Metabolic interactions with endosymbiotic photosynthetic dinoflagellate Symbiodinium spp. are fundamental to reef-building corals (Scleractinia) thriving in nutrient-poor tropical seas. Yet, detailed understanding at the single-cell level of nutrient assimilation, translocation, and utilization within this fundamental symbiosis is lacking. Using pulse-chase 15N labeling and quantitative ion microprobe isotopic imaging (NanoSIMS; nanoscale secondary-ion mass spectrometry), we visualized these dynamic processes in tissues of the symbiotic coral Pocillopora damicornis at the subcellular level. Assimilation of ammonium, nitrate, and aspartic acid resulted in rapid incorporation of nitrogen into uric acid crystals (after ~45 min), forming temporary N storage sites within the dinoflagellate endosymbionts. Subsequent intracellular remobilization of this metabolite was accompanied by translocation of nitrogenous compounds to the coral host, starting at ~6 h. Within the coral tissue, nitrogen is utilized in specific cellular compartments in all four epithelia, including mucus chambers, Golgi bodies, and vesicles in calicoblastic cells. Our study shows how nitrogen-limited symbiotic corals take advantage of sudden changes in nitrogen availability; this opens new perspectives for functional studies of nutrient storage and remobilization in microbial symbioses in changing reef environments.

The potential ecotoxicity of nanosized zinc oxide (ZnO), synthesized by the polyol process, was investigated using common Anabaena flos-aquae cyanobacteria and Euglena gracilis euglenoid microalgae. The photosynthetic activities of these microorganisms, after addition of ZnO nanoparticles, varied with the presence of protective agents such as tri-n-octylphosphine oxide (TOPO) and polyoxyethylene stearyl ether (Brij-76) used to control particle size and shape during the synthesis. In the case of Anabaena flos-aquae , the photosynthetic activity, after addition of ZnO, ZnO-TOPO, and ZnO-Brij-76, decreased progressively due to stress induced by the presence of the nanoparticles in the culture medium. After contact with ZnO-TOPO nanoparticles, this decrease was followed by cell death. On the other hand, after 10 days, a progressive increase of the photosynthetic activity was observed after contact with ZnO and ZnO-Brij-76 nanoparticles. In the case of Euglena gracilis , cell death was observed after contact with all nanoparticles. Transmission electron microscopy (TEM) analyses of ultrathin sections of microorganisms showed that polysaccharides produced by Anabaena flos-aquae avoid particle internalization after contact with ZnO and ZnO-Brij-76 nanoparticles. On the other hand, nanoparticle internalization was observed after contact with all nanoparticles in the presence of Euglena gracilis and also with ZnO-TOPO nanoparticles after contact with Anabaena flos-aquae .

The search for a better understanding of why cyanobacteria often dominate phytoplankton communities in eutrophic freshwater ecosystems has led to a growing interest in the interactions between cyanobacteria and bacteria. Against this background, we studied the location of bacteria within Microcystis colonies, and compared the structural and phylogenetic diversity of Microcystis-attached and free-living bacterial communities living in the same French lake, the Villerest reservoir. Using transmission electron microscopy, we show that most of the bacteria inside the colonies were located close to detrital materials that probably resulted from lysis of Microcystis cells. The 16S rRNA sequencing approach revealed a clear distinction between the attached and free-living communities at the levels of both their general structure and their operational taxonomic unit (OTU) composition. In particular, Microcystis colonies appeared to be depleted of Actinobacteria, but conversely enriched in Gammaproteobacteria, in particular when the bloom was declining. At the OTU level, a clear distinction was also found between attached and free-living bacteria, and new clades were identified among our sequences. All these findings suggest that Microcystis colonies constitute a distinct habitat for bacteria living in freshwater ecosystems, and that direct and indirect interactions (cell lysis, nutrient recycling, etc.) may occur between them inside these colonies.

Paraconiothyrium variabile, one of the specific endophytic fungi isolated from the host plant Cephalotaxus harringtonia, possesses the faculty to inhibit the growth of common phytopathogens, thus suggesting a role in its host protection. A strong antagonism between the endophyte P. variabile and Fusarium oxysporum was observed and studied using optic and electronic microscopies. A disorganization of the mycelium of F. oxysporum was thus noticed. Interestingly, the biological effect of the main secondary metabolites isolated from P. variabile against F. oxysporum did not account for this strong antagonism. However, a metabolomic approach of pure fungal strains and confrontation zones using the data analysis tool XCMS were analyzed and pointed out a competition-induced metabolite production by the endophyte in the presence of the phytopathogen. Subsequent MS/MS fragmentations permitted to identify one of the induced metabolites as 13-oxo-9,11-octadecadienoic acid and highlighted a negative modulation of the biosynthesis of beauvericin, one of the most potent mycotoxin of F. oxysporum, during the competition with the endophyte.

International audienceThe biomineralization process and skeletal growth dynamics of azooxanthellate corals are poorly known. Here, the growth rate of the shallow-water dendrophyllid scleractinian coral Balanophyllia regia was evaluated with calcein-labeling experiments that showed higher lateral than vertical extension. The structure, mineralogy and trace element composition of the skeleton were characterized at high spatial resolution. The epitheca and basal floor had the same ultrastructural organization as septa, indicating a common biological control over their formation. In all of these aragonitic skeletal structures, two main ultrastructural components were present: “centers of calcification” (COC) also called rapid accretion deposits (RAD) and “fibers” (thickening deposits, TD). Heterogeneity in the trace element composition, i.e., the Sr/Ca and Mg/Ca ratios, was correlated with the ultrastructural organization: magnesium was enriched by a factor three in the rapid accretion deposits compared with the thickening deposits. At the interface with the skeleton, the skeletogenic tissue (calicoblastic epithelium) was characterized by heterogeneity of cell types, with chromophile cells distributed in clusters regularly spaced between calicoblasts. Cytoplasmic extensions at the apical surface of the calicoblastic epithelium created a three-dimensional organization that could be related to the skeletal surface microarchitecture. Combined measurements of growth rate and skeletal ultrastructural increments suggest that azooxanthellate shallow-water corals produce well-defined daily growth step

Cyanobacteria are photosynthetic prokaryotes capable of synthesizing a large variety of secondary metabolites that exhibit significant bioactivity or toxicity. Microcystis constitutes one of the most common cyanobacterial genera, forming the intensive blooms that nowadays arise in freshwater ecosystems worldwide. Species in this genus can produce numerous cyanotoxins (i.e., toxic cyanobacterial metabolites), which can be harmful to human health and aquatic organisms. To better understand variations in cyanotoxin production between clones of Microcystis species, we investigated the diversity of 24 strains isolated from the same blooms or from different populations in various geographical areas. Strains were compared by genotyping with 16S-ITS fragment sequencing and metabolite chemotyping using LC ESI-qTOF mass spectrometry. While genotyping can help to discriminate among different species, the global metabolome analysis revealed clearly discriminating molecular profiles among strains. These profiles could be clustered primarily according to their global metabolite content, then according to their genotype, and finally according to their sampling location. A global molecular network of all metabolites produced by Microcystis species highlights the production of a wide set of chemically diverse metabolites, including a few microcystins, many aeruginosins, microginins, cyanopeptolins, and anabaenopeptins, together with a large set of unknown molecules. These components, which constitute the molecular biodiversity of Microcystis species, still need to be investigated in terms of their structure and potential bioactivites (e.g., toxicity).

International audienceThe design of cell-based bioreactors for inorganic particles formation requires both a better understanding of the underlying processes and the identification of most suitable organisms. With this purpose, the process of Au3+ incorporation, intracellular reduction, and Au0 nanoparticle release in the culture medium was compared for four photosynthetic microorganisms, Klebsormidium flaccidum and Cosmarium impressulum green algae, Euglena gracilis euglenoid and Anabaena flos-aquae cyanobacteria. At low gold content, the two green algae show maintained photosynthetic activity and recovered particles (ca. 10 nm in size) are similar to internal colloids, indicating a full biological control over the whole process. In similar conditions, the euglenoid exhibits a rapid loss of biological activity, due to the absence of protective extracellular polysaccharide, but could grow again after an adaptation period. This results in a larger particle size dispersity but larger reduction yield. The cyanobacteria undergo rapid cell death, due to their prokaryotic nature, leading to high gold incorporation rate but poor control over released particle size. Similar observations can be made after addition of a larger gold salt concentration when all organisms rapidly die, suggesting that part of the process is not under biological control anymore but also involves extracellular chemical reactions. Overall, fruitful information on the whole biocrystallogenesis process is gained and most suitable species for further bioreactor design can be identified, i.e., green algae with external coating

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