Understanding and quantifying the microbial contribution to particulate organic matter (POM) flux and carbon cycling in the ocean relies largely on the collection of sinking POM. A critical aspect of evaluating microbial communities on sediment trap-collected POM is their preservation until recovery and sample processing. While RNAlater has emerged as a recent preservative in sediment trap studies, no comparisons have been made between RNAlater and the more established preservative, formalin, to determine their effectiveness in capturing POM-attached microbial communities. Our work evaluates these preservatives through laboratory testing and sediment trap deployments (lasting days to months). Formalin-fixed POM required decrosslinking (undoing bonds between DNA and proteins) to extract DNA. When decrosslinking was included, no community differences were observed from subsequent DNA extraction or amplification when preserved in the lab. However, when preservatives were deployed inside sediment traps, significant differences were observed among unpreserved, RNAlater-preserved, and formalin-preserved POM. In these samples, some copiotrophs, such as Gammaproteobacteria, Verrucomicrobiae, and Flavobacteria, demonstrated greater relative abundances in RNAlater, and the overall magnitude of these differences was greater for RNAlater-preserved vs. formalin-fixed POM, indicating differing abilities of each preservative to halt microbial activity, contributing to the overrepresentation of some taxa. Together, these results suggest that formalin preservation is the best option for use in sediment traps, especially for long-term deployments (> 5 d), common for moored sediment trap timeseries studies. A skewed view of microbial community structure on POM has implications for our understanding of the taxa and metabolisms contributing to carbon turnover in the ocean.

Essential trace metals (e.g., Fe, Co, Cu) are vital for phytoplankton metabolism and marine biogeochemical cycles, while toxic metals and emerging contaminants (e.g., Cd, Pb, Hg, Li, microplastics) pose ecological risks. Their bioavailability and transfer through marine food webs depend partly on their chemical form (speciation between dissolved, colloidal, particulate) and biological uptake by plankton. These processes – chemical speciation and trophic transfer - can modify phytoplankton community structure and the entire marine food web with potential consequences for ecosystem functioning.To investigate how metals and contaminants transfer through the water column and plankton, a mesocosm experiment conditions was carried out. Mesocosms with the natural phytoplanktonic assemblage from Villefranche Bay (Mediterranean Sea, France) and copepods collected from the same location were exposed to a gradient of metals, Li and UV-degraded microplastics, ranging from present-day concentration to levels representative of plausible future environmental scenarios.This experiment was conducted in 9 x 300 L with a 1 m high water column allowing the development of export fluxes. Major nutrients were added in all mesocosm to insure the phytoplanktonic development before metals and contaminants additions of treatment. Treatments were as follow : 1 mesocosm control; 3 mesocosms with x1.5, x2 and x5 of natural metals concentrations (Fe, Mn, Zn, Co, Cd, Cu, Ni and Li); 2 mesocosms with addition of different concentrations of UV-degraded polypropylene (10 and 160 µg/L, size distribution centred at ~50 µm); and 3 mesocosms with different concentrations of both metals and microplastics. During the 17-day experiment biological parameters (nutrient concentrations, biovolume, particulate organic carbon and nitrogen, pigment concentration and cell abondance) were monitored throughout the experiment and first results indicate an initial exponential growth phase, followed by nutrient limitation and a transition toward heterotrophic conditions. Metals concentrations will be analysed in colloidal (3kDa – 0.22 µm), dissolved (< 0.22 µm) and particulate (> 0.22µm) fractions. The particulate fraction, included microphytoplankton, was rinsed with EDTA/oxalate to quantify intracellular metals. Zooplankton was sampled at the beginning and the end of the experiment to assess potential bioaccumulation. Exported material was collected daily to quantify and characterize export fluxes. These data will allow determination of partition coefficients and bioaccumulation. Daily photophysiological measurements - including the maximum quantum yield (Fv/Fm), absorption cross-section (SigmaPSII) of photosystem II, photosynthesis–irradiance curves and photoprotection capacity - will enable the detection of any potential adverse effects of the treatment on the physiological status of phytoplankton. This presentation will report on the experiment and present preliminary results on the partition coefficients and bioaccumulation of metals and contaminants, and will explore their potential impact on phytoplankton and zooplankton communities.

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.

Mesopelagic microbes and zooplankton, degrade, and attenuate >90% of the 10 billion tonnes of particulate organic carbon that sinks into the oceans' interior annually. Approaches such as particle interceptors/incubators (called c-respire) can isolate the microbial assemblage attached to particles from that of zooplankton, enabling quantification of microbially mediated particulate organic carbon flux attenuation. This metric yields patterns of particulate organic carbon degradation by microorganisms through the upper mesopelagic (200-500 m depth). Here, we investigate the temporal sequence of particulate organic carbon degradation in two steps. First, we intercept sinking particle assemblages from different depths (180-300 m) and hence with varying degrees of exposure to microbial activity. Second, we incubate these intercepted particles shipboard for 12 h (short-term) and track degradation using apparent respiratory quotients (dDIC/dDO2). We also conducted a 12-h shipboard incubation on a particle assemblage that had already undergone a 36-h in situ c-respire (long-term) incubation. At a subantarctic and two polar sites, apparent respiratory quotients (ARQs) from short-term incubations exhibited a significant decrease with depth, consistent with particles deeper in the upper mesopelagic being exposed to a longer period of degradation and flux attenuation (as they settle). ARQs from all long-term incubations had significantly lower ARQs, and smaller depth-dependent gradients, than the short-term incubations. We interpret these trends as being driven in part by sequential changes in the stoichiometry of the microbially altered particulate organic carbon (POC) substrates. ARQs of <0.5 (less than the theoretical minimum) were observed in long-term incubations suggesting a role for incomplete oxidation of dissolved substrates. This temporal sequence is used to conceptually explore what sets the limits on microbially mediated degradation of POC.

The biological carbon pump contributes to set the magnitude of carbon sequestration in the oceans' interior. Estimating the relative contribution of microbial versus zooplankton‐mediated processes to particulate organic carbon (POC) flux attenuation provides insights into how this pump functions. Our study took place during the high productivity summer period in the Subantarctic and Polar Front Zone. In the upper mesopelagic (i.e., 180–300 m depth), we concurrently measured the downward POC flux, particle size and morphology, microbial remineralization rates and estimated size‐specific sinking velocities. These concomitant measurements revealed two different export systems, dominated by fecal material in the Subantarctic, and phyto‐aggregates in polar waters. These two systems were characterized by similar low particle sinking velocities (∼10 m d −1 ), while microbial remineralization rates differed by an order of magnitude. Higher microbial remineralization rates in the Subantarctic (0.11 d −1 ), compared to polar waters (0.04 d −1 ), were likely driven by the confounding effect of temperature and particle characteristics. Despite this difference in microbial remineralization rates, these two export systems were characterized by relatively similar transfer efficiencies, suggesting that microbes had differing influences. A comparison of microbially mediated (i.e., scaled using observed remineralization rates) with total POC flux attenuation (i.e., driven by the dual impact of microbes and flux‐feeders) revealed a higher microbial contribution to the flux attenuation in the upper mesopelagic of the subantarctic compared to the polar region. This deconstruction of the flux attenuation revealed an increasing influence of microbes on POC degradation with depth to become the predominant actor in the lower mesopelagic.

Wildfires contribute significantly to biomass burning. The deposition of ash from wildfires into surface ocean waters is a source of iron (Fe), namely pyrogenic Fe, and may enhance primary production in Fe-limited domains. However, due to the low solubility of Fe and the operational definition of its dissolved fraction, a portion of the dissolved Fe (DFe) released during ash dissolution may reprecipitate as authigenic inorganic colloids. This process can lead to an overestimation of the bioavailable pyrogenic DFe. To remain in a soluble form, Fe must be complexed with organic ligands capable of undergoing biochemical processes such as bacterial degradation, direct uptake, or photoreduction, leading to potentially bioavailable forms of DFe. Among the diverse range of iron-binding ligands, humic-type ligands (LFeHS) are important. LFeHS are ubiquitous in seawater, soluble, and may lead Fe to a bioavailable form. LFeHS are ubiquitous in seawater, soluble, and keep Fe in a bioavailable form. Here we present results from dissolution experiments. Ash samples collected in 2009 after wildfire events in the Spanish Mediterranean region were put in contact with non-euxinic, filtered Mediterranean surface seawater in a 7-day batch experiment. Four deposition fluxes were tested. The concentrations of DFe, fluorescent dissolved organic matter (FDOM), LFeHS, and the amount of Fe complexed by humic-type ligands were measured. Our results indicate that ash dissolution induces an increase in LFeHS, proportional to the ash concentration in the experimental medium. FDOM measurements confirm a time-dependent increase in humic-type material of terrestrial origin. Additionally, the observed increase in protein-like FDOM (C4) suggests that ash deposition enhances the modification of dissolved organic matter by bacteria. Using a simple kinetic model, we determined the dissolution rate constant for the tested ash. This constant can be incorporated into global oceanic models such as PISCES or REcoM to improve predictions of pyrogenic Fe bioavailability and its impacts on marine ecosystems.

The passive sinking flux of particles, termed the biological gravitational pump (BGP), is an important component of the ocean’s biological carbon pump. In addition, carbon-rich particles are actively injected to depth through the diel vertical migration (DVM) of micronekton and mesozooplankton from the surface to the oceans’ twilight zone (200 m – 1000 m depth). This is known as the mesopelagic-migrant pump (MMP). We investigated the magnitude of the MMP at one subantarctic and two polar sites in summer by assessing particulate and dissolved carbon export below 200 m depth based on DVM and the composition of the mesopelagic community. Carbon injection potential (CIP) for the dominant taxa at each site was estimated through four pathways, i.e., excretion, respiration, fecal pellets, and carcass production. Blooms of two migratory tunicate species, the pyrosome Pyrosoma atlanticum (subantarctic) and the salp Salpa thompsoni (polar) dominated the micronekton biomass and MMP export ranged from 5.0 to 9.4 mg C m-2 d-1 across the three Southern Ocean sites. Mesozooplankton abundance was dominated by copepods, which contributed an additional 0.7 to 32.2 mg C m-2 d-1 to the MMP. Results from this summertime study suggest an increase in the relative importance of the MMP compared to the BGP south of the Polar Front, however, future work should target the seasonality of the MMP, which necessitates linking environmental drivers to micronekton and mesozooplankton community composition, life history, and DVM.

Sinking biogenic particles are central to transporting carbon to depth. To date, studies have focused on quantifying the downward export flux from the epipelagic (0-100 m), often neglecting particle fate in the mesopelagic (100–1000 m) due to sampling issues. Particle fate is set by sinking speed and flux attenuation which determine penetration depth. Characterising particle penetration depth is essential to quantify atmospheric return times for biologically-fixed carbon, hence the influence of the biological pump on climate. Here, a profiling float-based imaging sensor, measuring particle abundance over 14 size-classes (0.1–2.58 mm), revealed conspicuous particle export pulses, from two annual phytoplankton blooms, with size-dependent sinking speeds from 3 to 136 m d− 1. Penetration depth of small slow-sinking particles (< 0.6 mm) was < 200 m, accounting for 66% of POC attenuation across all size-classes over the mesopelagic. Larger particles, penetrating to > 900 m, resulted in only a small increase in POC attenuation to 77% at 200 m. This attenuation exceeded that from respiration (42%), derived from float-based oxygen measurements, suggesting that POC attenuation was jointly controlled by remineralisation and fragmentation. This float-based approach can assess the downward and return pathway of the biological pump.

The biological pump supplies carbon to the oceans' interior, driving long-term carbon sequestration and providing energy for deep-sea ecosystems1,2. Its efficiency is set by transformations of newly formed particles in the euphotic zone, followed by vertical flux attenuation via mesopelagic processes3. Depth attenuation of the particulate organic carbon (POC) flux is modulated by multiple processes involving zooplankton and/or microbes4,5. Nevertheless, it continues to be mainly parameterized using an empirically derived relationship, the 'Martin curve'6. The derived power-law exponent is the standard metric used to compare flux attenuation patterns across oceanic provinces7,8. Here we present in situ experimental findings from C-RESPIRE9, a dual particle interceptor and incubator deployed at multiple mesopelagic depths, measuring microbially mediated POC flux attenuation. We find that across six contrasting oceanic regimes, representing a 30-fold range in POC flux, degradation by particle-attached microbes comprised 7-29 per cent of flux attenuation, implying a more influential role for zooplankton in flux attenuation. Microbial remineralization, normalized to POC flux, ranged by 20-fold across sites and depths, with the lowest rates at high POC fluxes. Vertical trends, of up to threefold changes, were linked to strong temperature gradients at low-latitude sites. In contrast, temperature played a lesser role at mid- and high-latitude sites, where vertical trends may be set jointly by particle biochemistry, fragmentation and microbial ecophysiology. This deconstruction of the Martin curve reveals the underpinning mechanisms that drive microbially mediated POC flux attenuation across oceanic provinces.

Iron (Fe) is an essential micronutrient for phytoplankton growth, but its scarcity in seawater limits primary productivity across much of the ocean. Most dissolved Fe (DFe) in seawater is complexed with Fe-binding organic ligands, a poorly constrained fraction of dissolved organic matter (DOM), which increase Fe residence time and impact Fe bioavailability. Here, we present the conditional concentration (LFe) and binding-strength (log KFe'Lcond) of Fe-binding ligands in the Western Tropical South Pacific (WTSP) Ocean during the GEOTRACES TONGA cruise (GPpr14). The transect crossed the Lau basin, a region subject to shallow hydrothermal Fe inputs that fuel intense diazotrophic activity, the oligotrophic South Pacific gyre, and the Melanesian basin. Organic speciation was analyzed by competitive ligand exchange adsorptive cathodic stripping voltammetry (CLE-AdCSV) using salicylaldoxime at 25 µM. We found a high mean LFe of 5.2 ± 1.2 nMeqFe (n = 103) across the entire transect, predominantly consisting of intermediate strength L2 ligands (84%; mean log KFe'Lcond of 11.6 ± 0.4), consistent with humic-like substances. DFe correlated with the humic-like component of the fluorescent DOM (HS-like FDOM), yet the electroactive Fe-binding humic-like substances (LFeHS) accounted for only 20 ± 13% of LFe in the mixed layer and 8 ± 6% in deep waters. Ligands were in large excess compared to DFe (mean excess ligand eLFe = 4.6 ± 1.1 nMeqFe), suggesting poor stabilization of DFe inputs. High LFe (up to 9 nMeqFe) in samples close to hydrothermal sites could be due to detoxification strategies from plankton communities toward hydrothermally-fueled toxic trace metals other than Fe, with an apparent dilution of the DOM from the Lau basin into neighboring regions. We also observed a different peak potential of the Fe salicylaldoxime complex detected by CLE-AdCSV between the Lau and Melanesian basins, and between surface and deep waters. To our knowledge, this change in potential has not previously been reported; whether this represents a novel detection of specificities in DOM composition merits further investigation. Competition between Fe and competing metals for ligand binding sites could favor DFe oxidation and precipitation near hydrothermal vents and explain the absence of strong Fe stabilization in the WTSP.

The gravitational sinking of organic debris from ocean ecosystems is a dominant mechanism of the biological carbon pump (BCP) that regulates the global climate. The fraction of primary production exported downward, the e‐ratio, is an important but poorly constrained BCP metric. In mid‐ and high‐latitude oceans, seasonal and local variations of sinking particle fluxes strongly modulate the e‐ratio. These locally specific e‐ratio variations and their ecological foundations are here encapsulated in the term “export systems” (ES). ES have been partly characterized for a few ocean locations but remain largely ignored over most of the ocean surface. Here, in a fully conceptual approach and with the primary aim to understand rather than to estimate ocean carbon export, we combine biogeochemical (BGC) modeling with satellite observations to map ES at fine spatio‐temporal scales. We identify four plausible ES with distinct e‐ratio seasonalities across mid‐ and high‐latitude oceans. The ES map confirms the outlines of traditional BGC provinces and unveils new boundaries indicating where (and how) the annual relationship between carbon export and production changes markedly. At six sites where ES features can be partially inferred from in situ data, we test our approach and propose key ecological processes driving carbon export. In the light of our findings, a re‐examination of 1,841 field‐based e‐ratios could challenge the conventional wisdom that e‐ratios change strongly with latitude, suggesting a possible seasonal artifact caused by the timing of observations. By deciphering carbon export mechanistically, our conceptual ES map provides timely directions to emergent ocean robotic explorations of the BCP.

Iron is an essential nutrient that regulates productivity in ~30% of the ocean. Compared with deep (>2000 meter) hydrothermal activity at mid-ocean ridges that provide iron to the ocean's interior, shallow (<500 meter) hydrothermal fluids are likely to influence the surface's ecosystem. However, their effect is unknown. In this work, we show that fluids emitted along the Tonga volcanic arc (South Pacific) have a substantial impact on iron concentrations in the photic layer through vertical diffusion. This enrichment stimulates biological activity, resulting in an extensive patch of chlorophyll (360,000 square kilometers). Diazotroph activity is two to eight times higher and carbon export fluxes are two to three times higher in iron-enriched waters than in adjacent unfertilized waters. Such findings reveal a previously undescribed mechanism of natural iron fertilization in the ocean that fuels regional hotspot sinks for atmospheric CO2.

This study reports the only recent characterization of two contrasted wet deposition events collected during the PEACETIME (ProcEss studies at the Air–sEa Interface after dust deposition in the MEditerranean Sea) cruise in the open Mediterranean Sea (Med Sea) and their impact on trace metal (TM) marine stocks. Rain samples were analysed for Al, 12 TMs (Co, Cd, Cr, Cu, Fe, Mn, Mo, Ni, Pb, Ti, V and Zn) and nutrient (N, P, dissolved organic carbon) concentrations. The first rain sample collected in the Ionian Sea (Rain ION) was a typical regional background wet deposition event, whereas the second rain sample collected in the Algerian Basin (Rain FAST) was a Saharan dust wet deposition event. Even in the remote Med Sea, all background TM inputs presented an anthropogenic signature, except for Fe, Mn and Ti. The concentrations of TMs in the two rain samples were significantly lower compared to concentrations in rains collected at coastal sites reported in the literature, due to the decrease in anthropogenic emissions during the preceding decades. The atmospheric TM inputs were mainly dissolved forms, even in dusty Rain FAST. The TM stocks in the mixed layer (ML, 0–20 m) at the FAST station before and after the event showed that the atmospheric inputs were a significant supply of particulate TMs and dissolved Fe and Co for surface seawater. Even if the wet deposition delivers TMs mainly in soluble form, the post-deposition aerosol dissolution could to be a key additional pathway in the supply of dissolved TMs. At the scale of the western and central Mediterranean, the atmospheric inputs were of the same order of magnitude as ML stocks for dissolved Fe, Co and Zn, highlighting the role of the atmosphere in their biogeochemical cycles in the stratified Med Sea. In case of intense dust-rich wet deposition events, the role of atmospheric inputs as an external source was extended to dissolved Co, Fe, Mn, Pb and Zn. Our results suggest that the wet deposition constitutes only a source of some of dissolved TMs for Med Sea surface waters. The contribution of dry deposition to the atmospheric TM inputs needs to be investigated.

In the Western Tropical South Pacific, a hotspot of dinitrogen‐fixing organisms has been identified. The survival of these species depends on the availability of dissolved iron (DFe); however, the source of this DFe is still unclear. DFe was measured along a transect from 175°E to 166°W near 19–21°S. The distribution of DFe showed high spatial variability: low concentrations (∼0.2 nmol kg−1) in the South Pacific gyre and high concentrations (up to 50 nmol kg−1) in the west of the Tonga arc, indicating that this arc is a clear boundary between iron‐poor and iron‐rich waters. An optimal multiparameter analysis was used to distinguish the relative importance of physical transport relative to non‐conservative processes on the observed distribution. This analysis demonstrated that the shallow hydrothermal sources present along the Tonga‐Kermadec arc are responsible for the high concentrations observed in the photic layer. Nevertheless, in contrast to what has been observed for deep hydrothermal plumes, our results highlighted the rapid decrease in DFe concentrations near shallow hydrothermal sources. This is likely due to a shorter residence time of surface water masses combined with several biogeochemical processes at play (precipitation, scavenging, biological uptake, and photoreduction). This study clearly highlights the role of shallow hydrothermal sources on the DFe cycle within the Tonga‐Kermadec arc where a strong link to biological activity in surface waters can be assessed, despite the small but significant fraction of DFe ultimately stabilized. It also emphasizes the need to consider the impact of these sources for a better understanding of the global iron cycle.

N2 fixation rates were measured in the 0–1000 m layer at 13 stations located in the open western and central Mediterranean Sea (MS) during the PEACETIME cruise (late spring 2017). While the spatial variability in N2 fixation was not related to Fe, P nor N stocks, the surface composition of the diazotrophic community indicated a strong longitudinal gradient increasing eastward for the relative abundance of non-cyanobacterial diazotrophs (NCDs) (mainly γ-Proteobacteria) and conversely decreasing eastward for photo-heterotrophic group A (UCYN-A) (mainly UCYN-A1 and UCYN-A3), as did N2 fixation rates. UCYN-A4 and UCYN-A3 were identified for the first time in the MS. The westernmost station influenced by Atlantic waters and characterized by highest stocks of N and P displayed a patchy distribution of diazotrophic activity with an exceptionally high rate in the euphotic layer of 72.1 nmolNL-1d-1, which could support up to 19 % of primary production. At this station at 1 % PAR (photosynthetically available radiation) depth, UCYN-A4 represented up to 94 % of the diazotrophic community. These in situ observations of greater relative abundance of UCYN-A at stations with higher nutrient concentrations and dominance of NCDs at more oligotrophic stations suggest that nutrient conditions – even in the nanomolar range – may determine the composition of diazotrophic communities and in turn N2 fixation rates. The impact of Saharan dust deposition on N2 fixation and diazotrophic communities was also investigated, under present and future projected conditions of temperature and pH during short-term (3–4 d) experiments at three stations. New nutrients from simulated dust deposition triggered a significant stimulation of N2 fixation (from 41 % to 565 %). The strongest increase in N2 fixation was observed at the stations dominated by NCDs and did not lead on this short timescale to changes in the diazotrophic community composition. Under projected future conditions, N2 fixation was either increased or unchanged; in that later case this was probably due to a too-low nutrient bioavailability or an increased grazing pressure. The future warming and acidification likely benefited NCDs (Pseudomonas) and UCYN-A2, while disadvantaged UCYN-A3 without knowing which effect (alone or in combination) is the driver, especially since we do not know the temperature optima of these species not yet cultivated as well as the effect of acidification.

Earth and Space Science Open Archive This preprint has been submitted to and is under consideration at Global Biogeochemical Cycles. ESSOAr is a venue for early communication or feedback before peer review. Data may be preliminary.Learn more about preprints preprintOpen AccessYou are viewing the latest version by default [v1]DFe patterns impacted by shallow hydrothermal sources along a transect through the Tonga-Kermadec arcAuthorsChloéTillietteiDVincentTaillandierPascaleBouruet-AubertotNicolasGrimaiDChristopheMaesiDMarylineMontanesGeraldineSarthouiDMaria-ElenaVorrathiDVeronicaArnoneMatthieuBressacDavidGonzález-SantanaFrédéricGAZEAUCécileGuieuiDSee all authors Chloé TillietteiDCorresponding Author• Submitting AuthorSorbonne Université, CNRS, Laboratoire d'Océanographie de VillefrancheiDhttps://orcid.org/0000-0002-3009-9342view email addressThe email was not providedcopy email addressVincent TaillandierSorbonne Université, CNRS, Laboratoire d'Océanographie de Villefrancheview email addressThe email was not providedcopy email addressPascale Bouruet-AubertotPierre and Marie Curie Universityview email addressThe email was not providedcopy email addressNicolas GrimaiDFrench National Centre for Scientific Research (CNRS)iDhttps://orcid.org/0000-0002-1607-8975view email addressThe email was not providedcopy email addressChristophe MaesiDLOPS/IRDiDhttps://orcid.org/0000-0001-6532-7141view email addressThe email was not providedcopy email addressMaryline MontanesSorbonne Université, CNRS, Laboratoire d'Océanographie de Villefrancheview email addressThe email was not providedcopy email addressGeraldine SarthouiDInstitut Universitaire Europeen de la meriDhttps://orcid.org/0000-0001-6560-6669view email addressThe email was not providedcopy email addressMaria-Elena VorrathiDUniv Hamburg, Inst Geol, Bundesstr 55, D-20146 Hamburg, GermanyiDhttps://orcid.org/0000-0001-7208-1186view email addressThe email was not providedcopy email addressVeronica Arnone6Instituto de Oceanografía y Cambio Global, IOCAG, Universidad de Las Palmas de Gran Canariaview email addressThe email was not providedcopy email addressMatthieu BressacFrench National Centre for Scientific Researchview email addressThe email was not providedcopy email addressDavid González-SantanaUniv Brest, CNRS, IRD, Ifremer, LEMARview email addressThe email was not providedcopy email addressFrédéric GAZEAUUnknownview email addressThe email was not providedcopy email addressCécile GuieuiDLaboratoire d'Oceanographie de Villefranche (LOV)iDhttps://orcid.org/0000-0001-6373-8326view email addressThe email was not providedcopy email address

Dissolved iron (DFe) supply is pivotal in setting phytoplankton productivity and bloom dynamics in remote areas. In the Southern Ocean, phytoplankton benefit from vertical Fe supply from a subsurface reservoir (termed new Fe) which triggers the beginning of the phytoplankton growth season. The main physical supplies of DFe at play are storms mixing and eddies advection. But while the relevance of physical and chemical gradients for the biological dynamics is nowadays obvious, the underlying mechanisms to restock the DFe pool and the responses of phytoplankton biomass to transient vertical of this new DFe supply remains under question. A strong hypothesis is that remineralization process in the mesopelagic zone participate to restock nutrients, including Fe. We used a two-step experiment to simulate the seasonal DFe supply pathways on natural microbial residents acclimated to low DFe levels (late in the growth season), and to investigate their response towards a microbiologically regenerated DFe source. This study shows that regenerated DFe from subsurface particles enhances secondary production by bacteria and stimulates specific phytoplankton taxa to grow in surface waters. In particular, we present evidence that small species and non-siliceous cells were better able to take advantage of Fe

In the Subantarctic sector of the Southern Ocean, vertical entrainment of iron (Fe) triggers the seasonal productivity cycle but diminishing physical supply during the spring to summer transition forces microbial assemblages to rapidly acclimate. Here, we tested how phytoplankton and bacteria within an isolated eddy respond to different dissolved Fe (DFe)/ligand inputs. We used three treatments: one that mimicked the entrainment of new DFe (Fe-NEW), another in which DFe was supplied from bacterial regeneration of particles (Fe-REG), and a control with no addition of DFe (Fe-NO). After 6 days, 3.5 (Fe-NO, Fe-NEW) to 5-fold (Fe-REG) increases in Chlorophyll a were observed. These responses of the phytoplankton community were best explained by the differences between the treatments in the amount of DFe recycled during the incubation (Fe-REG, 15% recycled c.f. 40% Fe-NEW, 60% Fe-NO). This additional recycling was more likely mediated by bacteria. By day 6, bacterial production was comparable between Fe-NO and Fe-NEW but was approximately two-fold higher in Fe-REG. A preferential response of phytoplankton (haptophyte-dominated) relative to high nucleic acid (HNA) bacteria was also found in the Fe-REG treatment while the relative proportion of diatoms increased faster in the Fe-NEW and Fe-NO treatments. Comparisons between light and dark incubations further confirmed the competition between picophytoplankton and HNA for DFe. Overall, our results demonstrate great versatility by microorganisms to use different Fe sources that results in highly efficient Fe recycling within surface waters. This study also encourages future research to further investigate the interactions between functional groups of microbes (e.g. HNA and cyanobacteria) to better constraint modeling in Fe and carbon biogeochemical cycles.

Abstract. Mineral dust deposition is an important supply mechanism for trace elements in the low-latitude ocean. Our understanding of the controls of such inputs has been mostly built onto laboratory and surface ocean studies. The lack of direct observations and the tendency to focus on near surface waters prevent a comprehensive evaluation of the role of dust in oceanic biogeochemical cycles. In the frame of the PEACETIME project (ProcEss studies at the Air-sEa Interface after dust deposition in the MEditerranean sea), the responses of the aluminium (Al) and iron (Fe) cycles to two dust wet deposition events over the central and western Mediterranean Sea were investigated at a timescale of hours to days using a comprehensive dataset gathering dissolved and suspended particulate concentrations, along with sinking fluxes. Dissolved Al (dAl) removal was dominant over dAl released from dust. Fe / Al ratio of suspended and sinking particles revealed that biogenic particles, and in particular diatoms, were key in accumulating and exporting Al relative to Fe. By combining these observations with published Al / Si ratios of diatoms, we show that adsorption onto biogenic particles, rather than active uptake, represents the main sink for dAl in Mediterranean waters. In contrast, systematic dissolved Fe (dFe) accumulation occurred in subsurface waters (~100–1000 m), while dFe input from dust was only transient in the surface mixed-layer. The rapid transfer of dust to depth (up to ~180 m d−1), the Fe-binding ligand pool in excess to dFe in subsurface (while nearly-saturated in surface), and low scavenging rates in this particle-poor depth horizon are all important drivers of this subsurface dFe enrichment. At the annual scale, this previously overlooked mechanism may represent an additional pathway of dFe supply for the surface ocean through diapycnal diffusion and vertical mixing. However, low subsurface dFe concentrations observed at the basin scale (< 0.5 nmol kg−1) questions the residence time for this dust-derived subsurface reservoir, and hence its role as a supply mechanism for the surface ocean, stressing the need for further studies. Finally, these contrasting responses indicate that dAl is a poor tracer of dFe input in the Mediterranean Sea.

Lithogenic elements such as aluminum (Al), iron (Fe), rare earth elements (REEs), thorium (232Th and 230Th, given as Th) and protactinium (Pa) are often assumed to be insoluble. In this study, their dissolution from Saharan dust reaching Mediterranean seawater was studied through tank experiments over 3 to 4 d under controlled conditions including controls without dust addition as well as dust seeding under present and future climate conditions (+3 ∘C and −0.3 pH). Unfiltered surface seawater from three oligotrophic regions (Tyrrhenian Sea, Ionian Sea and Algerian Basin) were used. The maximum dissolution was low for all seeding experiments: less than 0.3 % for Fe, 1 % for 232Th and Al, about 2 %–5 % for REEs and less than 6 % for Pa. Different behaviors were observed: dissolved Al increased until the end of the experiments, Fe did not dissolve significantly, and Th and light REEs were scavenged back on particles after a fast initial release. The constant 230Th/232Th ratio during the scavenging phase suggests that there is little or no further dissolution after the initial Th release. Quite unexpectedly, comparison of present and future conditions indicates that changes in temperature and/or pH influence the release of Th and REEs in seawater, leading to lower Th release and a higher light REE release under increased greenhouse conditions.