Biology subjects

Hehemann, J.-H.

Publications and source records attributed to Hehemann, J.-H..

15 recordsLinked to original sources

bioRxiv2026

Widespread attraction of marine bacteria to the macroalgae-derived polysaccharide fucoidan

Fucoidan is the second most abundant polysaccharide in brown macroalgae, contributing up to 50% of the dissolved organic matter released daily from these algae into the ocean, substantially influencing the marine carbon cycle. Due to its complex chemical structure, fucoidan is highly resistant to microbial degradation and accumulates on the surface of macroalgae, where it contributes to regulating their microbiome. Here, we used a range of microfluidic assays to quantify the chemotactic and physiological responses of marine bacteria to fucoidan. Our results reveal that all the strains tested (14 out of 14) are attracted to fucoidan produced by two different species of brown macroalgae. Attraction occurred only at high fucoidan concentrations (0.1 to 1 mg/mL), such as those expected in the immediate vicinity of algal surfaces, whereas low concentrations elicited no response or modest chemorepulsion. Although fucoidan did not support bacterial growth under our experimental conditions, it did not impair the utilization of other carbon sources. Together, these findings suggest that fucoidan gradients can act as chemical cues guiding marine bacteria toward the nutrient-rich microenvironment of macroalgal surfaces, pointing to a potential ecological role of fucoidan in shaping macroalgal microbiota, bacterial behavior, and ultimately carbon cycling in the ocean.

microbiology↗

bioRxiv2026

Bacterial capture and lysis on diatom spines reveal a suspension-feeding strategy

Phytoplankton contribute nearly half of global primary production and play a central role in ocean biogeochemistry. Although traditionally viewed as phototrophs, a growing body of evidence indicates that many phytoplankton are mixotrophic, supplementing photosynthesis with heterotrophic acquisition of nutrients and organic matter. Yet, the mechanisms enabling heterotrophic feeding in diatoms remain poorly understood. Here, we show that the chitinous spines of the diatom Conticribra weissflogii can serve as attachment sites for the bacterium Marinobacter adhaerens. Cryo-electron tomography and viability staining revealed that a substantial fraction of spine-associated M. adhaerens undergoes lysis. Consistent with bacterial lysis on the spines, Raman spectroscopy showed that diatom cells bearing spine-associated bacteria acquired more bacteria-derived nitrogen than uncolonized cells. Although microfluidic assays revealed a low bacterial affinity for the spines, our results suggest that attachment can be promoted by fucoidan deposits identified on the spines - a compound for which bacteria showed significantly higher binding affinity than chitin - as well as by their geometry, which encounter rate calculations show to be particularly efficient at intercepting bacteria. Together, these findings reveal a previously undescribed form of suspension-feeding in phytoplankton, reminiscent of the role of zooplankton pseudopods in prey capture. Diatom spines intercept bacteria from the surrounding water, retain them near the cell surface, and facilitate subsequent nutrient acquisition, probably through bacterial lysis. Our results expand the functional repertoire of diatom spines beyond defense and buoyancy, identifying spine-mediated bacterial capture as a previously overlooked pathway contributing to phytoplankton mixotrophy.

microbiology↗

bioRxiv2026

Macroalgal fucoidan can activate the biological carbon pump

Macroalgae secrete complex carbohydrate polymers, their extracellular matrix, as protection against microbial degradation. By resisting breakdown, these carbohydrates can contribute to marine carbon sequestration, though mechanisms, extent, and timescales remain unknown. Using ship-based sampling and experiments, we found that brown macroalgae release 1.7-4.2% of carbon fixation as fucoidan, equivalent to 0.32-0.88 mg fucoidan per gram of dry seaweed tissue per day. A Bayesian model trained on our empirical data, coupled with Monte Carlo simulations suggests an annual global release of 13-37 megatons fucoidan carbon. Moreover, degradation resistance combined with surface-activity enabled fucoidan to act as glue that cross-linked allochthonous organic carbon including microbes and proteins into marine snow. Notably, substantial fucoidan exudation was universally conserved across all tested species and regions. Thus, any brown macroalgal species can be used e.g. via aquafarming to enhance the formation of marine snow.

ecology↗

bioRxiv2024

Roots of coastal plants stabilize carbon fixed by marine algae

Coastal vegetated ecosystems are key-nature based solutions for climate change mitigation. Mangroves, seagrass meadows and saltmarshes contribute to carbon sequestration not only through their photosynthetic activity but also by anchoring sediments with their extensive root systems. By modulating flow coastal vegetation creates a low energy environment for sediment that includes carbon to accumulate. These roots physically stabilize the sediment, prevent erosion and enhance long-term retention of organic carbon. Hence, we hypothesized marine, algae derived organic matter may especially accumulate in plant vegetated ecosystems. We used algal and plant glycans as carbon sequestration proxy to trace the input and stabilization from source to sink and found those molecules in 93 sediment cores across different coastal vegetated ecosystems from temperate to tropical regions. Specific monoclonal antibodies showed algal-derived fucoidans were present in sediments of coastal vegetated ecosystems. Our findings suggest that the restoration of plant ecosystems that fix carbon dioxide, protect coasts and enhance biodiversity should also be enumerated for the stored carbon from distant donors. Conclusively, carbon sequestration is a synergistic outcome of photosynthetic contributors acting in concert across different ecosystems.

biochemistry↗

bioRxiv2024

Selective preservation of fucose-rich oligosaccharides in the North Atlantic Ocean

The ocean has a substantial capacity to store carbon dioxide fixed via photosynthesis in dissolved organic molecules. An estimated 20% of the 660 Gt dissolved organic carbon in the ocean pool consists of structurally uncharacterized oligosaccharides, which appear to resist microbial degradation (Aluwihare et al., 1997). Current technologies lack the sensitivity and molecular resolution to identify these oligosaccharides. Here, we adapted graphitized carbon chromatography to extract and separate marine oligosaccharides for liquid chromatography high resolution mass spectrometry analysis. Using a newly-developed de novo annotation tool, we found 110 oligosaccharide structures in surface and deep ocean seawater at two distant locations in the North Atlantic Ocean. One group of the detected oligosaccharides was found only in surface seawater and consisted of larger and more abundant molecules detected by our analysis. A second group of smaller, less abundant oligosaccharides was detected in both the surface and deep ocean seawater of both sampled locations. The composition of oligosaccharides differed between the surface and deep ocean, with deep ocean samples relatively enriched in hard-to-metabolize deoxy-sugars, and xylose, amino sugars and uronic acids compared to simple hexoses. Notably the deoxy-sugar fucose constituted 35-40% of the monomers in deep-sea oligosaccharides, twice the percentage in surface ocean oligosaccharides. The ubiquity of deep ocean oligosaccharides indicates that they represent a preserved fraction of the carbohydrate pool. Their enrichment in specific monosaccharides suggests selective preservation of fucose-rich oligosaccharides in the deep ocean.

ecology↗

bioRxiv2024

Utilization and degradation of laminarin-based substrates by marine yeasts suggests their niche-specific role in microbial loop dynamics.

In the oceans, the diversity of phytoplankton primary products supports a wide range of microbial heterotrophs, including bacteria and fungi. The organic substrate dynamics within pelagic microbial communities are strongly controlled by microorganismal interactions, resulting in a dense interactome. While the role of bacteria in the microbial loop is well documented, the degradation capacity and substrate specificity of marine fungi, as well as their role and function in metabolic guilds with bacteria, is comparatively less understood. We chose the polysaccharide laminarin, a major product of marine primary production, as well as oligomeric laminarin subunits and monomeric glucose, to study the degradation capacity of eleven marine yeast isolates from the pelagic microbial community of Helgoland Roads. Our aim was to measure yeast growth and correlate degradation yields and putative intermediate degradation products with the size of laminarin-based organic precursor substrates. We developed a reproducible, temporally resolved, high-throughput growth protocol to measure resource-specific yeast growth. Measurement of temporally fine-scaled growth kinetic models of isolates were accompanied with qualitative and quantitative chemical analyses of substrates and degradation intermediates. Our data showed that yeast growth was negatively correlated with oligomer length. Fluorophore-assisted carbohydrate electrophoresis suggested the lack of enzymatic endo-activity for laminarin in yeasts under investigation, suggesting they may occupy a niche in the microbial loop, benefitting from extracellular hydrolysis of carbohydrates by other microorganisms. In terrestrial environments, namely forest soil ecosystems, yeasts have been assigned a similar niche, supporting a prominent role of yeasts in microbial interactomes.

microbiology↗

bioRxiv2024

SusC/D-like proteins in Gammaproteobacteria that utilize fructans

Fructans are ubiquitous in terrestrial ecosystems, however, these glycans are unexplored in the marine environment. We have discovered that the Antarctic gammaproteobacterium Pseudoalteromonas distincta is highly adapted to the degradation of fructose-containing substrates. This is enabled by proteins encoded in several genomic regions, including a fructan polysaccharide utilization locus (PUL). In addition to a glycoside hydrolase from family 32 (GH32), the fructan PUL encodes two proteins that have been described as specific for Bacteroidota and were previously unknown for Gammaproteobacteria: a glycan-binding SusD-like protein and a SusC-like TonB-dependent transporter (TBDT), which work as a complex in glycan import. Proteome analyses and biochemistry results suggest that the SusC/D-like proteins of P. distincta shuttle small-sized inulin-type fructans directly into the cell, where they are degraded by a periplasmic exo-active GH32. A SusD-like protein could provide a competitive adavantage in the absence of extracelluar endo-active inulinases. Comparative genomics identified further SusC/D-like proteins in Gammaproteobacteria, most of which are co-encoded with GH32s, indicative of fructan PULs, and are frequently associated with the marine habitat. Our study thus shows the first known exception to the paradigm that only Bacteroidota use SusC/D-like proteins. It further suggests that fructans contribute to the marine glycan pool.

molecular biology↗

bioRxiv2024

Polyelectrolyte mannan from diatoms reshapes sunlit ocean microbiome

Diatoms are a keystone phylum in Earths ecosystems, specializing in oxygen production and carbohydrate fixation that fuels global food webs. Diatoms host a microbiome, but how they preferentially collect bacteria with complementary traits remains unknown. Here we show that diatoms exude a C6-sulfated -1,3-mannan that serves as a selective carbon source for adapted bacteria. Its structure was resolved by NMR spectroscopy, chromatography, chemical synthesis, and enzymatic dissection. Biochemical, physiological, and structural analyses revealed that specialized Bacteroidota employ a four-enzyme pathway to metabolize this mannan. Metagenomic and transcriptomic data indicate that the mannan globally selects for bacteria carrying these enzymes and associated traits. Because the mannan provides only carbon, oxygen, sulfur, and hydrogen, bacteria must obtain other essential elements from alternative sources, reinforcing metabolic interdependence. We propose that diatoms use sulfated mannans to attract beneficial partners and exclude competitors, thereby engineering a microbiome that enhances their productivity and underpins carbon cycling. Significance statementEukaryotes host microbial partners that shape their health, yet how they selectively assemble beneficial microbes remains unclear. Using diatom microalgae as a model, we show they exude a sulfated mannan that nourishes highly adapted bacteria tracking them across the global ocean. Our findings suggest that single-celled eukaryotes can "domesticate" prokaryotes--analogous to how humans have domesticated animals--albeit on a microscopic scale. Dominating much of Earths aquatic surface, diatoms drive [~]20% of global photosynthesis. We propose that sulfated mannan contributes to this success by helping diatoms shape microbial partnerships that underpin planetary energy balance and atmospheric chemistry.

ecology↗

bioRxiv2024

Phosphate starvation stops bacteria digesting algal fucan that sequesters carbon

Algae provide a solar powered pathway to capture and sequester carbon by injecting stable fucan made from carbon dioxide into the ocean 1-4. Stability of the pathway is at odds with the presence of marine bacteria with genes of enzymes that can digest fucan and release the carbon dioxide 5. Biochemical explanations for stable fucan remain hypothetical 6. We assembled a biological carbon cycle model and found phosphate limitation enhanced fucan synthesis by algae, stopped digestion by bacteria and thereby stabilized the fucan carbon sequestration pathway. Marine microalgae Glossomastix sp. PLY432 increased synthesis of fucan, a part of its extracellular matrix, under nutrient-growth limiting conditions. Rate and extent of fucan digestion by a marine, isolated bacterium of the Akkermansiaceae family decreased with decreasing phosphate concentration. Phosphate starvation restricted bacterial growth rate, biomass yield and in turn increased the amount of stable fucan. Phosphate is universally required for growth but rare relative to glycan carbon in photosynthesis-derived ecosystems. The fact that phosphate is required for replication, transcription and translation explains why bacteria can digest gigatons of laminarin with a few enzymes, but not fucan during nutrient limited algal blooms. We conclude phosphate starvation constrains the ability of bacteria to digest fucan, which evolves to maintain stability around algal cells and consequentially also to keep carbon dioxide in the ocean.

microbiology↗

bioRxiv2023

Spatial heterogeneity in carbohydrates and their utilisation by microbial communities in the high North Atlantic

Carbohydrates are chemically and structurally diverse, represent a substantial fraction of marine organic matter and are key substrates for heterotrophic microbes. Studies on carbohydrate utilisation by marine microbes have been centred on phytoplankton blooms in temperate regions, while far less is known from high-latitude waters and during later seasonal stages. Here, we combine glycan microarrays and analytical chromatography with metagenomics and metatranscriptomics to show the spatial heterogeneity in glycan distribution and their utilisation by microbes in Atlantic waters of the Arctic during late summer. The composition and abundance of monomers and glycan structures in POM varied with location and depth. Complex fucose-containing sulfated polysaccharides, known to accumulate in the ocean, were consistently detected, suggesting limited degradation by microbes. In contrast, the more labile {beta}-1,3-glucan exhibited a patchy distribution, indicating local variations in primary productivity and rapid utilisation. Metatranscriptomics showed active and dynamic microbial populations that targeted specific glycans. Gene transcription of carbohydrate-active enzymes revealed narrow substrate niches for specialists, involving compounds such as -mannans and alginate, along with the targeting of communal substrates, such as laminarin, by multiple populations. The observed spatial heterogeneity indicates that local biological and physical processes continue to shape the carbohydrate pool during late summer in high latitude waters and microbial populations are active and responsive to such changes.

microbiology↗

bioRxiv2022

Grazers affect the composition of dissolved storage glycans and thereby bacterioplankton composition during a biphasic North Sea spring algae bloom

Blooms of marine microalgae play a pivotal role in global carbon cycling. Such blooms entail successive blooms of specialized clades of planktonic bacteria that remineralize algal biomass. We investigated the bacterioplankton response to a bloom in the German Bight in spring 2020. Metagenome sequencing at 30 time-points allowed reconstruction of 251 metagenome-assembled genomes (MAGs), 245 representing as yet uncultured species, while corresponding metatranscriptome sequencing highlighted 50 particularly active MAGs. Together with algae, copepod, protist and bacteria diversity and abundance data in combination with physico-chemical data and antibody-based saccharide measurements, we demonstrate (i) how dissolved primary photoassimilated algal and secondary bacterial storage glycans shape the bacterioplankton community composition, and (ii) how grazing on higher trophic levels determines the release of these abundant glycans. We thus elucidate principles governing how bacterioplankton clades respond to algal blooms and collectively remineralize gigatons of carbon annually on a global scale.

ecology↗

bioRxiv2022

Not digested: algal glycans move carbon dioxide into the deep-sea

Marine algae annually synthesize gigatons of glycans from carbon dioxide, exporting it within sinking particles into the deep-sea and underlying sea floor, unless those glycans are digested before by bacteria. Identifying algal glycans in the ocean remains challenging with the molecular resolution of conventional analytic techniques. Whether algal glycans are digested by heterotrophic bacteria during downward transport, before they can transfer carbon dioxide from the ocean surface into the deep-sea or the sea floor, remains unknown. In the Red Sea Shaban Deep, where at 1500 m water depth a brine basin acts as a natural sediment trap, we found its high salt and low oxygen concentration accumulated and preserved exported algal glycans for the past 2500 years. By using monoclonal antibodies specific for glycan structures, we detected fucose-containing sulfated polysaccharide, {beta}-glucan, {beta}-mannan and arabinogalactan glycans, synthesized by diatoms, coccolithophores, dinoflagellates and other algae living in the sunlit ocean. Their presence in deep-sea sediment demonstrates these algal glycans were not digested by bacteria. Instead they moved carbon dioxide from the surface ocean into the deep-sea, where it will be locked away from the atmosphere at least for the next 1000 years. Considering their global synthesis, quantity and stability against degradation during transport through the water column, algal glycans are agents for carbon sequestration. Significance statementAlgae and plants use the greenhouse gas carbon dioxide to synthesize polymeric carbohydrates, or glycans, for energy storage, structural support and as protection against invasion by microbes. Glycans provide protection, are carbon sinks and enable carbon sequestration for as long as they are not digested by bacteria or other organisms, which releases the carbon dioxide back in to the atmosphere. In this study, we show that non-digested algal glycans sink into the deep ocean and into marine sediment. Thus, glycans are more than food for animals and prebiotics for bacteria, they are also molecules that remove carbon dioxide from the atmosphere and transfer it to the deep-sea, where it can be stored for 1000 years and longer.

biochemistry↗

bioRxiv2021

Biocatalytic quantification of α-glucan in particulate marine organic matter

Marine algae drive the marine carbon cycle, converting carbon dioxide into organic material. A major component of this produced biomass is a variety of glycans; and yet their chemical composition and individual involvement in production, sedimentation and bacterial uptake remain largely unknown due to a lack of analytical tools for glycan-specific quantification. Marine -glucans include a range of storage glycans from red and green algae, bacteria, fungi and animals. Although these compounds are likely to account for a high amount of the carbon stored in the oceans they have not been quantified in marine samples so far. Here we present a method to extract and quantify -glucans in particulate organic matter from algal cultures and environmental samples using a sequential physicochemical extraction and enzymes as -glucan-specific probes. This enzymatic assay is more specific and less susceptible to side reactions than chemical hydrolysis. Using HPAEC-PAD to detect the hydrolysis products allows for a glycan quantification in particulate marine samples even at low concentration of {approx} 2-7 {micro}g/L -glucans. We measured -glucans (and compared their concentration with the {beta}-glucan laminarin) in three microalgae laboratory cultures as well as in marine particulate organic matter from the North Sea and western North Atlantic Ocean. While laminarin from diatoms and brown algae is an essential component of marine carbon turnover, our results further indicate the significant contribution of starch-like -glucans to marine particulate organic matter. Henceforth, the combination of glycan-linkage-specific enzymes and chromatographic hydrolysis product detection can provide a powerful tool in the exploration of marine glycans and their role in the global carbon cycle.

ecology↗

bioRxiv2020

Structural basis of ligand selectivity by a bacterial adhesin lectin involved in multi- species biofilm formation

Carbohydrate recognition by lectins governs critical host-microbe interactions. MpPA14 lectin is a domain of a 1.5-MDa adhesin responsible for a symbiotic bacterium-diatom interaction in Antarctica. Here we show MpPA14 binds various monosaccharides, with L-fucose and N-acetyl glucosamine being the strongest ligands (Kd ~ 150 M). High-resolution structures of MpPA14 with 15 different sugars bound elucidated the molecular basis for the lectins apparent binding promiscuity but underlying selectivity. MpPA14 mediates strong Ca2+-dependent interactions with the 3, 4 diols of L-fucopyranose and glucopyranoses, and binds other sugars via their specific minor isomers. Thus, MpPA14 only binds polysaccharides like branched glucans and fucoidans with these free end-groups. Consistent with our findings, adhesion of MpPA14 to diatom cells was selectively blocked by L-fucose, but not by N-acetyl galactosamine. With MpPA14 lectin homologs present in adhesins of several pathogens, our work gives insight into an anti-adhesion strategy to block infection via ligand-based antagonists.

biochemistry↗

bioRxiv2020

Quantifying fluorescent glycan uptake to elucidate strain-level variability in foraging behaviors of rumen bacteria

Gut microbiomes have vast catabolic potential and are essential to host health and nutrition. An in-depth understanding of the metabolic pathways in these ecosystems will enable us to design treatments (i.e. prebiotics) that influence microbiome structure and enhance host physiology. Currently, the investigation of metabolic pathways relies on inferences derived from metagenomics or in vitro cultivations, however, novel approaches targeting specific cell physiologies can illuminate the functional potential encoded within microbial (meta)genomes to accurately assess metabolic abilities. Here, we present a multi-faceted study using complimentary next-generation physiology and omics approaches to characterize the microbial adaptation to a prebiotic in the rumen ecosystem. Using fluorescently labeled polysaccharides, we identified bacteria that actively metabolize a glycan prebiotic in the rumen microbiome ex vivo. Subsequently, we characterized strain-level variability in carbohydrate utilization systems and predict metabolic strategies of isolated bovine-adapted strains of Bacteroides thetaiotaomicron using comparative whole genome sequencing, RNA-Seq, and carbohydrate-active enzyme fingerprinting.

microbiology↗