Pierre E. Galand

The Mediterranean basin faces enhanced wildfire risks associated with human‐driven climate and land use changes. Wildfire‐generated aerosols can reach the ocean, where they may subsequently impact marine prokaryotic communities, key drivers of global biogeochemical cycles. However, our understanding of the influence of wildfire airborne particles on the abundance and composition of marine microbes remains limited. We conducted experiments in which surface water from the northwestern Mediterranean Sea was incubated in 300 L minicosms amended with varying amounts of wildfire fine ash particles, previously collected during a Mediterranean wildfire. Wet deposition of wildfire fine ash particles had a short‐term effect on prokaryotes by increasing their abundance and diversity, likely due to the release of both inorganic and organic substrates, alleviating nutrient limitations. Ash deposition could also indirectly affect prokaryotic communities via changes in the composition of phytoplankton populations. These mechanisms induced changes in prokaryotic community composition, reflecting a succession of taxa likely adapted to different substrate qualities. Ash had a negative effect on Cyanobiaceae but promoted the growth of Flavobacteriaceae , Rhodobacteraceae and SAR11 clade I among other taxa. Our findings demonstrate that wildfire ash can alter Mediterranean prokaryotic communities during oligotrophic periods, further exacerbating the impact of wildfires on marine ecosystems.

Coral reefs are marine biodiversity hotspots that provide a wide range of ecosystem services ¹ . They are reservoirs of bioactive metabolites, many produced by microorganisms associated with reef invertebrate hosts ² . However, for the keystone species of coral reefs—the reef-building corals—we still lack a systematic assessment of their microbially encoded biosynthetic potential and the molecular resources at stake due to the alarming decline in reef biodiversity. Here we analysed microbial genomes reconstructed from 820 reef-building coral samples of three representative coral genera collected at 99 reefs across 32 islands throughout the Pacific Ocean ( Tara Pacific expedition) ³ . By contextualizing our analyses with the microbiomes of other reef species, we found that only 10% of the 4,224 microbial species and less than 1% of the 645 species exclusively identified in Tara Pacific samples had genomic information available. Furthermore, the biosynthetic potential of reef-building coral microbiomes rivalled or surpassed that of traditional natural product sources such as sponges. Among the biosynthetically rich bacteria in the reef microbiome, we identified new groups of Acidobacteriota that encode previously unknown enzymology, in turn opening promising avenues for functional protein engineering. Together, this study underscores the importance of conserving coral reefs as vital reservoirs of molecular diversity.

The persistence of coral reefs is dependent on the arrival and settlement of coral larvae followed by their post‐settlement growth and survival. Despite evidence showing that benthic algae have variable effects on corals, it is still unclear how benthic communities of the coral nursery habitat impact the early development of the coral microbiome and if these impacts relate to the survival and growth of newly settled corals. Here, we tested whether the survival and growth of Acropora cytherea recruits are impacted by surrounding algae, and whether specific algae influence their associated bacterial and Symbiodiniaceae communities. A 6‐month survey of coral larvae experimentally settled near different algae showed that crustose coralline algae enhanced recruit survival. However, despite variation in their microbiome, the presence of different algae did not impact the coral microbial community composition. The recruit microbiome was colonised by bacteria shared among all benthic substrates rather than bacteria unique to specific algae. Furthermore, the microbiome of coral larvae was different from that of the recruits. We conclude that the microbiome of corals in their early life stages is structured by host ontogeny rather than by their surrounding benthos, but that the surrounding benthos contributes to the transfer of opportunistic bacteria.

Marine microbial communities are fundamental to nutrient and biogeochemical cycling, with intricate networks of metabolic interdependencies influencing their structure and dynamics. Among these, vitamins B₁ (thiamin) and B₁₂ (cobalamin) play crucial roles as enzymatic cofactors in central metabolic pathways. Despite their importance, the temporal dynamics of vitamin production, bioavailability, and associated microbial interactions, remain poorly understood. Using a 7-year monthly metagenomic time series from the NW Mediterranean Sea (SOLA station), we found that vitamin B₁/B₁₂ auxotrophs (need for an exogenous vitamin source) were present throughout the year. Among B₁ auxotrophs, those requiring the thiamin precursor HMP were the most prevalent, with peak abundances in summer. Distinct metagenome-assembled genome co-abundance patterns between B₁ and B₁₂ producers/auxotrophs across seasons suggested mutualistic relationships. Double B₁/B₁₂ vitamin complementarities were more common in summer, and single vitamin complementarity was dominant in winter. As previously shown for vitamin B₁₂, which is limiting during winter, bioassays revealed variable availability of vitamin B₁ in winter seawater despite the abundance of its producers, suggesting potential transfer of vitamin B₁ among microorganisms. Finally, microcosm experiments showed that B₁ and B₁₂ amendments significantly influenced the composition of microbial communities, with temporal variations in their impact. In some cases, B₁₂ and B₁ amendments favored both vitamin auxotrophs and producers, highlighting complex interdependencies between B₁ and B₁₂ producers and consumers. Our findings highlight the complexity of B vitamin-mediated metabolic interactions that shape microbial community dynamics and underscore the need for long-term, high-resolution studies to better understand vitamin-driven ecological processes in marine systems.

The coupled Arctic system is in rapid transition and is set to undergo further dramatic changes over the coming decades. These changes will lead most likely to an ice-free ocean in summer, expected before mid-century. The Arctic will become more strongly influenced by atmospheric and oceanographic processes characteristic of mid-latitudes, increasing the prevalence of contaminants and new biological species. This ongoing transition of the Arctic to a new state necessitates systematic monitoring of all sentinels (variables that make an essential contribution to characterizing the Earth's state) to improve our understanding of the system, enhance forecasting and support knowledge-based decisions. Here, we describe a sustained multi-decadal observation program to be implemented on the Tara Polar Station between 2026 and 2046. The monitoring program is designed as a series of year-long drift expeditions, called Tara Polaris, in the central Arctic Ocean, covering all seasons. The multidisciplinary data will bridge ecological, geochemical, biological, and physical parameters and processes in the atmosphere, sea ice and ocean. In addition, data collected with consistent methodologies over a 20-year period will make it possible to distinguish long-term trends from seasonal and interannual variability. In this paper, we discuss specific measurement challenges in each compartment (i.e., atmosphere, sea ice and ocean) along key sentinels and the most pressing scientific questions to be addressed. The expected outcomes of the Tara Polaris program will enable us to understand and quantify the main feedbacks of the coupled Arctic system, with their seasonal and interannual trends and spatial variability.

Marine microbial populations play essential roles in ocean ecosystems, yet the processes shaping their genomic structure across space and time remain poorly understood. Here, we examined the population-scale patterns of 1,505 prokaryotic metagenome-assembled genomes (MAGs) retrieved from the Northwestern Mediterranean Sea in two long-term coastal time series, along 15 and 7 years in these sites, as well as in the global ocean. We found that populations were generally more genomically differentiated across large spatial scales than across long temporal scales. Among a subset of 389 MAGs well represented in all datasets, 68.4% showed weak population divergence over time but strong global-scale differentiation. This was evident in the abundant and widespread cyanobacteria Prochlorococcus and Synechococcus. In contrast, only 6.4% of the MAGs exhibited weak divergence over time and space, with SAR11 MAGs being a clear example, likely reflecting their high dispersal and recombination rates. Other groups, such as SAR86 and Flavobacteriales, showed strong divergence at temporal and spatial scales, suggesting seasonal and/or regional adaptation. Positive selection was more readily detectable in the long-term coastal observatories than in the global ocean, despite the more significant population divergence observed across broad geographic scales. Temperature consistently showed a significant association with the population structure of many MAGs. Overall, our results highlight the dominant influence of large geographic scales in shaping microbial population structure alongside taxon-specific responses to temporal variation, particularly seasonality. Altogether, our work advances the understanding of microbial population structure across broad spatial and temporal scales, a critical step toward predicting microbial dynamics in a changing ocean.

Crustose coralline algae (CCA) and their bacterial communities can emit chemical cues favoring coral larval settlement. Indeed, larvae of Eunicella singularis (white gorgonian) preferentially settle on CCA. Here, we investigated the effect of two Mediterranean CCA holobionts, Macroblastum dendrospermum and Lithophyllum stictiforme , on E. singularis larvae settlement and their bacterial communities, after warming and acidification treatments. We exposed CCA to temperature and pH expected for 2100 (SSP5‐8.5) and to a marine heatwave event. Larval settlement increased 1.8–2.7 times in the presence of CCA exposed to warming and acidification compared to non‐exposed CCA. High abundance of bacteria belonging to the Pirellulaceae family was observed in all CCA, while a higher abundance of monosaccharides was found in exudates of exposed CCA. Based on CCA‐related 16S rDNA metabarcoding and metabolomics results, we hypothesize that the enhanced larval settlement was driven by the Pirellulaceae breakdown and utilization of CCA polysaccharides, in combination with polysaccharide release through the CCA cell walls likely augmented by decalcification. Furthermore, CCA acted as sources of bacterial taxa that may establish and persist in the adult E. singularis holobiont, independently of climate change effects. We conclude that CCA are key for E. singularis recruitment success, especially under future climate conditions, and contribute to their microbiome development.

Understanding how marine microbiomes will respond to ongoing global change is crucial. Functional redundancy, the capacity of different microbes to perform the same function, is considered a key mechanism underpinning the stability and resilience of the ocean microbiome. Although the extent of functional redundancy remains debated, investigating its manifestation in environmentally similar and interconnected microbial communities may provide critical insights into its role in shaping microbial community dynamics. We hypothesized that examining the long-term synchrony and rhythmicity of temperate microbial communities in such locations could provide insight into the role of functional redundancy. High functional redundancy at the community level would manifest as rhythmic and synchronous metabolic functions across sites, even in the absence of synchrony or rhythmicity at finer organizational levels, such as individual genes or taxa, thereby contributing to community resilience. Conversely, low functional redundancy would imply that synchrony and rhythmicity extend to both the contributing genes and taxa, suggesting a greater vulnerability of the community to environmental variability. To test this framework, we analyzed the long-term synchrony and rhythmicity of two marine-coastal microbiomes in the Mediterranean Sea, separated by approximately 150 km and connected by a dominant southwest current. Monthly collected metagenomes from a seven-year period were examined at the levels of metabolic functions (e.g., KEGG pathways), predicted genes (open reading frames), and taxa. We found functions, genes, and taxa exhibiting high, low, or anti-synchrony, as well as displaying rhythmic or non-rhythmic patterns. Although rhythmic behavior was observed on average across all organizational levels, consistent with the seasonal dynamics expected in temperate Mediterranean waters, average synchrony across microbiomes remained low. Focusing specifically on 45 markers of key biogeochemical functions, we revealed that several functions exhibited high synchrony and rhythmicity, in sharp contrast to the low synchrony and rhythmicity among the most abundant genes and taxa contributing to those functions. This suggests that functional redundancy and complementary dynamics at lower organizational levels, with distinct taxa contributing to key metabolic functions at different times, lead to rhythmic and synchronous dynamics at higher levels through emergent self-organization. Together, our results highlight functional redundancy and emergent self-organized dynamics as key mechanisms supporting the stability and resilience of marine microbiomes under environmental change.

Crustose Coralline Algae (CCA) play a crucial role in coral reef ecosystems, contributing significantly to reef formation and serving as substrates for coral recruitment. The microbiome associated with CCAs may promote coral recruitment, yet these microbial communities remain largely understudied. This study investigates the microbial communities associated with a large number of different CCA species across six different islands of French Polynesia, and assess their potential influence on the microbiome of coral recruits. Our findings reveal that CCA harbor a large diversity of bacteria that had not been reported until now. The composition of these microbial communities was influenced by geographic location, and was also closely linked to the host species, identified at a fine taxonomic unit using the 16S rRNA gene of the CCA chloroplast. We demonstrate the usefulness of these ecologically meaningful units that we call CCA chlorotypes. Additionally, we observed a correlation between host phylogeny and microbiome composition (phylosymbiosis) in two CCA species. Contrary to expectations, the CCA microbiome did not act as a microbial reservoir for coral recruits. However, the microbial community of coral recruits varied according to the substrate on which they grew. The study significantly expands the number of characterized CCA microbiomes, and provides new insight into the extensive diversity of these microbial communities. We show distinct microbiomes between and within CCA species, characterized by specific chloroplast 16S rRNA gene sequences. We term these distinct groups “chlorotypes”, and demonstrate their utility to differentiate CCA. We also show that only few bacterial taxa were shared between CCA and coral recruits growing in contact with them. Nevertheless, we observed that the microbial community of coral recruits varied depending on the substrate they grew on. We conclude that CCA and their associated bacteria influence the microbiome composition of the coral recruits.