Lian-Hua Xu

Sheng-Li Zhang

Shi-Ying Guo

Xue-Ji Zhang

Serge Cosnier

Robert S Marks

Wen-Ju Wang

Hai-Bo Zeng

Dan Shan
email: danshan@njust.edu.cn

ATMP derived Cobalt-metaphosphate Complex as Highly Active Catalyst for Oxygen Reduction Reaction

Keywords: Amino trimethylene phosphonic acid (ATMP), Oxygen reduction reaction (ORR), Electrocatalysis, Cobalt-metaphosphate

Rational design and facile synthesis of highly active electrocatalysts with low cost for oxygen reduction reaction (ORR) are always of great challenge. Specifically, development of a new type of energy-saving materials with convenient method is regarded as the current bottleneck. Herein, an innovative strategy based on amino trimethylene phosphonic acid (ATMP) as chelating agent for cobalt-metaphosphate coordination polymer is reported to one-pot synthesis of a novel precursor in methanol for ORR electrocatalyst. Carbonization of the precursor at 900 o C at N 2 atmosphere results in the feasible formation of cobalt metaphosphate based composite (Co(PO 3 ) 2 /NC). A further step in the thermal cleavage at 650 o C at air for 4h, Co(PO 3 ) 2 /NC can be finally transformed into inorganic Co(PO 3 ) 2 . Advanced spectroscopic techniques and density function theory (DFT) calculations are applied to confirm the main catalytically active center and the physical properties of Co(PO 3 ) 2 /NC. This obtained Co(PO 3 ) 2 /NC nanocomposite exhibits superior electrocatalysis to Co(PO 3 ) 2 with an enhanced onset potential (0.906 V vs. RHE) and diffusion limiting current (5.062 mA cm -2 ), which are roughly close to those of commercial 20 % Pt/C (0.916 V, 5.200 mA cm -2 ).

INTRODUCTION

Fuel cells capable of directly converting chemical energy into electrical energy has attracted great attention due to their higher power generation efficiencies, higher energy conversion efficiencies, eco-friendly and longer lifespan [START_REF] Kim | Mechanisms of Two-Electron and Four-Electron Electrochemical Oxygen Reduction Reactions at Nitrogen-Doped Reduced Graphene Oxide[END_REF]. Nevertheless, the oxygen reduction reaction (ORR) is a sluggish step at the cathode of fuel cells, which limits the efficiency of the entire fuel cells [START_REF] Qiao | Hierarchically Ordered Porous Carbon with Atomically Dispersed FeN 4 for Ultraefficient Oxygen Reduction Reaction in Proton-Exchange Membrane Fuel Cells[END_REF]. Therefore, the development of cost-effective and efficient electrode materials for ORR are key priorities. With more and more efficient catalysts were exploited, the following critical issues still need to be resolved: (1) Finding a highly efficient chemical synthetic method to develop ORR alternative catalysts that are both high active, abundant, cost-effective and obtained feasibly. [START_REF] Qiao | Hierarchically Ordered Porous Carbon with Atomically Dispersed FeN 4 for Ultraefficient Oxygen Reduction Reaction in Proton-Exchange Membrane Fuel Cells[END_REF] Tailing a new catalytic material and mastering chemical synthetic mechanism of the catalysts based on the theory of electron transfer.

Cobalt oxide materials are promising alternatives to replace noble metal oxides for ORR [START_REF] Sun | Single iron atoms coordinated to g-C 3 N 4 on hierarchical porous N-doped carbon polyhedra as a high-performance electrocatalyst for the oxygen reduction reaction[END_REF][START_REF] Yang | Honeycomb-like porous carbon with N and S dual-doping as metal-free catalyst for the oxygen reduction reaction[END_REF][START_REF] He | Cobalt oxide doped with titanium dioxide and embedded with carbon nanotubes and graphene-like nanosheets for efficient trifunctional electrocatalyst of hydrogen evolution, oxygen reduction, and oxygen evolution reaction[END_REF]. For rationally designing electrocatalysts, coordination environment as well as the related electronic structure of the Co oxides materials should be considered to ensure a rapid procession of the reactions [START_REF] Cai | Bioinspired Engineering of Cobalt-Phosphonate Nanosheets for Robust Hydrogen Evolution Reaction[END_REF]. Transition metal metaphosphates/phosphates provide positive contributions for structural stability and can stabilize the intermediate state of the transition metal ions thus initiated wide-ranging interests [START_REF] Chang | Coralloid Co 2 P 2 O 7 Nanocrystals Encapsulated by Thin Carbon Shells for Enhanced Electrochemical Water Oxidation[END_REF]. Moreover, theoretical studies and experiments also indicate that the phosphate/metaphosphate group make good contributions in proton transfer, which is of great significance for catalysis. [START_REF] Vilciauskas | The mechanism of proton conduction in phosphoric acid[END_REF][START_REF] Mulloyarova | Conformational Mobility and Proton Transfer in Hydrogen-Bonded Dimers and Trimers of Phosphinic and Phosphoric Acids[END_REF] Many methods have been developed to synthesize cobalt phosphate/metaphosphate nanostructures, such as hydrothermal route [START_REF] Yuan | Cobalt phosphate nanoparticles decorated with nitrogen-doped carbon layers as highly active and stable electrocatalysts for the oxygen evolution reaction[END_REF][START_REF] Zhao | Phosphonate-derived nitrogen-doped cobalt phosphate/carbon nanotube hybrids as highly active oxygen reduction reaction electrocatalysts[END_REF][START_REF] Liu | Mesoporous Nanosheet Networked Hybrids of Cobalt Oxide and Cobalt Phosphate for Efficient Electrochemical and Photoelectrochemical Oxygen Evolution[END_REF], thermolytic molecular precursor (TMP) method [START_REF] Ahn | Electrocatalytic Water Oxidation at Neutral pH by a Nanostructured Co(PO 3 ) 2 Anode[END_REF], sol-gel process [START_REF] Pothiraja | Non-interpenetrating transition metal diorganophosphate 2-dimensional rectangular grids from their 1-dimensional wires: Structural, transformations under mild conditions[END_REF], freeze-drying method [START_REF] Zhan | The development of cobalt phosphate for bifunctional oxygen electrocatalysis in alkaline solution[END_REF] and combustion assisted argon-annealed route [START_REF] Gond | Sodium Cobalt Metaphosphate as an Efficient Oxygen Evolution Reaction Catalyst in Alkaline Solution[END_REF].

Although these cobalt phosphates/metaphosphates exhibit state-of-the-art performance for ORR [START_REF] Zhou | Nitrogen-doped cobalt phosphate@nanocarbon hybrids for efficient electrocatalytic oxygen reduction[END_REF], oxygen evolution reaction (OER) [START_REF] Ahn | Electrocatalytic Water Oxidation at Neutral pH by a Nanostructured Co(PO 3 ) 2 Anode[END_REF][START_REF] Gond | Sodium Cobalt Metaphosphate as an Efficient Oxygen Evolution Reaction Catalyst in Alkaline Solution[END_REF], and renewable energy technologies (e.g. as cathode for SIBs) [START_REF] Gond | Cubic Sodium Cobalt Metaphosphate [NaCo(PO 3 ) 3 ] as a Cathode Material for Sodium Ion Batteries[END_REF], the cost, durability and complexity of synthesis limit their sustainable application in a commercial scale [START_REF] Hoffmann | Investigation of thermal decomposition of phosphonic acids[END_REF].

Thus, seeking out an environmentally friendly and effectively synthetic path for cobalt phosphate/metaphosphate nanocomposites is a vital thing. Amino trimethylene phosphonic acid (ATMP) is a strong complexing agent for divalent cations with the morphology of tripodal tetradentate formed by combining a central atom 'N' with three ligands 'metaphosphate group'. The good chemical stability (melting point: 200 o C, boiling point: 746.2 o C), good chelation, lattice distortion as well as the cost-effective of ATMP has successfully caught our attention.

In the three phosphonic groups in ATMP, the phosphorus atom forms a σ bond with oxygen atom in form of sp 3 orbital, p-dπ bond which is perpendicular to the P-O σ bond formed between 3d 0 xz and 2pz as well as 3d 0 xy and 2py [START_REF] Hoffmann | Investigation of thermal decomposition of phosphonic acids[END_REF]. During the reaction, the oxygen atoms on the phosphonic group bond with Co (II) (3d 7 ) with three unpaired electron pairs to form a high spin complex, the σ and π orbitals of the ligand all participate in the molecular orbital of the combined complex. Thus, as depicted in Scheme 1, we firstly synthesize a novel metal organophosphate copolymer which can serve as an alternative precursor using ATMP as a chelating agent and then calcinated it to form a carbon based highly active catalyst. The main catalytically active center and the physical properties of cobalt metaphosphate based composite (Co(PO 3 ) 2 /NC) have been confirmed. Meanwhile, comparative analysis of cobalt oxide (CoO) and our synthesized Co(PO 3 ) 2 /NC reveals the potential structural features for efficient ORR. This study provides valuable insight into the design and synthesis of efficient ORR catalysts.

[Scheme 1]

Experimental section

Chemicals and materials

Cobalt (II) nitrate hexahydrate (Co(NO 3 ) 2 6H 2 O, AR, 99 %) and methanol (CH 3 OH, AR, 99 %) were purchased from Aladdin (China). The amino trimethylene phosphonic acid (ATMP, 50 wt. %) was bought from Shanghai Meryer Chemical Technology Co. Ltd (China). Cobalt (II) oxide was obtained from Energy Chemical (China). A commercially available 20 wt. % platinum-loaded carbon (Pt/C) loaded on carbon black (Johnson Matthey) was used as received. All other chemicals were of analytical grade and used without further purification. Distilled water was used throughout the experiment.

Preparation of Co(PO 3 ) 2 /NC, Co(PO 3 ) 2 and NC

The Co(PO 3 ) 2 /NC complex was prepared by a simple coordination reaction followed by calcination. In a typical preparation, 0.1 mol/L Co(NO 3 ) 2 6H 2 O methanol solution was prepared. Subsequently, 2 mL ATMP (50 wt. %) was added dropwise into 20 mL above solution and churned for 30 mins. The pH of the mixture is 4.54. Superfine pink powder formed during the process of mixing. Thus, the cobalt-metaphosphate coordination polymer was obtained and used as precursor for annealing treatment. The obtained sample was further heated from room temperature to 900 o C under the protection of N 2 flow with a heating rate of 5 °C min -1 and then held for 2 h at 900 o C. After it cooled down to room temperature, the black product denoted as Co(PO 3 ) 2 /NC was ground and collected for detection and characterization.

For comparison, Co(PO 3 ) 2 was obtained by reheating the Co(PO 3 ) 2 /NC at 650 °C for 4 h in air to oxidize and remove the carbon and nitrogen "cumbersome". NC were obtained by directly heating the ATMP at 900 °C for 2 h.

Materials characterization

The ultraviolet absorption spectra were measured by UV-vis spectrophotometry (UV-3600, Shimadzu) and solid UV-vis spectrophotometry (Evolution 220). A TENSOR Model 27 FT-IR spectrometer was used for FT-IR spectra test. Thermal gravimetric and differential thermal analyses (TG-DTA) analysis was investigated on Synchro Thermal Analyzer (STA 449 F1 Jupiter. NETZSCH). The X-ray diffraction (XRD) patterns were collected on X-ray diffractometer (Bruker D8) at 40 kV and 40 mA. With 532 nm solid laser as an excitation source, an iHR550 Raman microscope (HORIBA scientific) was used for Raman spectra test. The detailed morphology and size of all the catalysts were analyzed by transmission electron microscope (TEM, FEI Tecnai 20 electron microscope) and high-resolution transmission electron microscope (HRTEM, jeol jem2100f) and the element mapping images were carried out on Quanta 250F and FEI. The surface functional properties of all the catalysts were investigated by K-Alpha X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250XI). The binding energies were calibrated using the C 1s peak at 284.6 eV.

Electrochemical measurements

All electrochemical studies were performed with a conventional three-electrode cell using a graphite rod as counter electrode, a saturated calomel Hg/Hg 2 Cl 2 electrode as reference electrode. Cyclic voltammetry (CV) and rotating disk electrode (RDE, 0.196 cm 2 , Metrohm) were recorded on CHI660D electrochemical workstation (CH Instrument). Linear sweep voltammetry (LSV) experiments were executed at 10 mV s -1 under rotating speeds varying from 400 to 2025 rpm. During electrochemical measurements a flow of N 2 or O 2 was maintained over the electrolyte (0.1 mol L -1 KOH) for continuous N 2 or O 2 saturation. All catalysts were fabricated follow steps: Firstly, The catalyst ink (10 mg mL -1 ) of Co(PO 3 ) 2 /NC, Co(PO 3 ) 2 , CoO, NC and Pt/C were prepared by dispersing the catalyst powders in deionized water and ultrasonic dispersion for 5 minutes. Secondly, spreading 10 L of the catalyst ink onto pretreated RDE and dried at room temperature. Finally, 5 L of Nafion/ethanol solution (5 wt.%) was dropped on the surface of above-modified electrode and dried in air. Thus, the catalysts modified RDE were fabricated. Herein, all the potentials were converted to a reversible hydrogen electrode (RHE), E (RHE) = E (Hg/Hg 2 Cl 2 ) + 1.008 V.

The RRDE test of Co(PO 3 ) 2 /NC was conducted to verify and test the electron transfer number (n) and H 2 O 2 production yield. the results were calculated according to the eq (1) and ( 2):

(1) [START_REF] Qiao | Hierarchically Ordered Porous Carbon with Atomically Dispersed FeN 4 for Ultraefficient Oxygen Reduction Reaction in Proton-Exchange Membrane Fuel Cells[END_REF] where I D and I R reference the desk current and ring current, respectively. where n is the electron transfer number, H 2 O 2 % is the production yield during the ORR process, and N is the collection efficiency (0.35 in this study, the specific experiment is shown in Figure S1).

Calculation Method

All density function theory (DFT) calculations were performed based on the projector-augmented-wave method as implemented in the Vienna ab initio simulation package (VASP) [START_REF] Kresse | Ab initio molecular dynamics for liquid metals[END_REF][START_REF] Kresse | Ab initio molecular-dynamics simulation of the liquid-metal-amorphous-semiconductor transition in germanium[END_REF]. The Blöchl's all-electron-like projector augmented plane wave (PAW) method was used to describe the interactions between ion cores and valence electrons [START_REF] Blöchl | Projector augmented-wave method[END_REF]. The electronic exchange-correlation potential was treated within the functional of Perdew-Bu\rke-Ernzerhof (PBE) function for generalized gradient approximation (GGA). The plane-wave cutoff was set to 300 eV and the convergence threshold was set as 1×10 -4 eV in energy and 0.01 eVÅ -1 in force. The Monkhorst-Pack special k point was set with 3×4×4 grid meshes for structure optimization and 7×9×9 grid meshes for electronic property calculation. The optimized lattice constants for Co(PO 3 ) 2 are a = 11.04 Å, b = 8.09 Å, and c = 9.56 Å.

A slab with Co(PO 3 ) 2 and Co(PO 3 ) 2 /NC surfaces was constructed to simulate the reaction condition of Co(PO 3 ) 2 during ORR process, respectively. The supercell used for the N-doped carbon sheet consists of 127 carbon atoms with a single doped nitrogen atom at the center. The isolated Co(PO 3 ) 2 nanoparticle (Co 4 P 8 O 24 ) was optimized in a 30.0 Å cubic supercell in which the Brillouin zone integration was carried out for the Γ-point only. The ORR reaction pathways can be expressed as the following four steps:

(3) (4) (5) (6)
The computational hydrogen electrode model proposed by Nørskov and co-workers was used to express the thermochemistry of the sub-reactions at any given pH and applied potential U. The free energies were calculated from total energies using Eq. [START_REF] He | Cobalt oxide doped with titanium dioxide and embedded with carbon nanotubes and graphene-like nanosheets for efficient trifunctional electrocatalyst of hydrogen evolution, oxygen reduction, and oxygen evolution reaction[END_REF]. where i = 1, 2, 3, 4 corresponds to each step in Eq. [START_REF] Sun | Single iron atoms coordinated to g-C 3 N 4 on hierarchical porous N-doped carbon polyhedra as a high-performance electrocatalyst for the oxygen reduction reaction[END_REF][START_REF] Yang | Honeycomb-like porous carbon with N and S dual-doping as metal-free catalyst for the oxygen reduction reaction[END_REF][START_REF] He | Cobalt oxide doped with titanium dioxide and embedded with carbon nanotubes and graphene-like nanosheets for efficient trifunctional electrocatalyst of hydrogen evolution, oxygen reduction, and oxygen evolution reaction[END_REF][START_REF] Cai | Bioinspired Engineering of Cobalt-Phosphonate Nanosheets for Robust Hydrogen Evolution Reaction[END_REF]. ∆E is the reaction energy, ∆ZPE is the change of zero-point energy, T (298.15 K) is temperature, ∆S is the difference in entropy. The zero-point energies were calculated from the vibration frequencies. The entropies were taken from standard tables for gas-phase molecules [START_REF] Kresse | From ultrasoft pseudopotentials to the projector augmented-wave method[END_REF] Gas phase H 2 O at 0.035 bar was used as the reference state, because, at this pressure, gas phase H 2 O is in equilibrium with liquid water at 300 K. GORR and ηORR were then given by Eq. (7-9). ( 7)

(8) (9)

Result and discussion

Initially, a series of advanced spectroscopic techniques were used to confirm the successful coordination occurred between ATMP and Co(II). The UV-vis absorption measurements were performed in the range of 300-800 nm at room temperature is illustrated in Figure 1A. Herein, the precursor of Co(PO 3 ) 2 /NC was fabricated by mixing Co (II) and ATMP in H 2 O/CH 3 OH ( ) for 0.5 h. The absorption of ATMP and Co (II) can be found at 685 nm and 542 nm respectively.

Surprisingly, a distinct blue-shift was found in Co (II)-ATMP complex, showing a new peak at 510 cm -1 . An implication of this phenomenon is the possibility that Co ions have a strong combination with ATMP.

Figure 1B shows the FI-IR spectra of ATMP and the precursor of Co(PO 3 ) 2 /NC.

The band at 933 cm -1 and 1064 cm -1 are attributed to the stretching vibration of P-OH groups in organic phosphonic acid group in ATMP [START_REF] Chen | High temperature proton exchange membranes prepared from epoxycyclohexylethyltrimethoxysilane and amino trimethylene phosphonic acid as anhydrous proton conductors[END_REF]. The corresponding two absorption peaks in the precursor simultaneously occurred red-shift which located at 920 cm -1 and 1004 cm -1 respectively. A possible explanation for the shift might be that the two hydroxyl groups adjacent to the phosphorus atom in the ligands in ATMP dissociate the H + and then coordinate with the Co (II). Additionally, the band at 1204 cm -1 corresponds to the stretching vibration of P=O groups in ATMP [START_REF] Dołęga | Biomimetic zinc(ii) and cobalt(ii) complexes with tri-tert-butoxysilanethiolate and imidazole ligands-structural and spectroscopic studies[END_REF]. The position of this peak also shifted in the precursor which located in 1155 cm -1 . This discrepancy could be attributed to that the oxygen adjacent to phosphorus contains two pairs of lone electron pairs are prone to chelate with Co (II). Thermogravimetric analyses open interesting ways for the determination the thermal degradation behaviors of the metal complexes as well as structural elucidation.

TG-DTA experiments were performed on the precursor under an N 2 atmosphere with a heating rate of 10 o C min -1 . It can be seen from Figure 1D that the weight drops ∼13 % below 325 °C is ascribed to the evaporation of solvent (the mixed solution of methanol and water) and the self-polymerization under melting state of ATMP, associated with an intense exothermic peak at 325 o C in DTA curve [START_REF] Rajendran | Synergistic, antagonistic and biocidal effects of amino (trimethylene phosphonic acid), polyacrylamide and zn 2+ on the inhibition of corrosion of mild steel in neutral aqueous environment[END_REF]. The weight drops ∼6 % from 325 °C to 445 °C is attributed to the forming of 2-propane amine, cobalt-trimethyl phosphine and cobalt-trimethyl phosphine oxide [START_REF] Hoffmann | Investigation of thermal decomposition of phosphonic acids[END_REF]. Which deserving additional attention is that the reason for the weight loss (about 26 %) from 445 °C to 626 °C stemmed from the appearance of trimethylamine, cobalt-trimethyl phosphine oxide, tetramethylpyrrole, 4-dimethylaminophenol and multiple substituted benzene compounds [START_REF] Hoffmann | Investigation of thermal decomposition of phosphonic acids[END_REF]. The process accompanied by small amount of heat release.

With temperature increases, the weight drops ∼9 % from 626 °C to 900 °C reveal the gasification of the excessive ATMP and other organic compounds. Simultaneously, the free nitrogen atoms are far more likely to form pyrrole/pyridine rings with carbon which facilitates the electron transmission process. As for this N-doped carbon, the electric neutrality of carbon atoms could be destroyed due to the insertion of N atoms into the graphitic lattice [START_REF] Jiang | N and P co-functionalized three-dimensional porous carbon networks as efficient metal-free electrocatalysts for oxygen reduction reaction[END_REF]. The enlarged spin density and unevenly distributed charge density of carbon atoms could facilitate the adsorption of oxygen and subsequent reduction reaction on carbon [START_REF] Jiang | N and P co-functionalized three-dimensional porous carbon networks as efficient metal-free electrocatalysts for oxygen reduction reaction[END_REF][START_REF] Zhang | Encapsulation of Co-based nanoparticle in N-doped graphitic carbon for efficient oxygen reduction reaction[END_REF].

[Figure 1]

The composition and crystalline nature of the obtained pyrolysis products were identified by powder X-ray diffraction (XRD). As illustrated in Figure 2A (defective graphitic structures) and G band (E 2g -mode from the sp 2 carbon domain) at 1558 cm -1 peaks are attributed to the carbon material [START_REF] Zhang | Encapsulation of Co-based nanoparticle in N-doped graphitic carbon for efficient oxygen reduction reaction[END_REF][START_REF] Niu | One-pot synthesis of nitrogen-rich carbon dots decorated graphene oxide as metal-free electrocatalyst for oxygen reduction reaction[END_REF]. As shown in EDS mapping (Figure 2C), the existence of N can be demonstrated, and the homogenous distribution of C, O, Co, P and N can also be exhibited. Figure S3 shows the FI-IR spectra of Co(PO 3 ) 2 and Co(PO 3 ) 2 /NC. The weak absorption peaks at 1200 cm -1 is ascribed to the stretching vibration of N-C bond [START_REF] Chen | Nitrogen and phosphorus co-doped carbon nanosheets as efficient counter electrodes of dye-sensitized solar cells[END_REF], showing the successful doping of nitrogen in Co(PO 3 ) 2 /NC carbon materials. Since the N atom is inserted into the graphite lattice, the electric neutrality and the charge density of carbon atoms could be destroyed [START_REF] Yazdi | Nitrogen/sulfur co-doped helical graphene nanoribbons for efficient oxygen reduction in alkaline and acidic electrolytes[END_REF]. An increase in the spin density of carbon atoms could promote the adsorption of oxygen and subsequent reduction reaction on carbon [START_REF] Jiang | N and P co-functionalized three-dimensional porous carbon networks as efficient metal-free electrocatalysts for oxygen reduction reaction[END_REF][START_REF] Cai | 3D Co-N-doped hollow carbon spheres as excellent bifunctional electrocatalysts for oxygen reduction reaction and oxygen evolution reaction[END_REF].

The morphology of all the synthesized catalysts were examined by SEM and TEM images. synergistically enhance the overall electron transfer capability [START_REF] Yuan | Cobalt phosphate nanoparticles decorated with nitrogen-doped carbon layers as highly active and stable electrocatalysts for the oxygen evolution reaction[END_REF]. From the HRTEM images (Figure 2G) of Co(PO 3 ) 2 /NC nanocomposite, the lattice fringes are clearly visible in the bulk portion the particle, indicating single crystallinity of the particles.

The 1.47 Å spacing corresponds to the (121) plane of monoclinic Co(PO 3 ) 2 .

[Figure 2]

XPS was performed to investigate the bonding configuration and chemical state in Co(PO 3 ) 2 /NC. The full XPS survey spectra of NC (Figure S5A), Co(PO 3 ) 2 and Co(PO 3 ) 2 /NC (Figure 3A) both show three main peaks of C 1s (actually a trace amount of residual carbon existed in Co(PO 3 ) 2 , O 1s and P 2p at 284.15 eV, 533.0 eV and 133.9 eV, respectively. In addition, the content of nitrogen in Co(PO 3 ) 2 is close to zero, and no peak treatment is done. We first focus on the fine spectrum of Co(PO 3 ) 2 /NC, the XPS survey spectrum of Co(PO 3 ) 2 /NC appeared obvious N 1s peak, confirming successful N doped. As shown in Table S1, quantitative XPS analysis shows that the concentration of nitrogen is 2.02 for Co(PO 3 ) 2 /NC. As shown in Figure 3B, the N1 s XPS spectra of Co(PO 3 ) 2 /NC was decomposed into four dominant peaks, which correspond to pyridinic N (398.57 eV), Co-N x (400.21 eV), graphitic N (401.62 eV) and oxide N (404.51 eV), respectively. We are surprised to find that the nitrogen peak of NC (Figure S5B) is similar to that of Co(PO 3 ) 2 /NC, indicating that the main source of nitrogen in Co(PO 3 ) 2 /NC comes from NC. We also performed an elemental analysis of NC (Table S2), where the nitrogen content was 1.41%. It is considered that carbon materials with N doping promote the charge mobility in the carbon matrix, which can accelerate the catalytic activity [START_REF] Niu | One-pot synthesis of nitrogen-rich carbon dots decorated graphene oxide as metal-free electrocatalyst for oxygen reduction reaction[END_REF]. While for P 2p fine spectrum (Figure 3C), the deconvoluted two sub-peaks located at 134.60 and 135.50 eV are the characteristic P 2p which are beneficial to accelerate the catalytic activity [START_REF] Li | Nitrogen and Phosphorus Dual-Doped Graphene/Carbon Nanosheets as Bifunctional Electrocatalysts for Oxygen Reduction and Evolution[END_REF].

[Figure 3]

The ORR activity of all the catalysts were assessed using the RDE technique, with 4B shows the RDE voltammograms at a rotation speed of 1600 rpm for all samples. Co(PO 3 ) 2 /NC exhibits the enhanced performance with a relatively positive onset potential (E onset = 0.906 V vs. RHE), half wave potential (E 1/2 = 0.780 V vs. RHE) and diffusion limiting current (J L =5.062 mA cm -2 ) compared with NC (0.835 V, 0.726 V, 3.616 mA cm -2 ), CoO (0.827 V, 0.662 V, 2.232 mA cm -2 ), Co(PO 3 ) 2 (0.835 V, 0.726 V, 2.670 mA cm -2 ), and comparable to Pt/C (0.916 V, 0.800 V, 5.200 mA cm -2 ). The pronounced performance of Co(PO 3 ) 2 /NC is associated with the advantage of the intimate connection between N doped carbon and Co(PO 3 ) 2 NPs, which could trigger a synergistic effect thus accelerates electron transfer for the ORR process [START_REF] Zhou | Bio-inspired organic cobalt (II) phosphonates toward water oxidation[END_REF].

Moreover, in order to gain a further insight ORR kinetics of our synthesized catalysts, RDE tests at various rotations were performed (Figure S7). Electrochemical surface area (ECSA), reflecting the amount of catalyst in contact with the electrolyte, was obtained by measuring the CV curves with various scan rates with iR-corrected potentials of 0.9 V vs. RHE (Figure S9 and Figure S10). The double-layer capacitances of the Co(PO 3 ) 2 , CoO, NC and Co(PO 3 ) 2 /NC catalysts are calculated to be 0.1, 0.36, 8.2 and 12.6 F g -1 , respectively. After thermal cleavage at 650 o C at air for 4h, the carbon and nitrogen "cumbersome" drop off the surface of Co(PO 3 ) 2 /NC and the catalyst surface becomes much smoother and thus reduces the ECSA (Figure S3C). The formation of N-doped carbon on the surface of Co(PO 3 ) 2 could largely increases the electrochemical surface.

For the sake of evaluating the practical application of our catalyst, the durability and methanol-tolerance are key parameters. As performed in Figure 4C, slight performance attenuation with higher current retention can be achieved of Co(PO 3 ) 2 /NC with approximately 85.6 % relative current persisted after the test for 400 mins, whereas the commercial Pt/C retained only 83.8 % of its initial current. It indicates a longer-term stability of Co(PO 3 ) 2 /NC over the Pt/C in an alkaline electrolyte. As is known to all, fuel molecules (e.g., methanol) which may cause the fuel crossover effect when it passes through the membrane and poison the cathode catalyst. Herein, the methanol-tolerance of the Co(PO 3 ) 2 /NC and Pt/C catalysts were noted at 0.4 V (vs. RHE). As shown in Figure 4D, slight performance degradation (96 %) was achieved with Co(PO 3 ) 2 /NC after the injection of 3 M methanol into the electrolyte at 600 s. In contrast, the commercial Pt/C catalyst decreased more significantly. The results obviously confirm that the Co(PO 3 ) 2 /NC possesses a high resistance to methanol crossover.

The XPS, XRD and morphology characterization after ORR of Co(PO 3 ) 2 /NC have been conducted to support the good stability of Co(PO 3 ) 2 /NC. The composition of Co(PO 3 ) 2 /NC was firstly identified by XRD, as is shown in Figure S11, all diffraction peaks matched well with the Co(PO 3 ) 2 (PDF No. . This indicates that the crystal structure of the catalyst is mainly composed of Co(PO 3 ) 2 crystal after ORR test.

In addition, the surface composition after ORR reaction was observed by XPS (Figure S12). The same binding energies of N 1s (Figure S12A), P 2p (Figure S12B) and Co 2p (Figure S12C) were found for the recycled sample indicating that there is no change in the valence state of N 1s, P 2p and Co 2p.). Representative HRTEM images of the surface before and after ORR reaction are shown in Figure S13. The lattice fringes in the images of samples both before and after ORR are visible up. Such observations indicate no evidence of any phase transformation during the ORR reaction, supporting that our catalyst has good stability.

[Figure 4]

To further verify the main active site in our catalyst, one more control experiment was performed. The Co(PO 3 ) 2 /NC catalyst was acid treated in HCl (5 M) for two days to remove the cobalt metaphosphate nanoparticles as much as possible. The final product washed three times with HCl and DI water and then dried overnight in a vacuum. From the ICP test in Table S3, the content of Co (0.6 wt% Co remaining after etching) confirms that most of the cobalt metaphosphate was removed. As shown in Figure S14, the HCl-etched sample presents much inferior activity compared to the original Co(PO 3 ) 2 /NC, though XPS confirmed the presence of nitrogen (Figure S15). The calculated results are consistent with the experimental results. In our study, Co(PO 3 ) 2 groups as the main active site may be attributed to its excellent proton transfer properties, thus facilitating the ORR process. [START_REF] Vilciauskas | The mechanism of proton conduction in phosphoric acid[END_REF][START_REF] Mulloyarova | Conformational Mobility and Proton Transfer in Hydrogen-Bonded Dimers and Trimers of Phosphinic and Phosphoric Acids[END_REF] The N-doped graphitic carbon formed in situ can further enhance the catalytic activity of the system.

In our work, the ATMP not only acted as the metaphosphate source, but also thermally decomposed into N doped carbon coating on Co(PO 3 ) 2 NPs during the annealing process. We also proposed the feasible growth mechanism. After Co 2+ linked to ATMP in solution, under a high temperature treatment, Co(PO 3 ) 2 grew to be well crystalline nanoparticles and ATMP decomposed to be N doped carbon, gradually.

Notably, the N doped carbon was extruded out and coated on the surface of Co(PO 3 ) 2

NPs to form a network structure. The existence of NC has made great contribution to improving the stability of ORR. The catalytic properties of the materials in the same system as our catalytic materials are shown in Table S5 as the main active site may be attributed to its excellent proton transfer properties, thus facilitating the ORR process.

[Figure 5]

4. Conclusions

In summary, the Co(PO 3 ) 2 decorated with N-doped graphitic carbon have been designed to comprehend the relationship between the nanostructure and the electrocatalytic activity toward ORR. We offer a simple one step method to develop a new alternative precursor for efficiently catalyst for ORR. The proposed Co(PO 3 ) 2 /NC presents a better ORR properties than Co(PO 3 ) 2 in alkaline electrolyte which can be ascribed to the excited synergistic effect between N-doped carbon layer and Co(PO 3 ) 2 NPs which are beneficial to accelerate ORR. Our study shed light up an effective route to rational design and scalable fabrication of Co(PO 3 ) 2 with cost-effective, high-active, and long-term stability. It is currently working to modify specific metal center ions with different ligands to regulate their structural properties and further enhance the catalytic activity.

  Figure2B, the sharp peak at 671 cm -1 corresponds to the symmetric stretching

  Figure S4A, S4B and S4C show the SEM images of NC, CoO and Co(PO 3 ) 2 .It can be observed that the morphology of the NC exhibits a sheet-like structure, the CoO and Co(PO 3 ) 2 show a granular morphology. The surface of the CoO particles is relatively smooth, while the surface of the Co(PO 3 ) 2 particles is relatively rough, corresponding to the results of TEM (FigureS4D, S4E and S4F). In addition, there are some agglomerations of Co(PO 3 ) 2 . Figure2D and 2Eshow many ultrathin nanosheets are randomly compact and cross erectly together. It is observed that the single nanosheet has 20-50 nm in thickness. The lengths of the single erect plate are not uniform and they are in the range of 0.5-2 μm. Meanwhile, the rough surface on the carbon nanoplate in favor of exposing abundant marginal defective sites, which are beneficial for enhancing electrochemical performance[START_REF] Chen | Nitrogen and phosphorus co-doped carbon nanosheets as efficient counter electrodes of dye-sensitized solar cells[END_REF]. The representative TEM images (Figure2F) of Co(PO 3 ) 2 /NC reveal that there are many Co(PO 3 ) 2 NPs (about ~10 nm in diameter) are encapsulated by N-doped carbon layers, which act as bridges connect the Co(PO 3 ) 2 NPs. The tight connection of carbon and Co(PO 3 ) 2 NPs

  With increasing rotation speed from 400 to 2025 rpm, the current density increases gradually, indicating an increase of O 2 diffusion on the surface of electrode. The number (n) of electrons transferred of above catalysts can be obtained from the corresponding K-L plot, the value of n of Co(PO 3 ) 2 /NC is 3.92, indicating a dominated four-electron pathway in the ORR proceeded. The RRDE test of Co(PO 3 ) 2 /NC was conducted to verify and test the electron transfer number (n) and H 2 O 2 production yield. (Figure S8) We can see that, in the potential range from 0 to 1.0 V, the n value of Co(PO 3 ) 2 /NC remains between 3.7 and 4.0. This clearly reveals that the ORR on Co(PO 3 ) 2 /NC follows an efficient one-step, four-electron pathway over the entire potential range with a low H 2 O 2 production yield.

  Figure 5D. The previous theoretical studies demonstrated that the ORR activities are

Figure 1 .

 1 Figure 1. (A) UV-vis absorption spectra of ATMP, Co(II) and precursor in

Figure 2 .

 2 Figure 2. (A) XRD patterns of Co(PO 3 ) 2 /NC and Co(PO 3 ) 2 . (B) Raman spectrum of

Figure 3 .

 3 Figure 3. (A) Full XPS spectra and high resolution XPS spectra of (B) N 1s of

Figure 4 .

 4 Figure 4. (A) CV curve of Co(PO 3 ) 2 /NC in N 2 -saturated versus O 2 -saturated in 0.1 M

Figure 5 .Scheme 1 .

 51 Figure 5. (A) The band structure of Co(PO 3 ) 2 crystal, (B) The projected density of

Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Scheme 1

 123451 Figure 1

  The solid UV-vis absorption patterns of ATMP and the precursor of Co(PO 3 ) 2 /NC

	are illustrated in Figure 1C. What is surprising is that a significant difference appeared
	at 533 nm in the precursor, corresponding to O-donner octahedral Co 2+ complexes
	[4T1g4T1g(P)transition] [28]. This accords with our earlier observations, which
	showed that Co ions have successfully combined with oxygen which adjacent to
	phosphorus in ATMP through chelation.

  NC, CoO, Co(PO 3 ) 2 , Co(PO 3 ) 2 /NC and Pt/C being studied under the same experimental conditions for comparison. Figure 4A and Figure S6 present the CVs of NC, CoO, Co(PO 3 ) 2 Co(PO 3 ) 2 /NC and Pt/C in N 2 and O 2 saturated 0.1 M KOH at scan rate of 10 mV s -1 . For Co(PO 3 ) 2 /NC nanocomposite, the Ep of Co(PO 3 ) 2 /NC (0.816 V vs. RHE) was more positive than NC (0.767 V vs. RHE), CoO (0.724 V vs.

RHE

) and Co(PO 3 ) 2 (0.686 V vs. RHE), and it was in proximity to commercial Pt/C (0.858 V vs. RHE). It is suggested that the doping of N has greatly improve the electrochemical performance in this work. Interestingly, it can be found that two distinguishable oxidation peaks at around 0.902 and 1.090 V were found at the CV curve recorded with Co(PO 3 ) 2 /NC, which may be ascribed to the existence of Co at the lower valence state which in turn accelerates ORR. Figure

  . Compared with most materials, our proposed Co(PO 3 ) 2 /NC own many advantages such as high limiting current density, easily available source of phosphorus, easy to synthesize and structural stability. In addition, the cost of our catalyst is very low (the price of Co(PO 3 ) 2 /NC is about 13.8 RMB/g, which is much lower than commercial 20 wt. % Pt/C (about 800 RMB/g), see tableS6 and S7). We really hope to develop the potential of this kind of materials through more in-depth research in future.To further understand the influence of P on the length of Co-O bond and the angle of Co-O-P bond. we examined the crystal structure of CoO and Co(PO 3 ) 2 . The corresponding structural parameters are summarized in TableS8-S11. the distance of the Co-O bond (2.0285-2.1768 Å) in Co(PO 3 ) 2 is significantly longer than CoO (1.989 Å), and the angel of the Co-O-P bond (121.38 o -157.59 o ) in Co(PO 3 ) 2 is also

	longer than O-Co-O bond (109.47 o ) in CoO, resulting in a distorted coordination
	geometry. This result was consistent with previous reports and indicates that their
	structure contributes to stabilization of the reactive intermediate [38]. Based on the
	above advanced spectroscopic techniques and DFT calculations, there are three
	possible explanations for the high catalysis characteristics our synthesized
	cost-effective Co(PO 3 ) 2 /NC: (1) Compared with CoO, the introduction of P tunes the
	length of Co-O bond and the angel of O-Co-O bond, which provide positive
	contributions for structural stability, (2) the excited synergistic effect of the N-doped
	carbon layer and Co(PO 3 ) 2 NPs are beneficial to accelerate ORR, (3) Co(PO 3 ) 2 groups

Acknowledgements

This research was supported by National Natural Science Foundation of China (Grant No.21675086), the Fundamental Research Funds for the Central Universities (No.30918012202), and a project founded by the priority academic program development of Jiangsu Higher Education Institutions (PAPD). The authors wish also to acknowledge the support from the Sino-French international research network "New nanostructured materials and biomaterials for renewable electrical energy sources" for providing facilities.

AUTHOR INFORMATION

Corresponding Authors

Email: danshan@njust.edu.cn (D. Shan) zhangslvip@njust.edu.cn (S.L. Zhang) wangwenju1982@gmail.com (W.J. Wang) Fax: 0086-25-84303107

Notes

The authors declare no competing financial interest


Lavenia Ratnarajah
email: l.ratnarajah@liverpool.ac.uk

Stéphane Blain

Philip W Boyd

Marion Fourquez

Ingrid Obernosterer

Alessandro Tagliabue

Resource colimitation drives competition between phytoplankton and bacteria in the Southern Ocean

Keywords: Iron, Phytoplankton, Bacteria, Competition, Southern Ocean

Key points: (140 characters)

1.

Competition for iron between phytoplankton and bacteria is modulated by the colimiting effects of light and dissolved organic carbon 2. When enough labile dissolved organic carbon is present, bacteria can outcompete phytoplankton for iron 3. Phytoplankton exudation may be insufficient to stimulate bacterial dominance in the absence of other dissolved organic carbon sources Abstract (150 words) Across the Southern Ocean, phytoplankton growth is governed by iron and light, whilst 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, whilst increasing LDOC and decreasing light favours 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.

Plain summary

In large areas of the Southern Ocean, phytoplankton growth is controlled by the availability of iron and light whilst bacterial growth is controlled by the availability of iron and labile dissolved organic carbon (LDOC). We developed a mechanistic model to examine how phytoplankton and bacteria compete for iron under different light levels and LDOC supply.

We find that phytoplankton dominate as iron and light increases while increasing LDOC and decreasing light favours bacterial dominance. If enough LDOC is present, bacteria can outcompete phytoplankton for iron. More broadly, we find that while bacteria are a major consumer of iron in the Southern Ocean, seasonal changes in light drive phytoplankton dominance over bacteria, unless LDOC is supplied by other sources such as viral lysis, excretion etc., which strengthen competitive interactions.

Introduction

Phytoplankton growth in large areas of the Southern Ocean is regulated by the micronutrient iron (Fe) [START_REF] Martin | Iron deficiency limits phytoplankton growth in Antarctic waters[END_REF], Blain et al. 2007[START_REF] Boyd | Mesoscale iron enrichment experiments 1993-2005: synthesis and future directions[END_REF]), with seasonal co-limitation by light [START_REF] Mitchell | Light limitation of phytoplankton biomass and macronutrient utilization in the Southern Ocean[END_REF], Strzepek et al. 2012) due to the deep mixed layers and ice cover that are prevalent in early spring, autumn and winter. The low rates of primary production and large-scale upwelling of deep water containing low dissolved organic carbon (DOC) concentrations lead to some of the lowest surface DOC concentrations in the global ocean (~40 -50 µM, [START_REF] Hansell | Net removal of major marine dissolved organic carbon fractions in the subsurface ocean[END_REF]. While low surface dissolved Fe and DOC is thought to restrict heterotrophic bacterial (hereafter bacteria) growth in the Southern Ocean [START_REF] Church | Limitation of bacterial growth by dissolved organic matter and iron in the Southern Ocean[END_REF], Obernosterer et al. 2015), studies from Fe-limited regions have demonstrated that bacteria may also be significant consumers of dissolved Fe and can exhibit greater cellular Fe quotas than phytoplankton [START_REF] Tortell | The role of heterotrophic bacteria in iron-limited ocean ecosystems[END_REF][START_REF] Tortell | Marine bacteria and biogeochemical cycling of iron in the oceans[END_REF], Boyd et al. 2012, Fourquez et al. 2015, 2020). High bacterial Fe demand and uptake rates compared to phytoplankton suggest that bacteria could be significant competitors for dissolved Fe in the Southern Ocean (Fourquez et al., 2015, 2020[START_REF] Mazzotta | Characterization of the Fe Metalloproteome of a Ubiquitous Marine Heterotroph, Pseudoalteromonas (BB2-AT2): Multiple Bacterioferritin Copies enable significant Fe Storage[END_REF]).

Theoretically, two species competing for the same limiting resource cannot coexist at constant population levels [START_REF] Hardin | The competitive exclusion principle[END_REF]). However, colimitation can lead to stable coexistence [START_REF] Burson | Competition for nutrients and light: testing advances in resource competition with a natural phytoplankton community[END_REF] or alternative stable states, depending on ambient environmental factors (e.g. resource supply and lability, temperature, light and mortality) and the traits of the competing species [START_REF] Tilman | Resource competition and community structure[END_REF][START_REF] Brauer | The Nutrient-Load Hypothesis: Patterns of Resource Limitation and Community Structure Driven by Competition for Nutrients and Light[END_REF]) that affect resource uptake and growth [START_REF] Hutchinson | The concept of pattern in ecology[END_REF][START_REF] Titman | Ecological competition between algae: experimental confirmation of resource-based competition theory[END_REF]). In the Southern Ocean, the distinct seasonal light cycle regulates the timing of phytoplankton blooms, whereas Fe controls the magnitude of growth. While phytoplankton productivity then fluctuates according to season and location (Ardyna et al. 2017), the implications of changes in Fe, DOC and light over the seasonal cycle on phytoplankton-bacterial interactions are poorly understood. Upwelling of deep water is generally considered to supply a relatively refractory pool of DOC [START_REF] Hansell | Recalcitrant Dissolved Organic Carbon Fractions[END_REF]). However, the release of labile DOC via phytoplankton exudation [START_REF] Larsson | Phytoplankton exudate release as an energy source for the growth of pelagic bacteria[END_REF]Hagström 1979, Wood and[START_REF] Wood | Paradox lost? On the release of energy-rich compounds by phytoplankton[END_REF] or by other biotic processes commonly linked to phytoplankton production, such as sloppy feeding and faecal production by predators (Lampitt et In this study, we develop a mechanistic model to examine the competitive interactions between phytoplankton and bacteria in response to Fe, light and DOC colimitation. Our model was designed to assess the mechanisms behind these competitive interactions in an idealised setting and to explore the implications of these interactions on seasonal bloom dynamics in the Southern Ocean. We find resource competition influences the magnitude of phytoplankton and bacterial biomass and rates of Fe uptake, growth and carbon fixationwhich depend on secondary factors affecting the growth and biomass accumulation of phytoplankton and bacteria, such as light, DOC and luxury Fe uptake.

Methods

Biogeochemical model

The rate of change in phytoplankton carbon, chlorophyll-a and dissolved Fe is based on the Pelagic Integration Scheme for Carbon and Ecosystem studies (PISCES-v2, [START_REF] Aumont | PISCES-v2: an ocean biogeochemical model for carbon and ecosystem studies[END_REF]) model, which is commonly used as part of global studies of ocean Fe biogeochemistry (e.g. [START_REF] Tagliabue | How well do global ocean biogeochemistry models simulate dissolved iron distributions?[END_REF]. Here, we also explicitly account for the change in bacterial carbon and Fe biomass. Changes in phytoplankton and bacterial carbon and Fe biomass, as well as phytoplankton cellular chlorophyll-a are driven by rates of growth, carbon fixation, chlorophyll synthesis and Fe uptake in response to Fe, light (modelled here as the photosynthetic available radiation, PAR) and labile DOC (Fig. 1). We identify two sources of labile DOC: phytoplankton exudation and additional biotic sources which can be supplied via grazing processes (excretion, sloppy feeding), viral lysis etc. Detailed equations and specific parameterisations are described in the supplementary material (Supp. methods and Supp.

Table 1). Here we briefly describe the underlying processes considered and their dependencies.

Fe uptake by both phytoplankton and bacteria is modelled as a function of external Fe availability, including luxury Fe uptake and is down regulated when maximum cellular Fe quotas are reached. For simplicity, we assume phytoplankton and bacteria have the same maximum cellular Fe quota of 80 µmol Fe:mol C. This is based on the maximum phytoplankton cellular Fe quota used in PISCES-v2 [START_REF] Aumont | PISCES-v2: an ocean biogeochemical model for carbon and ecosystem studies[END_REF]) and maximum bacterial cellular Fe quota from a laboratory experiment under Fe-replete conditions [START_REF] Mazzotta | Characterization of the Fe Metalloproteome of a Ubiquitous Marine Heterotroph, Pseudoalteromonas (BB2-AT2): Multiple Bacterioferritin Copies enable significant Fe Storage[END_REF]). Labile DOC uptake by bacteria is modelled as a function of the temperature dependant maximum specific growth rate and bacterial Fe limitation. Light limitation of phytoplankton is identical to the PISCES-v2 model, explicitly accounting for phytoplankton carbon production and chlorophyll synthesis. Resource limitation for both phytoplankton and bacteria follow the quota model approach for Fe used in PISCES-v2 and a Monod style limitation form for labile DOC. Phytoplankton carbon fixation is then a function of the temperature dependant maximum growth rate and light and Fe limitation. DOC is cycled via exudation and mortality and Fe is cycled via mortality (Fig. 1). DOC exudation is a fixed proportion of phytoplankton carbon fixation (10%; [START_REF] Aumont | PISCES-v2: an ocean biogeochemical model for carbon and ecosystem studies[END_REF]. A quadratic mortality drives the recycling of cellular Fe, carbon and chlorophyll.

Experimental design

Using our model, we explore the role of Fe-light and Fe-DOC co-regulation of phytoplankton and bacteria using two sets of model simulations at a constant temperature of 1°C. We first run a set of simulations with phytoplankton alone, across a range of light and Fe levels (from 1 W m -2 to 40 W m -2 and 0 to 1 nM for PAR and Fe, respectively) and bacteria alone, across a range of labile DOC and Fe levels (from 0 to 100 µM and 0 to 1 nM for total DOC and total Fe, respectively). These initial experiments allow us to explore the separate responses of phytoplankton and bacteria to changing light, Fe and DOC levels. We then run a second set of experiments with both phytoplankton and bacteria competing under varying light and Fe To specifically address the potential role played by luxury Fe uptake in bacteria, we run a parallel simulation with variable cellular Fe quota for phytoplankton, but fixed the Fe quota for bacteria at a constant 10 µmol Fe:mol C [START_REF] Aumont | PISCES-v2: an ocean biogeochemical model for carbon and ecosystem studies[END_REF]. We use the model results after 35 days of integration (by which time the model has reached quasi steady state) in our analysis.

Results and Discussion

Response to resource co-limitation in the absence of competitive interactions

Increasing Fe and light leads to an expected increase in phytoplankton carbon biomass (Fig. 2a), with the phytoplankton cellular Fe quota (Fe:C ratio) increasing as ambient Fe concentration increases (Fig. 2b). However as light increases in parallel to Fe, phytoplankton accumulate less cellular Fe relative to carbon because of high rates of growth and carbon fixation (Fig. 2b,c). Increasing Fe and DOC similarly leads to an increase in carbon biomass of bacteria (Fig. 2d). Although bacteria reach their maximum cellular Fe quota in all our simulations, bacterial growth remained restricted at low DOC concentrations due to DOC limitation which leads to high Fe:C ratios due to low rates of C assimilation (Fig. 2e).

Increasing Fe and DOC in parallel results in a 7-fold increase in bacterial growth rate (Fig. 2f).

How does iron-light influence phytoplankton-bacterial interactions?

Competition for Fe between phytoplankton and bacteria affects growth rates, production and Fe uptake, which influences the relative accumulation of biomass. Under varying Fe and light, but fixed DOC, competition with bacteria decreases phytoplankton biomass by 3 -60% (Fig. 3a) relative to the simulations with only phytoplankton due to bacterial Fe uptake, with greatest change observed at the lowest Fe levels across all light levels. We denote the 1:1-line (i.e. region where phytoplankton biomass equals bacteria biomass), which separates regions of phytoplankton (blue) and bacterial (red) biomass dominance as the 'line of coexistence'. This line of coexistence when defined in terms of carbon biomass is very similar to that defined from the specific growth rate (d -1 , orange dash) and carbon production rate (orange solid) (Fig. 3b), due to their fundamental role in driving biomass accumulation in our model.

Below the line of coexistence (red area), bacteria are dominant, but their biomass remains low as low levels of phytoplankton productivity results in insufficient DOC supply from phytoplankton exudation. Above this line of coexistence (blue area), increasing light and Fe increases phytoplankton growth rate and carbon production resulting in overall phytoplankton dominance as they outcompete bacteria for Fe. As phytoplankton biomass increases, the DOC inventory increases via exudation, which stimulates bacterial growth in parallel, but bacterial biomass remains less than phytoplankton biomass (Supp. Fig. 1a,b). This is because the exudation of DOC by phytoplankton at the start of the experiment is insufficient for bacteria to outcompete phytoplankton. The low DOC levels allow phytoplankton to respond more strongly in general by increasing growth rates, carbon production and biomass (Supp. The difference in the rates of Fe uptake between phytoplankton and bacteria (Fig. 3c) show a slightly different form to that seen for biomass, growth and carbon fixation at low Fe and light. Bacteria consume up to 50% of the ambient Fe pool at low Fe (< 0.2 nM) and light (<8 W m -2 ) levels due to low phytoplankton biomass levels and higher Fe uptake efficiency (i.e. lower bacterial half saturation constant for Fe uptake, Supp. Table 1), but increasing Fe and light causes greater phytoplankton biomass to accumulate (Fig. 3c). We find that phytoplankton can outcompete bacteria for Fe uptake at high light and Fe (Fig. 3c) due to their higher biomass and growth rate (Supp. Fig. 1a and2a, b). In contrast, when bacteria rely solely on phytoplankton exudation as a DOC source, bacteria become relatively poor competitors for available Fe across these experiments.

How does iron-DOC influence phytoplankton-bacterial interactions?

Under varying Fe and DOC, but fixed light, bacteria receive two sources of DOC; DOC from phytoplankton exudation and DOC from additional biotic processes. In the natural environment, zooplankton excretion, viral lysis and sloppy feeding represent additional biotic sources of DOC as well as Fe. Whilst there are limited quantitative estimates on the contribution from each component, Antarctic krill were shown to excrete 188 -560 µM labile DOC that was rapidly utilised by the bacterial community within the 140 -160 hour experimental duration leading to an increase in bacterial biomass and production [START_REF] Arístegui | Krill excretion boosts microbial activity in the Southern Ocean[END_REF].

In contrast to the Fe-PAR simulation (where the only DOC was from phytoplankton exudation), phytoplankton-bacterial competition increases bacterial biomass ('+', Fig. 3d) more substantially (by up to 90% at low DOC levels), relative to the simulations with bacteria alone. The increase in bacterial biomass is driven by high phytoplankton biomass growth rates and carbon production (Supp. Fig. 1c, d and2d, d) which adds to the DOC inventory via phytoplankton exudation thereby facilitating bacterial growth (Fig. 3d,e). An exception is at high DOC and low Fe where competitive Fe uptake by phytoplankton results in a small (<6%) decrease in bacterial biomass ('-', Fig. 3d). Greater DOC levels increasingly shift the dominance from phytoplankton to bacteria throughout this experiment, pointing to the importance of DOC in driving the competitive fitness of bacteria. The biomass line of coexistence is similar to the 1:1 line for carbon production (orange solid), but phytoplankton growth rate at this point is ~10% greater than bacterial growth rate (Fig. 3e, Supp. Fig. 2). Fe uptake is decoupled from biomass levels and carbon production rates (Fig. 3f versus 3e).

Bacteria consume between 50 -80% of the ambient Fe pool except at low DOC (Fig. 3f).

While the Fe-PAR simulation suggests that bacteria are poor competitors for available Fe when they rely solely on phytoplankton exudation as a source of DOC, we find that bacteria are more effective competitors for Fe as total (phytoplankton exudation + additional biotic processes) DOC increases.

At the extremes, our simulations show phytoplankton dominance under increasing light and Fe (Fig. 3b and Supp. Fig. 1a and3b), but bacterial dominance under increasing total DOC and decreasing light (Fig. 3e and Supp. Fig. 1b, d and3a, b). We find that under moderate light levels (20 W m -2 ), coexistence between phytoplankton and bacteria can be facilitated when labile DOC increases in tandem with Fe up to around 20 -30 µM DOC. However, stable coexistence is only found in our simulations at low biomass levels where colimitation constrains both phytoplankton and bacterial populations (Fig. 3b,e). Overall, our simulations show that in the presence of sufficient labile DOC, bacteria are effective competitors with phytoplankton for Fe, exacerbating phytoplankton Fe limitation, most notably under moderate light levels.

Our model suggests that bacteria are an important consumer of Fe (Fig. 3c,f), in line with the field studies that demonstrated higher Fe uptake by bacteria compared to phytoplankton [START_REF] Tortell | The role of heterotrophic bacteria in iron-limited ocean ecosystems[END_REF][START_REF] Sarthou | The fate of biogenic iron during a phytoplankton bloom induced by natural fertilisation: impact of copepod grazing[END_REF], Boyd et al. 2012, Fourquez et al. 2015, 2020). Furthermore, we find that bacteria are unable to sustain high growth rates in the absence of luxury Fe uptake (Supp. Fig. 4). If phytoplankton were the only organisms undertaking luxury Fe uptake in our model, then phytoplankton would outcompete bacteria everywhere above the lowest light levels (2 W m -2 ) required to sustain phytoplankton growth, even at very high labile DOC levels (100 µM) as phytoplankton are able to rapidly consume and store any available Fe (Supp. Fig 4a -d). This highlights the importance of bacterial Fe storage in shaping the competition between phytoplankton and bacteria for Fe and is consistent with other approaches based on metalloproteins (Mazzatto et al. 2020).

Impact of covariance in Fe, PAR and DOC over the Southern Ocean seasonal cycle

There are two distinct regions within the Southern Ocean; the sub Antarctic zone (SAZ) and polar zone (PZ) which each encounter 4 different seasonal regimes; Winter (June -August), Spring (September -November), Summer (December -February), and Autumn (March -May). The availability of Fe, DOC and PAR is likely to vary between these 4 regimes and between the SAZ and PZ. We use satellite estimates of PAR (NASA Ocean Colour Data, VIIRS OBPG 2019) coupled with Fe fluxes from observations and modelling studies (Fig. 4) and the assumption that labile DOC exudation is ~10% of phytoplankton primary production, to initialise our model and examine the strength of phytoplankton-bacterial interactions in each seasonal regime.

In winter, deep mixed layers and ice cover results in complete darkness in the PZ, while PAR levels remain around 2 -7 W m -2 in the SAZ (Fig. 4). As the supply of deep Fe will be maximal at this time, dissolved Fe levels in seawater will be expected to reach their seasonal maxima of around 0.9 nM in the surface open ocean [START_REF] Tagliabue | Surfacewater iron supplies in the Southern Ocean sustained by deep winter mixing[END_REF]) and perhaps >1 nM closer to the shelf [START_REF] Tagliabue | A global compilation of dissolved iron measurements: focus on distributions and processes in the Southern Ocean[END_REF]. At this time of year, there is only a deep source of refractory DOC in large areas of the open ocean [START_REF] Hansell | Recalcitrant Dissolved Organic Carbon Fractions[END_REF]. Under these conditions, we find that phytoplankton and bacterial biomass are low in the SAZ where there is low light. Yet the very low biomass of phytoplankton take up ~98% of the Fe pool and deplete its concentration from 0.9 nM to 0.3 only after 35 days. Over this time, phytoplankton biomass can increase to from 0.1 to 6 µmol C L -1 but we observe no sustained increase in bacterial biomass despite a build-up of 1 µM of DOC in our experiment from phytoplankton exudation. Phytoplankton outcompeted bacteria because the low labile DOC at the onset of the experiment constrained bacterial growth, and instead allowed phytoplankton to drawdown Fe and moderately increase in biomass despite the apparent light limitation. To maintain seawater dissolved Fe levels of >0.7 nM throughout winter in the SAZ, either Fe must be continuously supplied via deep winter mixing [START_REF] Tagliabue | Surfacewater iron supplies in the Southern Ocean sustained by deep winter mixing[END_REF] or >50% of the particulate Fe in phytoplankton and bacteria needs to be rapidly recycled. In the PZ, despite the darkness, grazing by the resident zooplankton community (e.g. Antarctic krill, Walsh et al. 2020) under the winter ice can lead to the coupled release of Fe and DOC potentially driving competition between bacteria and low-light acclimated ice algae, a community of phytoplankton not modelled in this study.

In spring, PAR levels increase to 20 W m -2 in the PZ and 30 W m -2 in the SAZ (Fig. 4).

Shoaling of the mixed layer maintains high dissolved Fe stock of ~0.9 nM at the onset of spring [START_REF] Tagliabue | Surfacewater iron supplies in the Southern Ocean sustained by deep winter mixing[END_REF]). Initialising our model under these high Fe and light levels, coupled with the 1 µM of DOC that built up over the winter period, we find that phytoplankton outcompete bacteria at a ratio of 100:1 and consume >98% of the dissolved Fe Our analysis suggests that decreasing light and Fe, coupled with a small DOC inventory (5 µM) facilitates Fe uptake by bacteria, but microbial Fe uptake is decoupled from carbon production. Phytoplankton biomass is consistently greater than bacterial biomass across all seasons. Bacteria require >50 µM of labile DOC to outcompete phytoplankton biomass in spring. This decreases to 40 µM in summer, 15 µM in autumn and 8 µM in winter. Bacteria require less DOC as the season progresses because of the colimiting effects of Fe and light on phytoplankton growth. This suggests that sources such as ice, dust, sediments, recycling etc. with varying Fe and DOC levels may influence different components of the microbial community, with the strength of the interaction further compounded by seasonal effects on phytoplankton growth.

Competition under changing climate: uncertainties and future work

There is a strong interest in understanding the response of phytoplankton to a changing climate due to its role in the global carbon cycle and supporting ocean fisheries. Southern Ocean surface seawater is predicted to warm by 2 -3°C by the year 2100. Both phytoplankton and bacterial metabolism respond positively with an increase in temperature [START_REF] Rivkin | Microbial processes in cold oceans. I. Relationship between temperature and bacterial growth rate[END_REF]

Conclusions

Overall, we find that resource co-limitation and competition alters the response of phytoplankton and bacteria. Phytoplankton growth rate decreases due to competition for Fe uptake by bacteria. In contrast, the increase in DOC supplied via phytoplankton exudation leads to an increase in bacterial growth rate but is insufficient to stimulate bacterial biomass to an extent that bacteria outcompete phytoplankton. If additional DOC is supplied, for instance from recycling or sea ice melting, then bacteria can successfully outcompete phytoplankton for Fe and depress rates of phytoplankton productivity. As large areas of the Southern Ocean are characterised by low dissolved Fe-low labile DOC, the seasonal cycle of light and ensuing exudation, grazing, viral lysis and cell death which increases DOC inventory ultimately regulates phytoplankton-bacterial interactions. Carbon biomass relative to no competition simulation (a, d). Difference in biomass (black), growth rate (orange dash) and carbon production rate (orange solid) between phytoplankton and bacteria (b, e). Difference in Fe uptake rate between phytoplankton and bacteria (c, f).

All lines denote regions where phytoplankton and bacteria are equal and separate regions of phytoplankton (blue) and bacterial (red) dominance.

  al 1990, Møller et al. 2003, Møller 2007), viral lysis (Bratbak et al. 1993, Middleboe et al. 2002, Gobler et al. 2003, Poorvin et al. 2004) and phytoplankton cell mortality (Veldhuis et al. 2001) suggests that seasonal changes in phytoplankton production may facilitate phytoplankton and bacterial coexistence in the Southern Ocean by supplying labile DOC. While labile DOC is often rapidly utilised within hours (Fourquez et al. 2014, Obernosterer et al 2015), seasonal accumulation of DOC has been observed in the Bermuda Atlantic Time Series (BATS) site possibly due to phosphorus limitation of bacterioplankton growth (Cotner et al. 1997, Rivkin and Anderson 1997) or the accumulation of recalcitrant DOC that prevents rapid utilisation (Legendre and Le Fevre 1995, Carlson and Ducklow 1996, Carlson et al. 1998, Cherrier et al. 1996, Søndergaard et al. 2000). The turnover time for recalcitrant DOC ranges from months (semi-labile) -40,000 years (ultra-refractory) (Carlson and Ducklow 1995, Cherrier et al. 1996, Hansell et al. 2012, 2013, Lønborg et al. 2018). At a cellular level, phytoplankton can use mechanisms such as Fe storage protein (i.e. ferritin) and vacuolar storage that enable them to stockpile Fe for use when ambient Fe levels are low (e.g., Lampe et al. 2018, Marchetti et al. 2009, Cohen et al. 2018). A more limited set of observations indicates that although bacteria possess two types of ferritin-like molecules, the bacterial ferritins and bacterioferritin (Andrews et al. 2003, Rivera 2017, Debeljak et al. 2019), the regulation of the bacterioferritin gene is unclear (Carrondo 2003). Nevertheless, the higher Fe quotas observed in bacteria, relative to phytoplankton (Fourquez et al., 2015, 2020, Mazzotta et al. 2020, Tortell et al. 1996, Sarthou et al. 2008, Boyd et al. 2012) suggest that bacteria may require more Fe, or have the capacity for luxury Fe uptake, and hence may compete with phytoplankton for Fe. The parallel impact of other environmental regulatory factors, such as light or labile DOC, on the outcome of such a competition are unclear.

(

  Fe-PAR), with a fixed DOC level of 5 µM (broadly representative of typical labile DOC levels, Hansell et al. 2009), and varying Fe and DOC (Fe-DOC), with a fixed light level of 20 W m -2 (broadly representative of typical mixed layer average Southern Ocean light levels).

  Fig. 1a, b and 2a, b).

pool within 13 ( 2 (Fig. 4 )

 1324 PAR 30 W m -2 ) to 15 (PAR 20 W m -2 ) days because the low DOC at the onset of spring allows phytoplankton to outcompete bacteria. PAR continues to increase to the seasonal maximum of 30 W m -2 in the PZ and 40 W m -2 in the SAZ during summer, but dissolved Fe concentrations tend to decrease to ~0.15 nM in the surface open ocean as phytoplankton have depleted the winter stock and recycling turns over the dissolved Fe stock rapidly (Strzepek et al. 2005, Boyd et al. 2012, Tagliabue et al. 2014). Under low Fe and high light conditions, and assuming a labile DOC stock of ~5 µM in addition to continual DOC exudation (10% of primary production) (Hansell et al. 2009), phytoplankton take up ~80% of the ambient Fe and phytoplankton biomass ~16 times greater than bacteria. From late summer into autumn the mixed layer starts to deepen, PAR decreases from 20 to 0 W m -and Fe is at its seasonal minimum (~0.1 nM, Tagliabue et al. 2012). Here phytoplankton take up ~55% of the Fe and phytoplankton biomass is ~6 times greater than bacteria. The low standing stocks of Fe in spring/summer do not preclude fast recycling via bacterivory and herbivory coupled with rapid utilisation (Strzepek et al. 2005). Bacterivory and herbivory would lead to the coupled release of Fe and DOC. If we assume greater levels of Fe and labile DOC, that might be representative of the greater recycling (e.g. bacterivory; Strzepek et al. 2005), then the competitive fitness of bacteria is enhanced. Our results suggest that the amount of DOC necessary to switch the dominance from phytoplankton to bacteria varies as a non-linear function of Fe (black line in Fig 3e). At the low Fe levels typical of summer as little as 20-30 µM DOC would alter the competitive outcome, while at higher Fe levels typical of earlier in the bloom phase more DOC is needed (Fig 3e).

  , Pomeroy and Wiebe 2001, Deppeler and Davidson 2016, Toseland et al. 2013). But there is substantial disagreement on how temperature could regulate phytoplankton-bacterial interactions and DOC production and stimulation. Bacterial production could follow an increase in primary production and not temperature (Kirchman et al. 1995, Morán et al. 2006) or respond to an increase in temperature and not primary production (Hoppe et al. 2008). Our analysis finds that bacteria can outcompete phytoplankton leading to a decrease in phytoplankton biomass if sufficient labile DOC is available which can be found in regions of high recycling (Ruiz-Halpern et al. 2011, Arístegui et al. 2014) and ice melt (Norman et al. 2010, Underwood et al. 2010), where there is also concurrent release of Fe, a concept that has previously not been considered in our understanding of the Fe cycle in the Southern Ocean (Tagliabue et al., 2014). As ice melt increases with rising temperatures, the increase in Fe and DOC supply coupled with an increase in temperature may stimulate bacterial communities at the expense of phytoplankton. Furthermore, increased temperature can lead to increased DOC exudation by phytoplankton (Zlotnik and Dubinsky 1989) and greater microbial degradation of recalcitrant DOC (Lønborg et al. 2018), which suggests the potential for greater DOC availability under warmer climate. However, ice melt can strongly stratify the surface water, thereby relieving phytoplankton light limitation and strengthening competition with bacteria. Additional feedbacks, not explored in this study, are the interactions around the bacterial production of siderophores in response to Fe scarcity (e.g. Boiteau et al., 2016) and the weaker Fe binding agents associated with phytoplankton exudates, such as saccharides (Hassler et al., 2011) or heme groups (Louropoulou et al., 2020), or the role of bacterivory in recycling the Fe stored in bacteria (Tortell et al. 1996, Strzepek et al. 2005, Boyd et al. 2012, Boyd et al. 2015, Mazzotta et al. 2020). Furthermore, along the coastal Antarctic sea-ice edge communities, phytoplankton are limited by cobalamin (vitamin B 12 ) and Fe (Bertrand et al. 2015). As cobalamin is only produced by bacteria and archaea, this suggest that phytoplankton-bacterial interactions mediate micronutrient colimitation for nutrients other than Fe as well. As such, more experimental-modelling work is required to gain a better mechanistic understanding of how the competitive interactions between phytoplankton and bacteria could change in response to multiple concurrent environmental changes.

Figure 1 :Figure 2 :Figure 3 :

 123 Figure 1: Schematic of model configuration with 2 biological pools -phytoplankton and bacteria, and 3 resources -dissolved iron (Fe), light and dissolved organic carbon (DOC). Fe and DOC are taken up by phytoplankton and bacteria (solid lines) and recycled via mortality (dashed lines) and phytoplankton exudation (dotted).

Figure 4 :

 4 Figure 4: Top panel represents dissolved iron (Fe) fluxes from observations and modelling studies. Bottom panel demonstrates average mixed layer depth integrated light (PAR; based on VIIRS Ocean Colour Data, Ocean Biology Processing Group 2019 and de Boyer Montegut 2004) for the subAntarctic (solid line) and polar (dashed line) zones and assumptions of labile dissolved organic carbon (DOC) available for bacteria used to initialise the model. Highlighted rectangular regions represent austral spring and autumn.

Acknowledgements

This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (Grant agreement No. 724289). The model used in this study is based on the global 3D biogeochemical model PISCES as described in Aumont et al. 2015 and freely available as part of the NEMO modelling platform. The NEMO code is available from https://www.nemo-ocean.eu. The equations and parameters used in this study are provided in the supplementary material. Mixed layer depth data was sourced from deBoyer-Montegut et al. 2004 ( http://www.ifremer.fr/cerweb/deboyer/mld/Surface_Mixed_Layer_Depth.php) and PAR was sourced from NASA Ocean Color (VIIRS monthly climatological data at 4km resolution https://oceancolor.gsfc.nasa.gov/l3/).