roots fossilized by secondary carbonates, have been known for ages and are increasingly used for paleoenvironmental reconstructions. However, knowledge about their formation mechanisms remains limited. This study reports the mineralogical and chemical characterization of rhizoliths at different stages of mineralization and fossilization in the Late Pleistocene loess-paleosol sequence of Nussloch (SW Germany). Scanning electron microscopy coupled with elemental mapping and 13 C solid-state nuclear magnetic resonance were used to concomitantly characterize the mineral and organic matter of the rhizoliths. These joint analyses showed for the first time that large rhizoliths are not necessarily remains of single large roots but consist of numerous microrhizoliths as remains of fine roots, formed mainly by calcium carbonates with only low amounts of Mg and Si. They further revealed that the precipitation of secondary carbonates occurs not only around, but also within the plant root and that fossilization leads to the selective preservation of recalcitrant root biopolymers-lignin and suberin. The precipitation of secondary carbonates was observed to occur first around fine roots, the epidermis acting as a first barrier, and then within the root, within the cortex cells, and even sometimes around the phloem and within the xylem. This study suggests that the calcification of plant roots starts during the lifetime of the plant and continues after its death. This has to be systematically investigated to understand the stratigraphic context before using (micro)rhizoliths for paleoenvironmental reconstructions in terrestrial sediments.

Improved Isolation of SlaA and SlaB S-layer proteins in Sulfolobus acidocaldarius.

The Sulfolobus acidocaldarius S-layer is composed of two main proteins: SlaA, which forms the ordered structure of the S-layer matrix, and SlaB, which supports and anchors the S-layer into the tetraether lipid membrane. While SlaA has previously been purified by exploiting its thermotolerance and high resistance to detergents, SlaB has resisted isolation, particularly from the cell membrane. Removal of proteins other than those of the S-layer is especially difficult if large batch-scale culture volumes are unavailable. Here, we describe a benchtop-scale protocol for the purification of SlaA from S. acidocaldarius, enabling isolation of SlaB using size exclusion chromatography (gel filtration). Using this protocol, we were able to identify for the first time tetraether lipids strongly attached to SlaB via heat- and detergent-resistant interactions.

Photochemical studies on bis-sulfide and -sulfone tethered polyenic derivatives.

This study focusses on the [2 + 2]-photocycloaddition of a symmetric polyenic system tethered by an aryl bis-sulfide or sulfone platform. Using direct irradiation or photosensitization, no expected ladderane product was isolated. In most cases, only tricyclic products including a single cyclobutane moiety were formed. Irradiation of bis-acrylic precursors in water with encapsulation by a host (cyclodextrin or cucurbituril) provided a stereoselective access to valuable cyclobutyl adducts.

Disentangling interactions between microbial communities and roots in deep subsoil.

Soils, paleosols and terrestrial sediments serve as archives for studying climate change, and represent important terrestrial carbon pools. Archive functioning relies on the chronological integrity of the respective units. Incorporation of younger organic matter (OM) e.g. by plant roots and associated microorganisms into deep subsoil and underlying soil parent material may reduce reliability of paleoenvironmental records and stability of buried OM. Long-term effects of sedimentary characteristics and deep rooting on deep subsoil microbial communities remain largely unknown. We characterized fossil and living microbial communities based on molecular markers in a Central European Late Pleistocene loess-paleosol sequence containing recent and ancient roots with ages of several millenia. The molecular approach, comprising free and phospholipid fatty acids (FAs), core and intact polar glycerol dialkyl glycerol tetraethers (GDGTs), as well as 16S rRNA genes from bacterial DNA, revealed the presence of living microorganisms along the sequence, with bacterial community composition comparable to that of modern topsoils. Up to 88% redundancy between bacterial genetic fingerprint and molecular signature of fossil microorganisms suggested a time-integrated signal of the molecular markers accumulated over a time span potentially lasting from sedimentation over one or more rooting phases until today. Free FAs, core GDGTs and DNA, considered as remains of fossil microorganisms, corresponded with ancient and recent root quantities, whereas phospholipid FAs and intact polar GDGTs, presumably derived from living microorganisms, correlated only with living roots. The biogeochemical and ecological disequilibrium induced by postsedimentary rooting may entail long-term microbial processes like OM mineralization, which may continue even millenia after the lifetime of the root. Deep roots and their fossil remains have been observed in various terrestrial settings, and roots as well as associated microorganisms cause both, OM incorporation and mineralization. Therefore, these findings are crucial for improved understanding of OM dynamics and carbon sequestration potential in deep subsoils.