Stéphane Blain

TonB-dependent transporters (TBDTs) play a central role in siderophore-mediated iron acquisition in the marine environment. However, assigning substrate specificity to TBDTs remains a major bottleneck in understanding the ecological and mechanistic roles of siderophore production and uptake. Here, we identify in Marinobacter nauticus two TBDTs, OptA and OptB, that share low sequence identity yet both mediate Fe-petrobactin uptake. Despite this functional overlap, they underlie distinct regulatory mechanisms, suggesting differential utilization under specific environmental conditions. Phylogenetic analyses indicate that OptA likely co-evolved with the petrobactin biosynthetic gene cluster, whereas OptB appears to have followed a distinct evolutionary trajectory, acquiring the capacity to import petrobactin as a xenosiderophore. Using resuscitated communities from the Southern Ocean, we further show that Fe-petrobactin stimulated prokaryotic growth. Based on amino acid similarity, we identified OptB-like transporters encoded in metagenomes and MAGs and show that taxa harbouring these transporters, such as Pseudoaltermonas , respond specifically to Fe-petrobactin addition. OptB-like transporters were also detected in environmental metagenomes from the Southern Ocean, whereas the petrobactin biosynthetic gene clusters were not, indicating that petrobactin primarily functions as a xenosiderophore in natural communities.

The trace element iron (Fe) is a major constraint for microbially mediated processes in the Southern Ocean. The accelerated melting of glaciers could present a novel source of Fe, but whether glacial Fe is bioavailable to marine microbes is not known. We investigated the response of marine heterotrophic prokaryotes to Fe contained in colloids (20–200 nm) of glacial origin collected on Kerguelen Island (Southern Ocean). We followed prokaryotic growth in incubation experiments amended with colloids of either glacial or nonglacial origin and determined community composition and the abundance of genes involved in Fe-related processes in metagenomes and metagenome-assembled genomes (MAGs) at the final time point. Prokaryotic taxa belonging to Vibrionaceae and Pseudomonadaceae accounted together for 32% to 67% of the relative abundances in the glacial colloid–amended treatments, while Rhodobacteraceae, Flavobacteriaceae, and Alteromonadaceae were the dominant contributors to the communities in the incubations amended with nonglacial colloids. Metagenomic analysis revealed a higher abundance of genes involved in the biosynthesis of the siderophores pyoverdine and vibrioferrin as well as their respective transporters in the presence of glacial colloids compared to nonglacial colloids. Genes for the transport of both siderophores were present in diverse MAGs, while biosynthesis genes were detected in fewer MAGs. Our results suggest that the utilization of siderophores facilitates access to Fe from glacial colloids and points to the key role of specific prokaryotes in rendering this source of Fe available to Southern Ocean microbial communities.

Marine microbes are strongly interrelated to trace metals in the ocean. How the availability of trace metals selects for prokaryotic taxa and the potential feedback of microbial processes on the trace metal distribution in the ocean remain poorly understood. We investigate here the potential reciprocal links between diverse prokaryotic taxa and iron (Fe), manganese (Mn), copper (Cu), and nickel (Ni) as well as apparent oxygen utilization (AOU) across 12 well‐defined water masses in the Southern Indian Ocean (SWINGS—South West Indian Ocean GEOTRACES GS02 Section cruise). Applying partial least square regression (PLSR) analysis, we show that the water masses are associated with particular latent vectors that are a combination of the spatial distribution of prokaryotic taxa, trace elements, and AOU. This approach provides novel insights on the potential interactions between prokaryotic taxa and trace metals in relation to organic matter remineralization in distinct water masses of the ocean.

The Southern Ocean (SO) is known for its atypical bio‐optical regime. This complicates the interpretation of proxies measured from satellite and in situ platforms equipped with optical sensors, which occupy an important niche for monitoring the vast and remote SO. A ship‐based field study in concert with time series observations from BioGeoChemical‐Argo (BGC‐Argo) profiling floats were used to investigate spatial and temporal variations in bio‐optical relationships in the open ocean waters surrounding the Kerguelen Plateau in the Indian sector of the SO. Compared to other regions with similar chlorophyll concentrations, chlorophyll‐specific phytoplankton absorption in the blue waveband presented a consistent negative anomaly. The anomaly was uniform over deep mixed layers and correlated with phytoplankton size, photoacclimation and atypically high concentrations of fucoxanthin. The BGC‐Argo observation‐based proxies revealed that the blue absorption anomaly increased with chlorophyll concentration both spatially and temporally and, while particularly pronounced in the naturally iron‐fertilized waters, was also found in the High Nutrient Low Chlorophyll region. While phytoplankton size was an important driver of the anomaly, photoacclimation associated with self‐shading of phytoplankton cells was also involved during intense booms. The backscattering coefficient exhibited negative and positive anomalies in the low and high biomass regimes, respectively. The large positive anomaly in high biomass regimes was attributed to the variable non‐algal particles characteristics associated with a relatively high production of bloom by‐products. With clear understanding of the bio‐optical anomalies, BGC‐Argo floats stand as unique tools for monitoring the bio‐optical spatio‐temporal complexity of the SO.

The Agulhas Current in the southwest Indian Ocean is the strongest western boundary current on Earth. The major role of the Agulhas Current in driving significant heat and salt fluxes is well known, yet its biogeochemical fluxes remain largely uncharacterised. Here, we use nitrate isotopes (δ15N, δ18O, and Δ(15-18) = δ15N-δ18O) to evaluate nutrient supply mechanisms that ultimately support new production in the southwest Indian Ocean. Across the greater Agulhas region, thermocline nitrate-δ15N is lower (4.9-5.8‰) than the underlying Subantarctic Mode Water source (δ15N of 6.9‰) and the upstream source regions (where nitrate-δ15N ranges from 6.4-7.0‰), which we attribute to local N2 fixation. Using a one-box model to simulate the newly-fixed nitrate flux, we estimate a local N2 fixation rate of 7-25 Tg N.a-1, amounting to ~30-95% of the whole Indian Ocean nitrogen gain estimated by models. Thermocline and mixed-layer nitrate Δ(15-18) is also low, due to both N2 fixation and coupled partial nitrate assimilation and nitrification. This local nitrogen cycling imprints an isotopic signal on Indian Ocean nitrate that persists in Agulhas rings that “leak” into the South Atlantic and are subsequently transported northwards. If this signal is retained in calcifying organisms (e.g., foraminifera) deposited on the seafloor, it could be used to trace past Agulhas leakage, yielding quantitative insights into the strength of the Atlantic Meridional Overturning Circulation over time. In addition to local N2 fixation, the nitrate isotopes reveal three physical mechanisms of subsurface nitrate supply: i) inshore upwelling driven by the current and winds, ii) entrainment at the edges of a mesoscale eddy, and iii) density-driven overturning at the current edge induced by strong horizontal velocity and density shears. All these nitrate supply mechanisms are evident as incidences of relatively high-Δ(15-18) nitrate in the thermocline and surface yet the intensity and subsurface expression of some of them is not apparent in the physical data, highlighting the utility of the nitrate isotopes for exploring physical ocean processes. The high mesoscale variability that likely drives subsurface nitrate supply to Agulhas Current surface waters is common to all western boundary currents, implying that vertical nitrate entrainment is quantitatively significant in all such systems. We posit that along with N2 fixation, physical mechanisms of upward nitrate supply enhance ocean fertility and possibly carbon export in the South Indian Ocean. Higher rates of warming, and thus thermal stratification, are expected to decrease Indian Ocean productivity more rapidly in the future than that of other ocean basins. However, a coincident increase in eddy kinetic energy across boundary currents may enhance the upward nutrient supply, partially offsetting the stratification-driven decline in productivity.

The greater Agulhas Current region is an important component of the climate system, yet its influence on carbon and nutrient cycling is poorly understood. Here, we use nitrate isotopes (δ¹⁵N, δ¹⁸O, Δ(15–18) = δ¹⁵N–δ¹⁸O) to trace regional water mass circulation and investigate nitrogen cycling in the Agulhas Current and adjacent recirculating waters. The deep and intermediate waters record processes occurring remotely, including partial nitrate assimilation in the Southern Ocean and denitrification in the Arabian Sea. In the thermocline and surface, tropically sourced waters are biogeochemically distinct from adjacent subtropically sourced waters, confirming inhibited lateral mixing across the current core. (Sub)tropical thermocline nitrate δ¹⁵N is lower (4.9–5.8‰) than the sub‐thermocline source, Subantarctic Mode Water (6.9‰); we attribute this difference to local N₂ fixation. Using a one‐box model to simulate the newly fixed nitrate flux, we estimate a local N₂ fixation rate of 7–25 Tg N.a⁻¹, with the upper limit likely biased high. In the mixed layer, nitrate δ¹⁵N and δ¹⁸O rise in unison, indicating that phytoplankton nitrate assimilation dominates in surface waters, with nitrification restricted to deeper waters. Because nitrate assimilation and nitrification are vertically decoupled, the rate of nitrate assimilation plus N₂ fixation can be used to approximate carbon export. Thermocline and mixed‐layer nitrate Δ(15–18) is low, due to both N₂ fixation and coupled partial nitrate assimilation and nitrification. Similarly low‐Δ(15–18) nitrate in Agulhas rings indicates leakage of low‐δ¹⁵N nitrogen into the South Atlantic, which should be recorded in the organic matter sinking to the seafloor, providing a potential tracer of past Agulhas leakage.

Trace elements (TE) are tracers of multiple biotic and abiotic processes in the ocean and some of them are essential for marine life. Vertical export by particles is a major removal process of a large fraction of TE from the surface ocean. However, the seasonal export dynamics and its controlling factors, critical for the understanding of the internal TE cycling, remain poorly constrained. Here, we report and discuss the seasonal export of 15 TE in sinking particles collected by a sediment trap deployed in a highly productive region of the Southern Ocean. Basalt material was the main carrier phase for the export flux of 9 TE, and its dynamic was characterized by a strong decrease over time. TE export driven by biological carriers such as diatom spores and vegetative cells added pulsed seasonal dynamics to the lithogenic signal, while the contribution of fecal pellets was less variable over the season. For each TE, we were able to decipher the biological carrier phases that represent the most dominant export pathway. We discuss this partitioning with regards to the known metabolic functions of the different trace metals or TE of biological interest.

Trace elements (TE) are tracers of multiple biotic and abiotic processes in the ocean and some of them are essential for marine life. Vertical export by particles is a major removal process of a large fraction of TE from the surface ocean. However, the seasonal export dynamics and its controlling factors, critical for the understanding of the internal TE cycling, remain poorly constrained. Here, we report and discuss the seasonal export of 15 TE in sinking particles collected by a sediment trap deployed in a highly productive region of the Southern Ocean. Basalt material was the main carrier phase for the export flux of 9 TE, and its dynamic was characterised by a strong decrease over time. TE export driven by biological carriers such as diatom spores and vegetative cells added pulsed seasonal dynamics to the lithogenic signal, while the contribution of fecal pellets was less variable over the season. For each TE, we were able to decipher the biological carrier phases that represent the most dominant export pathway. We discuss this partitioning with regards to the known metabolic functions of the different trace metals or TE of biological interest.

As marine microorganisms and environmental conditions coevolved over geological timescales, metals have been incorporated into all essential metabolic processes. In the modern ocean, metals are present from trace amounts limiting microbial growth to toxic concentrations. Dissolved trace metals are a major bioavailable reservoir. However, the acquisition of metals from marine particles remains largely unexplored. Here, we combined chemical characterization and a comparative metatranscriptomics approach to investigate the availability of nine metals of biological importance on particles collected in the region of Heard Island (Indian sector of the Southern Ocean). Elemental ratios identified particulate matter as a potential source of metals for prokaryotes. The expression of genes for the uptake of metals through various mechanisms demonstrated that particles are a bioavailable reservoir. But genes involved in the control of resistance to metal toxicity, storage, sensing, and regulation were also highly expressed. Our observations suggest that homeostasis associated with a diverse prokaryotic community is the overarching mechanism that enhances the trace element processing on particles. These results provide clues that microbial activity on particles is critical in the redistribution of trace elements between different fractions and chemical forms in the ocean.

Across the Southern Ocean, phytoplankton growth is governed by iron and light, while bacterial growth is regulated by iron and labile dissolved organic carbon (LDOC). We use a mechanistic model to examine how competition for iron between phytoplankton and bacteria responds to changes in iron, light, and LDOC. Consistent with experimental evidence, increasing iron and light encourages phytoplankton dominance, while increasing LDOC and decreasing light favors bacterial dominance. Under elevated LDOC, bacteria can outcompete phytoplankton for iron, most easily under lower iron. Simulations reveal that bacteria are major iron consumers and suggest that luxury storage plays a key role in competitive iron uptake. Under seasonal conditions typical of the Southern Ocean, sources of LDOC besides phytoplankton exudation modulate the strength of competitive interactions. Continued investigations on the competitive fitness of bacteria in driving changes in primary production in iron‐limited systems will be invaluable in refining these results.