Jorge Michel Flores

The viral shunt is a fundamental ecosystem process which diverts the flux of organic carbon fixed through photosynthesis during algal bloom events from heterotrophic grazers to bacteria. Through the extracellular release of metabolites, lytic viral infections supply 2 to 10% of photosynthetically fixed carbon in the ocean for bacterial respiration. Despite its significance for the carbon cycle, we lack tools to detect the viral shunt in the natural environment and assess its ecological impact. Here, we investigated the use of exometabolites as biomarkers for the viral shunt by applying molecular, metabolomics, and oceanographic tools to study bloom dynamics of the cosmopolitan microalga Gephyrocapsa huxleyi (formerly Emiliania huxleyi ) across the Atlantic Ocean, spanning four biogeochemical provinces between Iceland and Patagonia. We mapped the distinct metabolic footprint of its viral infections using exo- and endometabolomics and detected nineteen organohalogen metabolites across the blooms, showing their global distribution. A time-resolved comparison of particulate and dissolved metabolite pools during an induced mesocosm bloom revealed that virocells—actively infected host cells—were the source of the halogenated metabolites. Three trichloro-iodo metabolites were present during the demise of all virus-infected oceanic blooms, highlighting them as suitable metabolic biomarkers for the viral shunt. The environmental stability of these halometabolites in the dissolved organic matter pool over a few days can recapitulate viral infections at earlier stages of phytoplankton bloom succession. The chloro-iodo metabolites thereby expand the existing repertoire of metabolic biomarkers for viral infections at sea and may advance efforts to trace the biogeochemical impact of alga–virus interactions in the ocean.

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.

Understanding the factors that influence the production of sea spray aerosols (SSA) is crucial for better understanding their impact on our climate. However, the role of oceanic microbial activity in contributing to SSA production is still underexplored. Here, we investigated the dynamics of SSA number concentration (N_SSA) during induced algal blooms within three airtight mesocosm enclosures. Monitoring algal abundance throughout a 24-day experiment revealed two main blooms: a mixed algal bloom followed by an extensive bloom of the coccolithophore Gephyrocapsa huxleyi and its subsequent demise. We observed two main patterns in N_SSA: (1) a diurnal variation with higher daytime emissions; and (2) a progressive decrease in N_SSA throughout the bloom succession, with peak N_SSA during the first mixed algal bloom and a decline during the G. huxleyi bloom and its demise. We hypothesize that photosynthetic processes could contribute to the observed diurnal changes, potentially through effects of enhanced bubble formation and subsequent SSA production. The suppression of N_SSA during the G. huxleyi bloom and its demise correlates with the accumulation of particulate organic carbon and transparent exopolymer particles, which can act as surfactants and potentially suppress SSA production by altering surface tension dynamics. Our findings underscore the complex interplay between algal bloom dynamics and SSA production, with implications for understanding aerosol dynamics in marine environments, particularly under changing climate conditions.

Diurnal variations in sea surface temperature (SST) influence the atmospheric boundary layer and the hydrological cycle. Using a decade of satellite data, we identify global patterns of diurnal SST variability, capturing both warming and cooling phases. We highlight diurnal cooling as a distinct and previously underrecognized phenomenon, accounting for over 38% of observed cases. By focusing on extreme cooling and warming events, defined as diurnal SST changes exceeding from the mean, we explore the spatial distribution, seasonality, and mechanisms driving transitions between three SST states: diurnal warming, a balanced state, and diurnal cooling. These states correspond to distinct upper‐ocean mixing regimes: minimal, neutral, and strong, each shaped by atmospheric forcing, including wind stress, cloud cover, and precipitation. In the tropics and mid‐latitudes, extreme warming events dominate, occurring primarily during the transition and summer seasons under calm winds, clear skies, and little to no rain. In contrast, high‐latitude regions are characterized by frequent diurnal cooling during winter, with passing storms emerging as key modulators of SST variability. Storms that occur during the day typically trigger strong cooling, while those passing at night can occasionally result in warming. By integrating seasonal context and focusing on extreme events, this study provides new insights into the atmospheric drivers of diurnal SST variability, an important step in constructing and tuning models that capture diurnal layer dynamics. These findings have implications for understanding energy budgets, air–sea interactions, and feedbacks in the coupled climate system.

The viral shunt is a fundamental ecosystem process which diverts the flux of organic carbon from grazers to heterotrophic microorganisms. Through the extracellular release of metabolites, lytic viral infections supply 2-10% of photosynthetically fixed carbon in the ocean for bacterial respiration. Despite its significance for the carbon cycle, we lack tools to detect the viral shunt in the natural environment and assess its ecological impact. Here, we study the use of exometabolites as biomarkers for the viral shunt by applying molecular, metabolomics, and oceanographic tools in blooms of the cosmopolitan microalga Gephyrocapsa huxleyi across the Atlantic Ocean, spanning four biogeochemical provinces between Iceland and Patagonia. We mapped the distinct metabolic footprint of its viral infections using exo- and endometabolomics approaches and detected nineteen organohalogen metabolites across the blooms, showing their global formation. Time-resolved comparison of particulate and dissolved metabolite pools during an induced mesocosm bloom indicated virocells – actively infected host cells – as the source of the halogenated metabolites. Three trichloro-iodo metabolites were present during demise of all virus-infected blooms, highlighting them as suitable metabolic biomarkers. The environmental stability of these halometabolites in the DOM pool over a few days can recapitulate viral infections at earlier stages of phytoplankton bloom succession.

Coral reefs are severely threatened by global and local environmental changes. However, susceptibility to perturbations and subsequent mortality varies among coral species. In this study, we tested the contribution of genetic and environmental conditions to coral’s phenotypic response inPocilloporaspp. andPoritesspp. sampled together at a large ecological and temporal scale throughout the Pacific Ocean. We assessed coral phenotype signatures using a multi-biomarker approach (animal and symbiont biomasses, protein carbonylation and ubiquitination and total antioxidant capacities). In both genera, we highlighted a strong anticorrelation between the redox state and the animal and symbiont biomasses. In addition,Pocilloporaexhibited high phenotypic plasticity, responding to various environmental variables such as temperature, nutrients, phosphate, and carbonate chemistry. In contrast,Poritesdisplayed more robust phenotypes influenced by both genetics and past climate events. In conclusion, co-located coral species display different phenotypic response strategies that are influenced by different environmental conditions.

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.

Heat waves are causing declines in coral reefs globally. Coral thermal responses depend on multiple, interacting drivers, such as past thermal exposure, endosymbiont community composition, and host genotype. This makes the understanding of their relative roles in adaptive and/or plastic responses crucial for anticipating impacts of future warming. Here, we extracted DNA and RNA from 102 Pocillopora colonies collected from 32 sites on 11 islands across the Pacific Ocean to characterize host-photosymbiont fidelity and to investigate patterns of gene expression across a historical thermal gradient. We report high host-photosymbiont fidelity and show that coral and microalgal gene expression respond to different drivers. Differences in photosymbiotic association had only weak impacts on host gene expression, which was more strongly correlated with the historical thermal environment, whereas, photosymbiont gene expression was largely determined by microalgal lineage. Overall, our results reveal a three-tiered strategy of thermal acclimatization in Pocillopora underpinned by host-photosymbiont specificity, host transcriptomic plasticity, and differential photosymbiotic association under extreme warming.

Coral reefs are among the most diverse ecosystems on Earth. They support high biodiversity of multicellular organisms that strongly rely on associated microorganisms for health and nutrition. However, the extent of the coral reef microbiome diversity and its distribution at the oceanic basin-scale remains to be explored. Here, we systematically sampled 3 coral morphotypes, 2 fish species, and planktonic communities in 99 reefs from 32 islands across the Pacific Ocean, to assess reef microbiome composition and biogeography. We show a very large richness of reef microorganisms compared to other environments, which extrapolated to all fishes and corals of the Pacific, approximates the current estimated total prokaryotic diversity for the entire Earth. Microbial communities vary among and within the 3 animal biomes (coral, fish, plankton), and geographically. For corals, the cross-ocean patterns of diversity are different from those known for other multicellular organisms. Within each coral morphotype, community composition is always determined by geographic distance first, both at the island and across ocean scale, and then by environment. Our unprecedented sampling effort of coral reef microbiomes, as part of theTaraPacific expedition, provides new insight into the global microbial diversity, the factors driving their distribution, and the biocomplexity of reef ecosystems.

Coral reef science is a fast-growing field propelled by the need to better understand coral health and resilience to devise strategies to slow reef loss resulting from environmental stresses. Key to coral resilience are the symbiotic interactions established within a complex holobiont, i.e. the multipartite assemblages comprising the coral host organism, endosymbiotic dinoflagellates, bacteria, archaea, fungi, and viruses. Tara Pacific is an ambitious project built upon the experience of previous Tara Oceans expeditions, and leveraging state-of-the-art sequencing technologies and analyses to dissect the biodiversity and biocomplexity of the coral holobiont screened across most archipelagos spread throughout the entire Pacific Ocean. Here we detail the Tara Pacific workflow for multi-omics data generation, from sample handling to nucleotide sequence data generation and deposition. This unique multidimensional framework also includes a large amount of concomitant metadata collected side-by-side that provide new assessments of coral reef biodiversity including micro-biodiversity and shape future investigations of coral reef dynamics and their fate in the Anthropocene.