Low Silicate Research Articles

Secondary lanthanide phosphate mineralisation in weathering profiles of I-, S- and A-type granites

AbstractRare earth elements (REEs, ‘lanthanides’) constitute a vital commodity for technological applications. Although these elements occur at trace levels in many minerals, they can comprise major constituents of low abundance phosphate, carbonate, silicate and oxide minerals, some of which form during granite weathering. REE-phosphate phases can be a source of phosphorus for essential biomolecules and certain REEs are required by some bacterial enzymes involved in the oxidation of methanol, an important compound in the global biogeochemical carbon cycle. The mechanisms that promote the dissolution of lanthanide phosphate minerals are largely unknown, but probably vary with the lanthanide phosphate mineralogy of weathered rock and soil. Here, we studied weathering of five I-type, three S-type and one A-type granite to determine the extent of weathering of primary REE- and/or P-bearing minerals apatite, allanite and monazite, and the formation of secondary REE/P-bearing minerals. We found evidence for greater mobilisation of REE and P in weathered I-type and A-type granites than in S-types, reflecting the higher solubility of apatite and allanite relative to monazite. Although monazite persisted in highly weathered S-type granites, some alteration was detected. Secondary REE/P-bearing minerals were not detected in two S-type profiles, while spherical secondary REE/P-bearing mineral aggregates were abundant throughout the third S-type profile. Secondary euhedral REE/P-bearing crystals were abundant even in the slightly weathered I-type and A-type granite material, yet they were not detected in the highly weathered material, indicating that these minerals had dissolved. Our findings indicate that mineralogy constrains substantially, but does not control completely, lanthanide availability as a function of degree of weathering. These results have implications for predicting REE and phosphate bioavailability in soils derived from granitic rock types and suggest that highly weathered I-type granites may provide inocula for bioleaching experiments.

Comparative Proteomic Analysis Reveals New Insights Into the Common and Specific Metabolic Regulation of the Diatom Skeletonema dohrnii to the Silicate and Temperature Availability.

Silicate (Si) and temperature are essential drivers for diatom growth and development in the ocean. Response of diatoms to these particular stress has been investigated; however, their common and specific responses to regulate intracellular development and growth are not known. Here, we investigated the combination of physiological characteristics and comparative proteomics of the diatom Skeletonema dohrnii grown in silicate- and temperature-limited conditions. Results show that cell carbon and lipid quotas were higher at lower-temperature cells, whereas cellular phosphate was higher in cells grown with lower Si. In silicate-limited cells, nitrate transporters were downregulated and resulted in lower nitrate assimilation, whereas the phosphate transporters and its assimilation were reduced in lower-temperature conditions. In photosynthesis, lower silicate caused impact in the linear electron flow and NADPH production, whereas cycling electron transport and ATP production were affected by the lower temperature. Concerning cell cycle, imbalances in the translation process were observed in lower-silicate cells, whereas impact in the transcription mechanism was observed in lower-temperature cells. However, proteins associated with carbon fixation and photorespiration were downregulated in both stress conditions, while the carbohydrate and lipid synthesis proteins were upregulated. Our results showed new insights into the common and specific responses on the proteome and physiology of S. dohrnii to silicate and temperature limitation, providing particular nutrient (Si)- and temperature-dependent mechanisms in diatoms.

Predictive Model of Setting Times and Compressive Strengths for Low-Alkali, Ambient-Cured, Fly Ash/Slag-Based Geopolymers

The effects of curing temperature, blast furnace slag content, and Ms on the initial and final setting times, and compressive strengths of geopolymer paste and mortars are examined. The present work demonstrates that ambient-cured geopolymer pastes and mortars can be fabricated without requiring high alkalinity activators or thermal curing, provided that the ratios of Class F fly ash (40–90 wt%), blast furnace slag (10–60 wt%), and low alkalinity sodium silicate (Ms = 1.5, 1.7, 2.0) are appropriately balanced. Eighteen mix designs were assessed against the criteria for setting time and compressive strength according to ASTM C150 and AS 3972. Using these data, flexible and reproducible mix designs in terms of the fly ash/slag ratio and Ms were mapped and categorised. The optimal mix designs are 30–40 wt% slag with silicate modulus (Ms) = 1.5–1.7. These data were used to generate predictive models for initial and final setting times and for ultimate curing times and ultimate compressive strengths. These projected data indicate that compressive strengths >100 MPa can be achieved after ambient curing for >56 days of mixes of ≥40 wt% slag.

Influence of two kinds of low dimensional nano-sized silicate clay on the flame retardancy of polypropylene

Nano-rolled and nano-sheeted silicates were fabricated from nature kaolinite (Kaol). And their structure was characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscope (SEM) and so on. The two nano-sized silicates acted as synergists to enhance fire retardant performance of polypropylene (PP) with intumescent flame-retardants (IFR) together. The limited oxygen index (LOI) of PP is only 18%, incorporation of 25 wt% IFR lifted the LOI to 31.1%, further increase to 34.5 and 35.5% by 1.5 wt% substitution of nano-rolled Kaol (N-Kaol) or exfoliated Kaol (E-Kaol) with same amount of IFR, respectively. Moreover, each modified Kaol can effectively control melt dripping to help PP reach UL-94 V-0 grade. In cone test, the resultes showed that the peak heat release rate (pHRR) values of PP/IFR/N-Kaol (269 kW/m2) and PP/IFR/E-Kaol (319 kW/m2) composites were reduced by 38.6 and 27.2% compared to that of PP/IFR (438 kW/m2), respectively; the smoke release was similar to the heat release. Continue increased char percentage documented the promotion on charring from E-Kaol and N-Kaol. The PP/IFR/N-Kaol exhibited the best char strength. Morphology observation on macro/micro-structure of char and real-time FTIR demonstrated that N-Kaol can be beneficial to forming homogenous and compact intumescent char layer through “double effect” of barrier and pillar-like support during burning, especially after 340 s, compared with the PP/IFR containing E-Kaol. This special structure of char layer can effectively limit the flow and transfer of inflammable gases and heat between underlying matrix and outside environment.

Silver doped glasses from the system BaO/SrO/ZnO/SiO2 – The influence of Sb, Sn, and Ta on the formation of core-shell structures

Three glasses from the base system BaO/SrO/ZnO/SiO2 have been synthesized using conventional melt quenching technique and different concentrations of Ag, Sn, Sb, and Ta. After two-step heat treatments, two glasses showed volume crystallization, while the third glass crystallized solely from the surface. The different crystallization mechanisms were studied by differential scanning calorimetry and both scanning and transmission electron microscopy. It was found that the formation of LEAZit (Low Expansion Alkaline Earth Zinc Silicates) volume crystals runs parallel to the formation of core-shell structures inside the glassy matrix. Such a core-shell structure consists of a metallic Ag core, which is surrounded by a shell composed of Zn2SiO4. Into this crystalline phase, Sb, Sn, as well as Ta can be incorporated. Glasses without Sb show the formation of metallic and spherically shaped Ag particles without any shell, being unsuitable to trigger volume crystallization in the studied glasses. Hence, the presence of Ag, Sn, Sb, or Ta in a suitable ratio is decisive for the nucleation and crystallization mechanism of the studied glasses.

Expression and Applications of HriCFP in E. coli: A Novel Biosensing Fluorescent Protein

Metalloids and heavy metal contamination in the environment have become a global problem. Therefore, there is a dire need to develop effective and inexpensive approaches that can facilitate efficient monitoring of the hazardous level of these environmental pollutants. Microbial cell-based and fluorescent protein-based biosensors offer relatively convenient and inexpensive tools for the analysis of environmental pollutants as opposed to traditional instrumental approaches. Small size fluorescent proteins can withstand exposure to denaturants, high temperature and a wide pH range. These characteristics, along with their potential of sensing different toxic analytes, makes them a suitable candidate for developing on-site detection biosensors. The current study exploits the biosensing potential of a novel fluorescent protein called HriCFP. HriCFP was expressed in the prokaryotic system (gram-negative E. coli), which showed stable and discreet expression in bacterial cells. Whole-cell biosensors (WCB) were developed by immobilization of HriCFP expressing non-pathogenic E. coli via nitrocellulose membrane, low melting agarose and sodium silicate gel. These immobilized biosensors were tested for their sensitivity of detection for environmental pollutants, i.e., heavy metals (Cu(II), Hg(II), As(III)). These WCBs exhibited profound fluorescent quenching when exposed to a range of heavy metals. These biosensors remained active for 12 days at 4°C, demonstrating their potential for long-term stability and storage. This study implies that HriCFP may have a significant advantage over other larger and multimeric proteins as it has a minimal impact on host strain metabolism and hence, increasing its sustainability for a longer period.

Interannual variability of Emiliania huxleyi blooms in the Barents Sea: In situ data 2014–2018

Using in situ data of 2014–2018, the summers Emiliania huxleyi blooms in the Barents Sea were studied. The blooms were recorded in the upper mixed layer in July and August every year, during which they spread to cover large areas and were associated with Atlantic waters. The E. huxleyi abundance revealed interannual variability, with the highest values (up to 12 × 106 cells/L) in July 2016. Bloom is characterized by a sharp seasonal thermocline, water surface temperature of about 7.14–11.7 °C, low silicate (0.45 ± 0.08 μM) and nitrogen (0.74 ± 0.16 μM) concentration, high phosphorus concentration (0.09 ± 0.01 μM) and nitrogen to phosphorus ratio significantly below the Redfield ratio. Data confirming the hypothesis of limiting the growth of diatoms by nitrogen concentration are presented. When E. huxleyi bloomed, its biomass exceeded 70% of the total phytoplankton biomass, species diversity was low, and diatoms were practically absent, and dinoflagellates were usually represented by large species.

Physiological silicon incorporation into bone mineral requires orthosilicic acid metabolism to SiO44.

Under physiological conditions, the predominant form of bioavailable silicon (Si) is orthosilicic acid (OSA). In this study, given Si's recognized positive effect on bone growth and integrity, we examined the chemical form and position of this natural Si source in the inorganic bone mineral hydroxyapatite (HA). X-ray diffraction (XRD) of rat tibia bone mineral showed that the mineral phase was similar to that of phase-pure HA. However, theoretical XRD patterns revealed that at the levels found in bone, the 'Si effect' would be virtually undetectable. Thus we used first principles density functional theory calculations to explore the energetic and geometric consequences of substituting OSA into a large HA model. Formation energy analysis revealed that OSA is not favourable as a neutral interstitial substitution but can be incorporated as a silicate ion substituting for a phosphate ion, suggesting that incorporation will only occur under specific conditions at the bone-remodelling interface and that dietary forms of Si will be metabolized to simpler chemical forms, specifically [Formula: see text]. Furthermore, we show that this substitution, at the low silicate concentrations found in the biological environment, is likely to be a driver of calcium phosphate crystallization from an amorphous to a fully mineralized state.

Environmental factors controlling the dynamics of phytoplankton communities during spring and fall seasons in the southern Sunda Shelf.

Phytoplankton play important roles in marine ecosystems and have major impacts on the global biogeochemical cycles, yet the knowledge of the processes for oceanic environments controlling the phytoplankton dynamics still needs to be improved, particularly in the coastal ecosystems. Monitoring diversity of phytoplankton communities, including micro/nano- and pico-forms, as well as their dynamics in relation to environmental factors, is thus of critical significance. Here, we conducted two cruises in the southern Sunda Shelf during spring and fall 2016 to investigate the community structure and dynamics of phytoplankton assemblages in relation to environmental factors combining microscopy and flow cytometry. Micro/nanophytoplankton communities changed from Trichodesmium spp. and diatoms co-dominance in spring to communities dominated by Trichodesmium spp. in fall. Diatoms were a highly diverse group, with 78 species, followed by dinoflagellates (35 species). Differences in average abundance of micro/nanophytoplankton between spring and fall might be attributed to the shift in dominance of Trichodesmium spp. and diatoms. Picophytoplankton characterized by a great abundance of picocyanobacteria were dominated by Synechococcus. The dynamics of picophytoplankton during spring and fall might be associated with the differences in dominant Synechococcus strains. Environmental variables had significant influence on the dynamics of phytoplankton communities, especially nitrate and silicate. Silicate was a major driver responsible for the variations of micro/nanophytoplankton. Low silicate potentially limited the diatom growth during fall, leading to the overwhelming dominance of Trichodesmium spp. Picophytoplankton showed positive correlations with nitrate and silicate, suggesting that these nutrients collectively controlled variations in picophytoplankton communities during spring and fall.

Dynamics of nutrient and phytoplankton in Epe Lagoon, Nigeria: possible causes and consequences of reoccurring cyanobacterial blooms

The impact of environmental variables and processes of nutrient enrichment on phytoplankton community at Epe lagoon was studied for 18 months (November 2012–April 2014). Two cyanobacterial bloom incidences were observed in the two dry seasons during this period. Rainfall pattern regulated nitrogen limitation which triggered cyanobacterial bloom development and influenced the bloom duration. Nutrient-laden influx from agricultural practices accounted for the increase in nitrate, phosphate and the relatively low silicate. Hypoxic conditions during the two bloom episodes reflected the vulnerability of this lagoon ecosystem. The cyanobacterial blooms were dominated by potentially toxic species; Anabaena circinalis, A. flos-aquae, A. limnetica and A. spiroides. A comparison of ambient nutrient ratios with the Redfield ratio (N/P/Si = 16:1:16) showed clear temporal variations that coincided with phytoplankton dynamics and the bloom regime. Low N/P ratios were recorded during the first 8 months, and there was a shift to higher ratios during the subsequent 8 months, followed by low N/P ratios in the last 2 months. Higher silicate values coincided with decreased cyanobacterial biomass, cyanobacteria bloom collapse and proliferation of diatoms during the first annual cycle. Cyanobacterial abundance exceeded the alert level 1 and almost reached alert level 2 which call for continuous environmental monitoring and management of coastal waters.

Community composition and photosynthetic physiology of phytoplankton in the western subarctic Pacific near the Kuril Islands with special reference to iron availability.

The western subarctic Pacific (WSP) is known as one of the most productive regions among the world's oceans in spring. However, its oceanic waters are also known as a High Nutrient, Low Chlorophyll (HNLC) region during summer due to low iron (Fe) availability in seawater. Indeed, recent studies have demonstrated that the distribution of Fe in the WSP is complex and heterogeneous. This study thus investigated the effects of Fe availability on the community composition and photophysiology of surface phytoplankton from coastal to offshore waters in the WSP in the summer of 2014. Although relatively high concentrations (>2 mg m-3) of chlorophyll (chl) a were found in the Sea of Okhotsk and some coastal waters, low chl a concentrations (<1 mg m-3) were commonly observed in offshore waters. Based on dissolved Fe and macronutrient concentrations, we deduced that low Fe availability limited phytoplankton growth in offshore waters, whereas low silicate and/or nitrate levels limited growth in the shelf areas. Scanning electron microscopy also revealed that the centric diatom Chaetoceros exclusively dominated the diatom assemblages in the shelf and coexisted with pennate diatoms in offshore waters, respectively. Primary productivity in surface waters was negatively correlated with the bottom of the euphotic layer or the light saturation index of the photosynthesis-irradiance curve, which indicates that the phytoplankton assemblages were well acclimated to in situ light conditions regardless of the water masses.

Pervaporative desalination of concentrated brine solution employing crosslinked PVA/silicate nanoclay membranes

Laponite nanodiscs are low cost synthetic silicate clay with extreme hydrophilic behavior. Therefore, laponite can be used to enhance pervaporation membranes’ performance. Poly(vinyl alcohol), (PVA)/laponite pervaporation membranes were fabricated via simple exfoliation method of the nanoclay in the polymer solution followed by chemical crosslinking using glutaraldehyde (GA). The fabricated membranes were employed to study the pervaporation desalination performance when treating reverse osmosis (RO) concentrated brine solution. The effect of nanofiller concentration on membrane properties, as well as the water diffusion coefficient through the membrane, swelling measurements was investigated. The highest rejection of ∼99.98% was achieved using 7 wt% laponite nanocomposite membrane at 40 °C with a flux of 31.2 kg/m2 h. Increasing the feed concentration resulted in a decrease in water flux, whereas the salt rejection was negligibly changed. The influence of the operating temperature and feed salt concentration on the desalination performance 7 wt% laponite nanocomposite membrane (PVA7) were obtained at different temperatures in the range between 30 °C and 70 °C and feed concentration up to 100,000 ppm the temperature profile is expressed by Arrhenius relationship.

Effects of alccofine and curing conditions on properties of low calcium fly ash-based geopolymer concrete

In order to employ the waste from plants, geopolymer concrete was manufactured with low calcium fly ash, alccofine, sodium silicate and sodium hydroxide as main ingredients. This study investigates the effect of alccofine percentages on the properties of fresh and hardened low-calcium fly ash-based geopolymer concrete. Alccofine was utilized in geopolymer concrete mixtures in 0%, 5% & 10% as partial replacement of fly ash (by weight) and cured at ambient (27 °C) as well as heat (90 °C) temperatures. The results of this study revealed that alccofine enhanced the slump, compaction, density and strength values in comparison to standard geopolymer concrete values with 0% alccofine, in contrary, air voids decreased. Further, X-ray diffraction and Scanning electron microscope results indicated better polymerization in the presence of alccofine which led to the formation of CSH, CASH, NASH products and better densification. Thus, geopolymer concrete produced using low calcium fly ash with different percentages of alccofine can be used for cast in situ practices and at ambient temperature.

Effect of waste glass incorporation on the properties of geopolymers formulated with low purity metakaolin

Given the worldwide abundant resources of low purity kaolin and urban waste glass, their use to produce alkali activated cements seem to be a promising area in the search for alternative sustainable cements with environmental advantages over Portland cement. Under this perspective, the properties of silicate activated low purity metakaolin (MK) and flat soda lime silicate waste glass (WG) at MK/WG mass ratios of 100/0, 85/15 and 70/30 were investigated. The chemical composition of the pastes was varied in terms of modulus (Ms = 0.8–1.2) SiO2/Na2O of the activating solution with additions of 12 and 16 wt% Na2O. Cubic samples dry cured at 20 °C were used to follow the compressive strength from 1 to 90 days, which reached from 17 to 56 MPa; the highest values were attained by samples activated with Ms = 0.8–1 and 12%Na2O. The products of the reactions were evaluated by X-ray diffraction, infrared spectroscopy, scanning electronic microscopy and nuclear magnetic resonance. The microstructures included unreacted MK, WG and N-A-S-H, while the incorporation of WG favored the formation of a more complex reaction product similar to a (N,C)-A-S-H type gel.

Preforming abundant surface cobalt hydroxyl groups on low crystalline flowerlike Co3(Si2O5)2(OH)2 for enhancing catalytic degradation performances with a critical nonradical reaction

Low crystalline flowerlike cobalt silicate hydroxide (Co3(Si2O5)2(OH)2, CoSiOx) is synthesized by a one-pot hydrothermal approach. Abundant highly active CoOH+ is preformed on the surface of CoSiOx, while the low crystallinity of CoSiOx leads to the formation of active 1O2 caused by the generation of oxygen vacancy in the valence change cycle between Co2+ and Co3+. The optimal CoSiOx with a high specific surface area of 665.2 m2 g−1 could expose surface CoOH+, thus rapidly activate peroxymonosulfate (PMS) and accelerate catalytic efficiencies. As a result, at least 11 kinds of organic pollutants could be degraded with 33–242% higher efficiencies than reported. In addition to the common SO4- and OH, large amounts of active 1O2 endow the CoSiOx/PMS system with superb catalytic performances in diverse conditions, suggesting the excellent adaptability for practical usage. This work provides a new protocol to advance the PMS activation process for high-performance cobalt based heterogeneous catalysts.

Nutrient Controls on Export Production in the Southern Ocean

We use observations from novel biogeochemical profiling floats deployed by the Southern Ocean Carbon and Climate Observations and Modeling program to estimate annual net community production (ANCP; associated with carbon export) from the seasonal drawdown of mesopelagic oxygen and surface nitrate in the Southern Ocean. Our estimates agree with previous observations in showing an increase in ANCP in the vicinity of the polar front (∼3 mol C m−2 y−1), compared to lower rates in the subtropical zone (≤ 1 mol C m−2 y−1) and the seasonal ice zone (&lt;2 mol C m−2 y−1). Paradoxically, the increase in ANCP south of the subtropical front is associated with elevated surface nitrate and silicate concentrations, but decreasing surface iron. We hypothesize that iron limitation promotes silicification in diatoms, which is evidenced by the low silicate to nitrate ratio of surface waters around the Antarctic polar front. High diatom silicification increases the ballasting effect of particulate organic carbon and overall ANCP in this region. A model‐based assessment of our methods shows a good agreement between ANCP estimates based on oxygen and nitrate drawdown and the modeled downward organic carbon flux at 100 m. This agreement supports the presumption that net biological consumption is the dominant process affecting the drawdown of these chemical tracers and that, given sufficient data, ANCP can be inferred from observations of oxygen and/or nitrate drawdown in the Southern Ocean.

Decomposition of environmentally persistent cyclic methylsiloxanes in subcritical water

The cyclic methylsiloxanes octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5) were reacted in subcritical water at temperatures of 150–350 °C under argon, under O2, or in the presence of H2O2. When the reactions were performed in pure subcritical water (under argon) at 300 °C for 6 h, both D4 and D5 were almost completely hydrolyzed to water-soluble and hexane-non-extractable compounds such as dimethylsilanediol (DMSD). However, the high percentages of total organic carbon (TOC) remained in the aqueous phase [87% and 81% of the carbon atoms in the initial D5 (26.6 μmol) and D4 (30.6 μmol), respectively] and the low silicate yields (both 3% of the silicon atoms in the initial D5 and D4) indicated that little mineralization had occurred. Although the presence of O2 had no significant effect on the reactivity of these substrates in subcritical water at temperatures up to 300 °C, the presence of H2O2 enhanced the decomposition of these substrates. Raising the temperature to 350 °C greatly enhanced the mineralization. Specifically, when the reactions were performed in 2.0 M H2O2 for 2 h, the percentages of TOC that remained in the aqueous phase decreased markedly (to 2% and 10% of the carbon atoms in the initial D5 and D4, respectively) and IR spectral measurements indicated that most of the substrates had been oxidized to silicate.

A FTIR study of silanol nests over mesoporous silicate materials

IR spectroscopy and DFT calculation were used to investigate the adsorption of water and toluene on aerosil calcined at a temperature of 250 °C as well as on silica produced by high-power electron-beam evaporation. It was shown that isolated SiOH groups (3746-3750 cm−1) on the surface of silicate materials weakly retain water and are easily displaced by toluene. An amount of adsorbed water raises slightly with vapor pressure. On the surface of aerosil there exist silanol nests that are characterized by two pairs of absorption bands: 3540, 3730 cm−1 and 3657, 3740 cm−1, which correspond to stretching vibrations of isolated and H-bonded hydroxyl groups, respectively. Vicinal OH groups in the nests adsorb molecular water according to the stoichiometry 2H2O: 1SiOH even at low water pressures. Owing to a higher acidity of the silanol nests, water is adsorbed much stronger, so that toluene cannot displace them. As a result, it was explained why MFI zeolites with a low silicate modulus are more hydrophobic than similar zeolites with a high modulus. Finally, a reliable spectral criterion of silanol nests existence in silicate materials was established; it consists in the presence of a pair of bands in IR spectra: a broad band in the region below 3600 cm−1 and a narrow band in the range of 3700–3740 cm−1.

Sea-ice properties and nutrient concentration as drivers of the taxonomic and trophic structure of high-Arctic protist and metazoan communities

In the Arctic Ocean, sea-ice decline will significantly change the structure of biological communities. At the same time, changing nutrient dynamics can have similarly strong and potentially interacting effects. To investigate the response of the taxonomic and trophic structure of planktonic and ice-associated communities to varying sea-ice properties and nutrient concentrations, we analysed four different communities sampled in the Eurasian Basin in summer 2012: (1) protists and (2) metazoans from the under-ice habitat, and (3) protists and (4) metazoans from the epipelagic habitat. The taxonomic composition of protist communities was characterised with 18S meta-barcoding. The taxonomic composition of metazoan communities was determined based on morphology. The analysis of environmental parameters identified (i) a ‘shelf-influenced’ regime with melting sea ice, high-silicate concentrations and low NOx (nitrate + nitrite) concentrations; (ii) a ‘Polar’ regime with low silicate concentrations and low NOx concentrations; and (iii) an ‘Atlantic’ regime with low silicate concentrations and high NOx concentrations. Multivariate analyses of combined bio-environmental datasets showed that taxonomic community structure primarily responded to the variability of sea-ice properties and hydrography across all four communities. Trophic community structure, however, responded significantly to NOx concentrations. In three of the four communities, the most heterotrophic trophic group significantly dominated in the NOx-poor shelf-influenced and Polar regimes compared to the NOx-rich Atlantic regime. The more heterotrophic, NOx-poor regimes were associated with lower productivity and carbon export than the NOx-rich Atlantic regime. For modelling future Arctic ecosystems, it is important to consider that taxonomic diversity can respond to different drivers than trophic diversity.

Coccolithophore distributions of the North and South Atlantic Ocean

We present average coccolithophore and associated biogeochemical results from ten Atlantic Meridional Transect (AMT) cruises between 50oN-50oS, results that show a number of unique observations across this massive data set. Lowest concentrations of coccolithophores were consistently found in equatorial waters. Highest concentrations of coccolithophore cells and coccoliths were associated with temperate, sub-polar environments. Concentrations of detached coccoliths were dispersed to ∼300m depth and show low inter-cruise variance. Coccolithophore cell concentrations in the subtropics remain highest at < 200m depth in the S. Atlantic and 100m in the N. Atlantic, near the 26.5 to 27 sigma-theta (σθ) isopycnal, within the Subantarctic Mode Water (SAMW). Diversity and species richness of coccolithophore cells was greater than for the detached coccoliths and generally greater in surface populations than in the deep chlorophyll maximum (DCM). Placolith-bearing coccolithophore species dominated over the umbelliform and floriform coccolithophore groups in surface waters of the Southern Ocean at 50oS, as well as the DCM from 50oS to the equator. The DCM was strongly associated with the σθ, of ∼27 kg m-3, coincident with the top of the nitracline, within the SAMW. Concentrations of coccolithophore cells and detached coccoliths were much less related to the density field than chlorophyll was. Highest integrated euphotic coccolith concentrations were observed when the integrated POC:chlorophyll ratio was highest. PIC:POC ratios increased with depth and approached maximum values near the top of the nitracline. Great Calcite Belt waters (upper 300m) showed biogenic silica:particulate inorganic carbon ratios (BSi:PIC) >1 while all other waters showed BSi:PIC ratios <1. Peaks in PIC and BSi were observed within, and above the SAMW, in waters with low phosphate and silicate concentrations. The results of this study suggest that coccolithophore-rich SAMW in the Southern Ocean is being conditioned after its formation, such that by the time it upwells in the Atlantic equatorial region, the water is no longer conducive to coccolithophore growth.