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Abstract Over the past decades, human impacts have changed the structure of tropical benthic reef communities towards coral depletion and macroalgal proliferation. However, how these changes have modified chemical and microbial waterscapes is poorly known. Here, we assessed how the experimental removal of macroalgal assemblages influences the chemical and microbial composition of two reef boundary layers, the benthic and the momentum. Chemical and microbial waterscapes were spatially structured, both horizontally and vertically, according to macroalgal dominance and boundary layers. Microbes associated with reef degradation were enriched in the boundary layers surrounding macroalgal-dominated substrata. Dominant macroalgae were surrounded by a distinct chemical pool of diverse lipid classes (e.g., diterpenoids and glycerolipids) and labile organic matter (e.g., organooxygen compounds), which diffused from algal tissues to boundary layers according to their polarity. Finally, our results highlighted strong co-variations between specific algal-derived metabolites and planktonic microbes, giving insight into their roles in coral reef functioning and resilience.

Abstract Coral reefs are considered one of the most emblematic ecosystems in our oceans, but their existence is increasingly threatened by climate change. In this study, natural populations of two reef-building coral genera, Pocillopora spp. and Porites spp., and one hydrocoral Millepora cf. platyphylla from two different marine provinces in the Pacific Ocean were investigated using a multi-omics approach as part of the Tara Pacific expedition. Here, we propose a standardised method consisting of a biphasic extraction method followed by metabolomics analysis using mass spectrometry for the lipidome and 1H nuclear magnetic resonance for hydrophilic metabolites. Our study assessed a broad range of the metabolome and is the first to identify and add 24 compounds by NMR and over 200 lipids by MS analyses for corals. Metabolic profiles were distinct among genera but not within genotypes of the cnidarian corals. Although endosymbiotic dinoflagellates of the family Symbiodiniaceae are known to play a central role in the metabolomic signature of the coral holobiont, they did not account for all differences. This suggests that a combined effect by different members of the coral holobiont and an interaction with the environment might be at play. Our study provides foundational knowledge on the coral holobiont metabolome.

Chemical composition and herbicidal, antifungal, antibacterial and molluscicidal activities of essential oils from Choukzerk, Eryngium triquetrum, and Alexander, Smyrnium olusatrum, from western Algeria were characterized. Capillary GC-FID and GC/MS were used to investigate chemical composition of both essential oils, and the antifungal, antibacterial, molluscicidal and herbicidal activities were determined by % inhibition. Collective essential oil of E. triquetrum was dominated by falcarinol (74.8%) and octane (5.6%). The collective essential oil of S. olusatrum was dominated by furanoeremophilone (31.5%), furanodiene+curzurene (19.3%) and (E)-β-caryophyllene (11%). The E. triquetrum oil was tested and a pure falcarinol (99%) showed virtuous herbicidal and antibacterial activities against potato blackleg disease, Pectobacterium atrosepticum, and Gram-negative soil bacterium, Pseudomonas cichorii (85 and 100% inhibition, respectively), and high ecotoxic activity against brine shrimp, Artemia salina, and the freshwater snail, Biomphalaria glabrata, with an IC50 of 0.35 µg/mL and 0.61 µg/mL, respectively. Essential oil of S. olusatrum showed interesting antibacterial and ecotoxic activity and good herbicidal activity against watercress seeds, Lepidium sativum (74% inhibition of photosynthesis, 80% mortality on growth test on model watercress), while the furanoeremophilone isolated from the oil (99% pure) showed moderate herbicidal activity. Both oils showed excellent antifungal activity against Fusarium. Both oils and especially falcarinol demonstrated good potential as new biocontrol agents in organic crop protection.

Miconia calvescens is a dominant invasive alien tree species that threatens several endemic plants in French Polynesia (South Pacific). While most analyses have been performed at the scale of plant communities, the effects on the rhizosphere have not been described so far. However, this compartment can be involved in plant fitness through inhibitory activities, nutritive exchanges, and communication with other organisms. In particular, it was not known whether M. calvescens forms specific associations with soil organisms or has a specific chemical composition of secondary metabolites. To tackle these issues, the rhizosphere of six plant species was sampled on the tropical island of Mo’orea in French Polynesia at both the seedling and tree stages. The diversity of soil organisms (bacteria, microeukaryotes, and metazoa) and of secondary metabolites was studied using high-throughput technologies (metabarcoding and metabolomics, respectively). We found that trees had higher effects on soil diversity than seedlings. Moreover, M. calvescens showed a specific association with microeukaryotes of the Cryptomycota family at the tree stage. This family was positively correlated with the terpenoids found in the soil. Many terpenoids were also found within the roots of M. calvescens, suggesting that these molecules were probably produced by the plant and favored the presence of Cryptomycota. Both terpenoids and Cryptomycota were thus specific chemicals and biomarkers of M. calvescens. Additional studies must be performed in the future to better understand if they contribute to the success of this invasive tree.