Larger Size Fractions Research Articles

Global Trends in the Distribution of Biogenic Minerals in the Ocean

AbstractThe cycling of marine particulate matter is critical for sequestering carbon in the deep ocean and in marine sediments. Biogenic minerals such as calcium carbonate (CaCO3) and opal add density to more buoyant organic material, facilitating particle sinking and export. Here, we compile and analyze a global data set of particulate organic carbon (POC), particulate inorganic carbon (PIC, or CaCO3), and biogenic silica (bSi, or opal) concentrations collected using large volume pumps (LVPs). We analyze the distribution of all three biogenic phases in the small (1–53 μm) and large (>53 μm) size classes. Over the entire water column 76% of POC exists in the small size fraction. Similarly, the small size class contains 82% of PIC, indicating the importance of small‐sized coccolithophores to the PIC budget of the ocean. In contrast, 50% of bSi exists in the large size fraction, reflecting the larger size of diatoms and radiolarians compared with coccolithophores. We use PIC:POC and bSi:POC ratios in the upper ocean to document a consistent signal of shallow mineral dissolution, likely linked to biologically mediated processes. Sediment trap PIC:POC and bSi:POC are elevated with respect to LVP samples and increase strongly with depth, indicating the concentration of mineral phases and/or a deficit of POC in large sinking particles. We suggest that future sampling campaigns pair LVPs with sediment traps to capture the full particulate field, especially the large aggregates that contribute to mineral‐rich deep ocean fluxes, and may be missed by LVPs.

Achieving highest Young's modulus in Al-Li by tracing the size and bonding evolution of Li-rich precipitates

For decades, it has been well accepted that every 1 wt.% Li addition to Al will reduce Al alloy's density by 3% and increase its Young's modulus by 6%. However, the fundamental mechanism of modulus improvements stays controversial though all studies agreed that the contribution of such a substantial boosting comes from Li-rich clusters either in solid solution or precipitations. In this study, we experimentally produce nano-sized Li-rich clusters by non-equilibrium solidification using centrifugal casting and trace their evolutions as a function of subsequent heat treatments. High-resolution transmission electron microscopy (HRTEM) reveals a further decrease in the lattice constants of Li-rich regions from the as-cast (0.406 nm), solid solution (0.405 nm) to the aged state (0.401 nm), while Young's modulus of the Al-Li alloy reaches 89.16 GPa. Small-angle neutron scattering (SANS) experiments and first-principle calculations based on density functional theory have shown both the bond strength around precipitates and the size of those Li-rich region dominate Young's modulus. At the beginning, it is volumetric compression due to Li addition that increases modulus, tightening the Al-Al potential curves. In the end, it is the Al-Al and Al-Li valence bonds in Al3Li at large size and high-volume fraction which increase its second derivative of internal energy and thus Young's modulus.

Sampling Procedure, Characterization, and Quantitative Analyses of Industrial Aluminum White Dross

Up to 10% of all metallic aluminum (Al) produced is lost due to dross generation during production. It also causes an environmental problem due to hazardous waste generation following treatment, as well as toxic gas generation when in contact with water/humidity. As a result, it is desirable to identify the parameters contributing to the generation of the Al white dross mixture. In view of this, a sampling procedure for collecting representative dross samples directly from the casthouse holding furnace has been developed together with a methodology for analyzing and evaluating the results. XRD was used for phase/chemical composition analyses of sieved dross samples, and both ring milling and cryomilling were used during sample preparation. Cryomilling proved to be the superior method allowing dross fractions < 1.25 and 1.25–4.5 mm to be pulverized. The fractions were analyzed and revealed that the sampling location inside the furnace plays a vital role as the injection of primary-produced Al into the furnace influences the dross characteristics. From Location 1 (closest to the injection point) to Location 4 (furthest away from the injection point), the metallic Al content in the dross decreased simultaneously as the oxide content increased. The results also confirmed that the larger size fraction had a higher metallic Al concentration, which correlates well with literature findings. By adopting a methodical sampling procedure followed by consistent routines for sample preparation, characterization, and analyses, process operations can be studied and thereby potentially used to minimize the loss of Al due to dross formation.Graphical

Salt‐Induced Flocculation of Dissolved Organic Matter and Iron Is Controlled by Their Concentration and Ratio in Boreal Coastal Systems

AbstractRivers and estuaries are important conduits and reactors for organic matter (OM). Despite the substantial export of terrestrial OM by rivers, only a small proportion of this material reaches the open ocean. One potentially important mechanism contributing to the removal of terrestrial dissolved OM (DOM) is flocculation; a process that transforms DOM into larger size fractions that can settle into sediment pools. Here we investigate the role of flocculation in adjacent boreal coastal systems over three consecutive seasons. Flocculation experiments, which include the artificial salting of freshwater DOM to mid (12 psu) and high salinity (25 psu), and a 1:1 mixture of freshwater DOM with their respective saline endmember, highlight a potentially important source of particulate carbon (PC) in boreal estuaries. Along with a 3.5% ± 1.0% removal of dissolved organic carbon (DOC) and 44% ± 16% dissolved iron (DFe), the potential for flocculation of these constituents significantly increased with increasing salinity from 12 to 25 psu. The particulate matter (PM) produced by salt‐induced flocculation was comparable to in situ PC concentrations (in situ PC = 27.5 μmol L−1 and flocculated PC = 15.0 μmol L−1) and stable carbon isotopic signatures (in situ PM = −28.8‰ and flocculated PM = −28.3‰). DFe:DOC and Sr were the only parameters that could significantly explain the degree of carbon and iron flocculation. This demonstrates the importance of DOC, DFe, and optical properties, and the predictive value of DFe:DOC for understanding DOM susceptibility to flocculation and its relevance and contribution to regional and global carbon budgets.

Implications of particle size on the respective solid-state properties of naltrexone in PLGA microparticles

A thorough understanding of the complexities in formulating and manufacturing polymeric microspheres is required for new and generic drug applications. Specifically, for an ANDA application for polymeric microsphere-based products, the applicant must meet Q1 (qualitative) and Q2 (quantitative) sameness, and in some cases, Q3 (e.g., microstructural) sameness. Herein, we report the naltrexone crystallinity in a PLGA microparticle system prepared from a dichloromethane-benzyl alcohol solvent system results in a crystallinity dependence as a function of microparticle size from the same batch – illustrating intrabatch microstructural variability. As the particle size increases, the crystallinity increases, with additional polymorphic forms more readily noted at the large particle sizes. Furthermore, during dissolution, a polymorphic transition and/or crystallization occurs at larger size fractions. This study highlights the importance of controlling the manufacturing parameters during microparticle formation, specifically solvent extraction and particle size control. Furthermore, with the approval of generic microparticles formulations on the horizon, this study highlights the importance of Q3, the same components in the same concentration with the same arrangement of matter, whereby microparticles can have varying microstructural properties across particle sizes from the same batch.

Phytoplankton Sources and Sinks of Dimethylsulphoniopropionate (DMSP) in Temperate Coastal Waters of Australia.

The ecologically important organic sulfur compound, dimethylsulfoniopropionate (DMSP), is ubiquitous in marine environments. Produced by some species of phytoplankton and bacteria, it plays a key role in cellular responses to environmental change. Recently, uptake of DMSP by non-DMSP-producing phytoplankton species has been demonstrated, highlighting knowledge gaps concerning DMSP distribution through the marine microbial food web. In this study, we traced the uptake and distribution of DMSP through a natural marine microbial community collected from off the eastern coastline Australia. We found a diverse phytoplankton community representing six major taxonomic groups and conducted DMSP-enrichment experiments both on the whole community, and the community separated into large (≥8.0 µm), medium (3.0–8.0 µm), and small (0.2–3.0 µm) size fractions. Our results revealed active uptake of DMSP in all three size fractions of the community, with the largest fraction (>8 µm) forming the major DMSP sink, where enrichment resulted in an increase of DMSPp by 144%. We observed evidence for DMSP catabolism in all size fractions with DMSP enrichment, highlighting loss from the system via MeSH or DMS production. Based on taxonomic diversity, we postulate the sources of DMSP were the dinoflagellates, Phaeocystis sp., and Trichodesmium sp., which were present in a relatively high abundance, and the sinks for DMSP were the diatoms and picoeucaryotes in this temperate community. These findings corroborate the role of hitherto disregarded phytoplankton taxa as potentially important players in the cycling of DMSP in coastal waters of Australia and emphasize the need to better understand the fate of accumulated DMSP and its significance in cellular metabolism of non-DMSP producers.

Hydrographic Feature Variation Caused Pronounced Differences in Planktonic Ciliate Community in the Pacific Arctic Region in the Summer of 2016 and 2019.

Planktonic ciliates are an important component of microzooplankton, but there is limited understanding of their responses to changing environmental conditions in the Pacific Arctic Region. We investigated the variations of ciliate community structure and their relationships with environmental features in the Pacific Arctic Region in the summer of 2016 and 2019. The Pacific water was warmer and more saline in 2019 than in 2016. The abundance and biomass of total ciliate and aloricate ciliate were significantly higher in 2019 than those in 2016, while those of tintinnid were significantly lower. The dominant aloricate ciliate changed from large size-fraction (> 30 μm) in 2016 to small size-fraction (10–20 μm) in 2019. More tintinnid species belonging to cosmopolitan genera were found in 2019 than in 2016, and the distribution of tintinnid species (Codonellopsis frigida, Ptychocylis obtusa, and Salpingella sp.1) in 2019 expanded by 5.9, 5.2, and 8.8 degrees further north of where they occurred in 2016. The environmental variables that best-matched tintinnid distributions were temperature and salinity, while the best match for aloricate ciliate distributions was temperature. Therefore, the temperature might play a key role in ciliate distribution. These results provide basic data on the response of the planktonic ciliate community to hydrographic variations and implicate the potential response of microzooplankton to Pacification as rapid warming progresses in the Pacific Arctic Region.

Sources and processes of organic aerosol in non-refractory PM1 and PM2.5 during foggy and haze episodes in an urban environment of the Yangtze River Delta, China

Organic aerosol (OA) generally accounts for a large fraction of fine particulate matter (PM2.5) in the urban atmosphere. Despite significant advances in the understanding their emission sources, transformation processes and optical properties in the submicron aerosol fraction (PM1), larger size fractions - e.g., PM2.5 - still deserve complementary investigations. In this study, we conducted a comprehensive analysis on sources, formation process and optical properties of OA in PM1 and PM2.5 under haze and foggy environments in the Yangtze River Delta (eastern China), using two aerosol chemical speciation monitors, as well as a photoacoustic extinctiometer at 870 nm. Positive matrix factorization analysis - using multilinear engine (ME2) algorithm - was conducted on PM1 and PM2.5 organic mass spectra. Four OA factors were identified, including three primary OA (POA) factors, i.e., hydrocarbon-like OA (HOA), cooking OA (COA), and biomass burning OA (BBOA), and a secondary OA (SOA) factor, i.e., oxidized oxygenated OA (OOA). An enhanced PM1-2.5 COA concentration was clearly observed during cooking peak hours, suggesting important contribution of fresh cooking emissions on large-sized particles (i.e., PM1-2.5). The oxidation state and concentration of PM2.5 HOA were higher than that in PM1, suggesting that large-sized HOA particles might be linked to oxidized POA. High contribution (44%) of large-sized OOA to non-refractory PM2.5 mass was observed during haze episodes. During foggy episodes, PM1 and PM2.5 OOA concentrations increased as a positive relationship over time, along with an exponential increase in the PM2.5-OOA to PM1-OOA ratio. Meanwhile, OOA loadings increased with the aerosol liquid water content (ALWC) during foggy episodes. Random forest cross-validation analysis also supported the important influence of ALWC on OOA variations, supporting substantial impact of aqueous process on SOA formation during haze and/or foggy episodes. Obtained results also indicated high OOA contributions (21%–36%) and low POA contributions (6%–14%) to the PM2.5 scattering coefficient during haze and foggy episodes, respectively. Finally, we could illustrate that atmospheric vertical diffusion and horizontal transport have important but different effects on the concentrations of different primary and secondary OA factors in different particle size fractions.

Contrasting diversity patterns of prokaryotes and protists over time and depth at the San-Pedro Ocean Time series

Community dynamics are central in microbial ecology, yet we lack studies comparing diversity patterns among marine protists and prokaryotes over depth and multiple years. Here, we characterized microbes at the San-Pedro Ocean Time series (2005–2018), using SSU rRNA gene sequencing from two size fractions (0.2–1 and 1–80 μm), with a universal primer set that amplifies from both prokaryotes and eukaryotes, allowing direct comparisons of diversity patterns in a single set of analyses. The 16S + 18S rRNA gene composition in the small size fraction was mostly prokaryotic (>92%) as expected, but the large size fraction unexpectedly contained 46–93% prokaryotic 16S rRNA genes. Prokaryotes and protists showed opposite vertical diversity patterns; prokaryotic diversity peaked at mid-depth, protistan diversity at the surface. Temporal beta-diversity patterns indicated prokaryote communities were much more stable than protists. Although the prokaryotic communities changed monthly, the average community stayed remarkably steady over 14 years, showing high resilience. Additionally, particle-associated prokaryotes were more diverse than smaller free-living ones, especially at deeper depths, contributed unexpectedly by abundant and diverse SAR11 clade II. Eukaryotic diversity was strongly correlated with the diversity of particle-associated prokaryotes but not free-living ones, reflecting that physical associations result in the strongest interactions, including symbioses, parasitism, and decomposer relationships.

RNA Viruses in Aquatic Ecosystems through the Lens of Ecological Genomics and Transcriptomics.

Massive amounts of data from nucleic acid sequencing have changed our perspective about diversity and dynamics of marine viral communities. Here, we summarize recent metatranscriptomic and metaviromic studies targeting predominantly RNA viral communities. The analysis of RNA viromes reaffirms the abundance of lytic (+) ssRNA viruses of the order Picornavirales, but also reveals other (+) ssRNA viruses, including RNA bacteriophages, as important constituents of extracellular RNA viral communities. Sequencing of dsRNA suggests unknown diversity of dsRNA viruses. Environmental metatranscriptomes capture the dynamics of ssDNA, dsDNA, ssRNA, and dsRNA viruses simultaneously, unravelling the full complexity of viral dynamics in the marine environment. RNA viruses are prevalent in large size fractions of environmental metatranscriptomes, actively infect marine unicellular eukaryotes larger than 3 µm, and can outnumber bacteriophages during phytoplankton blooms. DNA and RNA viruses change abundance on hourly timescales, implying viral control on a daily temporal basis. Metatranscriptomes of cultured protists host a diverse community of ssRNA and dsRNA viruses, often with multipartite genomes and possibly persistent intracellular lifestyles. We posit that RNA viral communities might be more diverse and complex than formerly anticipated and that the influence they exert on community composition and global carbon flows in aquatic ecosystems may be underestimated.

Monitoring microplastics in drinking water: An interlaboratory study to inform effective methods for quantifying and characterizing microplastics

California Senate Bill 1422 requires the development of State-approved standardized methods for quantifying and characterizing microplastics in drinking water. Accordingly, we led an interlaboratory microplastic method evaluation study, with 22 participating laboratories from six countries, to evaluate the performance of widely used methods: sample extraction via filtering/sieving, optical microscopy, FTIR spectroscopy, and Raman spectroscopy. Three spiked samples of simulated clean water and a laboratory blank were sent to each laboratory with a prescribed standard operating procedure for particle extraction, quantification, and characterization. The samples contained known amounts of microparticles within four size fractions (1–20 μm, 20–212 μm, 212–500 μm, >500 μm), four polymer types (PE, PS, PVC, and PET), and six colors (clear, white, green, blue, red, and orange). They also included false positives (natural hair, fibers, and shells) that may be mistaken for microplastics. Among laboratories, mean particle recovery using stereomicroscopy was 76% ± 10% (SE). For particles in the three largest size fractions, mean recovery was 92% ± 12% SD. On average, laboratory contamination from blank samples was 91 particles (± 141 SD). FTIR and Raman spectroscopy accurately identified microplastics by polymer type for 95% and 91% of particles analyzed, respectively. Per particle, FTIR spectroscopy required the longest time for analysis (12 min ± 9 SD). Participants demonstrated excellent recovery and chemical identification for particles greater than 50 μm in size, with opportunity for increased accuracy and precision through training and further method refinement. This work has informed methods and QA/QC for microplastics monitoring in drinking water in the State of California.

The molecular size continuum of soil organic phosphorus and its chemical associations

The chemical nature of most organic P (Porg) in soil remains ‘unresolved’ but is accounted for by a broad signal in the phosphomonoester region of solution 31P nuclear magnetic resonance (NMR) spectra. The molecular size range of this broad NMR signal and its molecular structure remain unclear. The aim of this study was to elucidate the chemical nature of Porg with increasing molecular size in soil extracts combining size exclusion chromatography (SEC) with solution 31P NMR spectroscopy. Gel-filtration SEC was carried out on NaOH-EDTA extracts of four soils (range 238–1135 mg Porg/kgsoil) to collect fractions with molecular sizes of < 5, 5–10, 10–20, 20–50, 50–70, and > 70 kDa. These were then analysed by NMR spectroscopy.Organic P was detected across the entire molecular size continuum from < 5 to > 70 kDa. Concentrations of Porg in the > 10 kDa fraction ranged from 107 to 427 mg P/kgsoil and exhibited on average three to four broad signals in the phosphomonoester region of NMR spectra. These broad signals were most prominent in the 10–20 and 20–50 kDa fractions, accounting for on average 77% and 74% of total phosphomonoesters, respectively. Our study demonstrates that the broad signals are present in all investigated molecular size fractions and comprise on average three to four components of varying NMR peak line width (20 to 250 Hz).The stereoisomers myo- and scyllo-inositol hexakisphosphates (IP6) were also present across multiple molecular size ranges but were predominant in the 5–10 kDa fraction. The proportion of IP associated with large molecular size fractions > 10 kDa was on average 23% (SD = 39%) of total IP across all soils. These findings suggest that stabilisation of IP in soil includes processes associated with the organic phase.

Morpho-Functional Traits Reveal Differences in Size Fractionated Phytoplankton Communities but Do Not Significantly Affect Zooplankton Grazing.

The recent emergence of approaches based on functional traits allows a more comprehensive evaluation of the role of functions and interactions within communities. As phytoplankton size and shape are the major determinants of its edibility to herbivores, alteration or loss of some morpho-functional phytoplankton traits should affect zooplankton grazing, fitness and population dynamics. Here, we investigated the response of altered phytoplankton morpho-functional trait distribution to grazing by zooplankton with contrasting food size preferences and feeding behaviors. To test this, we performed feeding trials in laboratory microcosms with size-fractionated freshwater phytoplankton (3 size classes, >30 µm; 5–30 µm and <5 µm) and two different consumer types: the cladoceran Daphnia longispina, (generalist unselective filter feeder) and the calanoid copepod Eudiaptomus sp. (selective feeder). We observed no significant changes in traits and composition between the controls and grazed phytoplankton communities. However, community composition and structure varied widely between the small and large size fractions, demonstrating the key role of size in structuring natural phytoplankton communities. Our findings also highlight the necessity to combine taxonomy and trait-based morpho-functional approaches when studying ecological dynamics in phytoplankton-zooplankton interactions.

Evolution mechanism of recrystallization in a nickel-based single crystal superalloy under various cooling rates during heat treatment

The recrystallization behaviors of a nickel-based single crystal superalloy during heat treatment at 1,200 °C for 4 h with various cooling rates were studied. Results show that the thickness of recrystallization layer decreases with the increase of cooling rate. In addition, the microstructures of γ′ phase in the recrystallization region are different in various cooling rates. In the high cooling rates (70, 100 °C·min-1), small size and high volume fraction of γ′ phases are formed in the recrystallization region. It is also found that irregular fine secondary γ′ phases are precipitated between matrix channels with an average size of 150 nm in the original matric (100 °C·min-1). The sizes of the secondary γ′ phase decrease with the increase of cooling rate. In contrast, large size and small volume fraction of γ′ phases are formed in the recrystallization region, and a grain boundary layer is formed under a low cooling rate (10 °C·min-1). The evolution mechanism of recrystallization at various cooling rates during heat treatment is analyzed.

Raman spectroscopic evaluation of crystallinity, chemical composition and stability of pharmaceutical powder aerosols

Raman spectroscopy was used to evaluate the effects of temperature and humidity on the physicochemical stability of inhalable spray-dried (SD) powders containing budesonide (BUD) and crystalline or amorphous lactose. Powders prepared by spray drying of BUD-lactose solution or suspension containing lactose crystals in BUD solution were stored for 0, 1 and 7 days at 25 °C/60 RH or 40 °C/75 RH. Bulk powders along with the large and small particle size fractions collected on stages 2 and 5, respectively, of the Next Generation Impactor (NGI) were chemically characterised. SD powder from solution contained BUD and lactose in amorphous form and both components were homogeneously distributed in bulk and in the particles collected from the two NGI stages. In contrast, SD powder from suspension showed heterogeneous distribution of lactose and drug in the particles containing crystalline lactose. After 1 day of storage at either condition, recrystallisation and changes in the chemical composition of the particles for the SD powder from solution occurred. The number of drug-only particles increased by 70 on stage 5, whereas most particles on stage 2 still contained both drug and lactose. These changes were not observed in the SD powder from suspension after storage, confirming superior stability of the SD powder obtained from suspension.

Speciation and pH- and particle size-dependent solubility of phosphorus in tropical sandy soils

Growing demands for crops have expanded agricultural production to sandy soils with low available phosphorus (P) content and high P mobility. Knowledge of P speciation and solubility in such soils is poorly understood. Here we investigated: i) the P speciation in the clay size fraction of sandy soils, ii) the pH-dependent solubility of soil orthophosphate (PO4–P), and iii) the particle size-dependent solubility of the soil P. The studied loamy sand soils were low in organic matter (2.6–7.9 g kg−1) and deficient in available P (1.5–8.5 mg kg−1), and kaolinite (21%) was the only clay mineral identified using X-ray diffraction technique in the clay-sized particles. The P K-edge X-ray absorption near edge structure analysis revealed that P associated with Al phases, 45% adsorbed to gibbsite and 35% in variscite (AlPO₄·2H₂O), dominated the P speciation in the clay-sized particles. The very low PO4–P solubilities in the investigated bulk soils exhibited a pronounced positive pH-dependent pattern that was consistent with the pH-dependent solubility of Al3+ and Fe3+. Geochemical modeling suggested that the maximum PO4–P solubilities were linked to the solubility of variscite and strengite (FePO₄·2H₂O), indicating that PO4–P solubilities were controlled by the solubility of Al3+ and Fe3+ as restricted by the pH-dependent solubility of Al phases and Fe (hydr)oxides. The analysis of particle size fractions revealed that the largest size fraction (1000–2000 µm), containing visible plant debris and contributing the least to total soil mass (<0.126%), contained most of the total soil P stocks (50–60%). The small size factions (<250 µm) were the key to available PO4–P. Our data highlighted the pivotal role of Al/Fe (hydr)oxides and kaolinite in controlling the pH-dependent solubility of PO4–P in these tropical sandy soils having very limited contents of total and available P and a high risk of P loss.

Soluble, Colloidal, and Particulate Iron Across the Hydrothermal Vent Mixing Zones in Broken Spur and Rainbow, Mid-Atlantic Ridge.

The slow-spreading Mid-Atlantic Ridge (MAR) forms geological heterogeneity throughout the ridge system by deep crustal faults and their resultant tectonic valleys, which results in the existence of different types of hydrothermal vent fields. Therefore, investigating MAR hydrothermal systems opens a gate to understanding the concentration ranges of ecosystem-limiting metals emanating from compositionally distinct fluids for both near-field chemosynthetic ecosystems and far-field transport into the ocean interiors. Here, we present novel data regarding onboard measured, size-fractionated soluble, colloidal, and particulate iron concentrations from the 2018 R/V L’Atalante – ROV Victor research expedition, during which samples were taken from the mixing zone of black smokers using a ROV-assisted plume sampling. Iron size fractionation (<20, 20–200, and >200nm) data were obtained from onboard sequential filtering, followed by measurement via ferrozine assay and spectrophotometric detection at 562nm. Our results showed the persistent presence of a nanoparticulate/colloidal phase (retained within 20–200nm filtrates) even in high-temperature samples. A significant fraction of this phase was retrievable only under treatment with HNO3 – a strong acid known to attack and dissolve pyrite nanocrystals. Upon mixing with colder bottom waters and removal of iron in the higher parts of the buoyant plume, the larger size fractions became dominant as the total iron levels decreased, but it was still possible to detect significant (micromolar) levels of nanoparticulate Fe even in samples collected 5m above the orifice in the rising plume. The coolest sample (<10°C) still contained more than 1μM of only nitric acid-leachable nanoparticle/colloidal, at least 200 times higher than a typical Fe concentration in the non-buoyant plume. Our results support previous reports of dissolved Fe in MAR vent plumes, and we propose that this recalcitrant Fe pool – surviving immediate precipitation – contributes to maintaining high hydrothermal iron fluxes to the deep ocean.

Heterotrophic bacterial diazotrophs are more abundant than their cyanobacterial counterparts in metagenomes covering most of the sunlit ocean.

Biological nitrogen fixation contributes significantly to marine primary productivity. The current view depicts few cyanobacterial diazotrophs as the main marine nitrogen fixers. Here, we used 891 Tara Oceans metagenomes derived from surface waters of five oceans and two seas to generate a manually curated genomic database corresponding to free-living, filamentous, colony-forming, particle-attached, and symbiotic bacterial and archaeal populations. The database provides the genomic content of eight cyanobacterial diazotrophs including a newly discovered population related to known heterocystous symbionts of diatoms, as well as 40 heterotrophic bacterial diazotrophs that considerably expand the known diversity of abundant marine nitrogen fixers. These 48 populations encapsulate 92% of metagenomic signal for known nifH genes in the sunlit ocean, suggesting that the genomic characterization of the most abundant marine diazotrophs may be nearing completion. Newly identified heterotrophic bacterial diazotrophs are widespread, express their nifH genes in situ, and also occur in large planktonic size fractions where they might form aggregates that provide the low-oxygen microenvironments required for nitrogen fixation. Critically, we found heterotrophic bacterial diazotrophs to be more abundant than cyanobacterial diazotrophs in most metagenomes from the open oceans and seas, emphasizing the importance of a wide range of heterotrophic populations in the marine nitrogen balance.

The evolution of coarse grains and its effects on weakened basal texture during annealing of a cold-rolled magnesium AZ31B alloy

The nucleation, grain growth of 34 coarse grains during annealing were tracked using a quasi-in-situ EBSD method. These 34 coarse grains had different orientations and most grains were non-basal orientated. No preferable grain growth or special types of grain boundaries were identified. Only 9 coarse grains nucleated from deformed grain boundaries due to initial large grain size and limited grain boundary volume fraction. The main nucleation site of 34 coarse grains was dislocation cells or subgrains in deformed grain interiors. Their recrystallisation behaviour can be illustrated by abnormal subgrain growth (AsGG) rarely reported in Mg alloys. The coarse basal grains showed no growth advantage in terms of grain size or number over other non-basal grains, leading to a weak basal texture in AZ31B alloy.

Planktonic ciliate trait structure variation over Yap, Mariana, and Caroline seamounts in the tropical western Pacific Ocean

Trait structure is increasingly used in plankton ecology to understand diversity and biogeography. However, our knowledge of microzooplankton (e.g. planktonic ciliates) trait structure and its variation with hydrography is limited. In this study, we analyzed planktonic ciliate trait structure in waters with different hydrography and deep Chlorophyll a maximum (DCM) layers over three seamounts: Yap, Mariana, and Caroline seamounts. Mariana seamount had a lower surface temperature than the Yap and Caroline seamounts. DCM layers over Mariana and Caroline seamounts were deeper than Yap seamount. There was a weak upwelling in upper 50 m around top of Mariana seamount. The ciliate distribution showed bimodal pattern (high abundance appeared in the surface and DCM layers) over three seamounts. At surface layer, the large size-fraction (>30 µm) abundance proportion to aloricate ciliate over Yap seamount (44.4%) was higher than Mariana (32.8%) and Caroline (36.1%) seamounts. For tintinnid abundance proportion to total ciliate, Mariana (12.0%) and Caroline (11.5%) seamounts at about 100-m depth were higher than that of Yap seamount (6.4%). Vertically, tintinnid could be divided into 4 groups over the three seamounts. At 30-m depth, group I (species occurring from surface to 100 m only) was dominant component over Yap and Caroline seamounts, while group IV (species occurring at every depth) changed into dominant component over Mariana seamount, the weak upwelling might be the reason. Salpingella faurei was the top dominant species, which corresponded to deeper DCM layers over Mariana and Caroline seamounts. Our results showed that the upwelling and the deeper DCM could influence the planktonic ciliate trait structure.