William De

Rezende Locatel

Chetna Mohabeer

Dorothée Laurenti

Yves Schuurman

Nolven Guilhaume
email: nolven.guilhaume@ircelyon.univ-lyon1.fr

Co-pyrolysis of beech wood and polyamide-6 : Impact of plastic concentration and wood/plastic synergistic effects

Keywords: Biomass pyrolysis, polyamide pyrolysis, co-pyrolysis, Bio-oils, Catalytic upgrading

Biomass residues and waste can be converted into bioliquids by pyrolysis, which can contribute to both the production of renewable fuels and the recycling of waste. In this paper, the catalytic copyrolysis of mixtures of beech wood (BW) and polyamide-6 (PA6) in different ratios was investigated to assess the effect of PA6 concentration on the properties of bio-oils and on the catalyst performances in upgrading the bio-oils. The catalytic treatment was applied to the pyrolysis vapors that were driven across a H-ZSM-5 catalyst bed before their condensation into liquid products. Increasing the PA6 concentration in the BW+PA6 blends led to an increase in the average molar mass of bio-oils and to an increase in their N-content. Caprolactam and 5-cyano-1pentene were the main products derived from PA6 pyrolysis, and their ratio also changed with the PA6 concentration. The production of fully deoxygenated compounds was strongly reduced in the presence of PA6, as the result of catalyst deactivation, also evidenced by the increase in sugars and decrease in furans concentrations. The amount of coke deposited on the catalyst decreased as the PA6 concentration increased, but this coke contained more nitrogen. Synergistic effects in the BW+PA6 pyrolysis were evidenced by the positive or negative deviation of different parameters (mass of gas, liquid, solid products, mass of coke on the catalyst, C, N, O composition of bio-oils) from a simple arithmetic combination of the same parameters for pure BW or pure PA6 weighted by their concentrations, in particular the N-content in the bio-oils was much higher than expected.

Introduction

Co-pyrolysis of wood and plastics could be considered in the near future to recycle waste biomass, in particular municipal waste and construction wood, which generally also contain plastics. Among these plastics, N-or Cl-containing plastics such as polyamide or polyvinylchloride have been little studied and should also be considered, given their wide use in many applications, but also because waste biomass contains various types of plastics that cannot be easily sorted and separated according to their composition. In a previous study [START_REF] De Rezende Locatel | Copyrolysis of beech wood and polyamide-6: Effect of HZSM-5 catalyst on the properties of pyrolysis oils[END_REF], the copyrolysis of beech wood (BW) and polyamide-6 (PA6) chips was investigated and the effect of ex-situ treatment of the pyrolysis vapors using a HZSM-5 catalyst was assessed. However, the biomass/plastic ratio is also an important parameter that affects the pyrolysis process and the products quality [START_REF] Zhang | Catalytic co-pyrolysis of lignocellulosic biomass with polymers: a critical review[END_REF], through interactions between the wood chips and the plastic particles, or among their degradation products, that can lead to synergistic effects.

Most of the studies dealing with synergistic effects in co-pyrolysis were carried out using polyolefin-based polymers like PP and PE, or polyaromatics like PS, which do not contain heteroatoms (Cl, N) that could affect the pyrolysis process and impair the catalyst performances, when a catalytic treatment is applied. For instance, aiming to identify the mechanisms responsible for this synergistic effect, Cheng and Huber [START_REF] Cheng | Production of targeted aromatics by using Diels-Alder classes of reactions with furans and olefins over ZSM-5[END_REF] studied the conversion of furanic compounds (furan, 2-methylfuran, furfural, and furfuryl alcohol) and olefins (ethylene and propylene) over ZSM-5 in a fixed-bed reactor operating between 450°C and 600°C. An increase in aromatics selectivity was observed when propylene was added to the feed, due to the occurrence of Diels-Alder reactions. The authors also reported that furfural and furfuryl alcohol underwent decarbonylation to produce furan and CO. Finally, a decrease in coke and CO2 yields was observed, suggesting that the presence of hydrogen-rich reactants such as olefins modified the reaction pathways for oxygen removal, shifting from decarbonylation and decarboxylation to dehydration.

Liu and coworkers [START_REF] Liu | On deconvolution for understanding synergistic effects in co-pyrolysis of pinewood and polypropylene[END_REF] also evaluated the synergistic effects observed during co-pyrolysis of pinewood and polypropylene carried out without a catalyst in a fixed-bed reactor at 900°C. The authors reported an increase in syngas (H2 and CO) production and a decrease in the formation of non-condensable hydrocarbons and in chars. These effects were assigned to the interaction between oxygen-containing radicals in the volatiles evolved from pinewood with hydrogen-rich radicals from the polymer that act as H-donor species, which promoted pinewood volatilization and inhibited the condensation and repolymerization reactions, hence reducing the char formation.

Li et al. [START_REF] Li | Enhancing the production of renewable petrochemicals by co-feeding of biomass with plastics in catalytic fast pyrolysis with ZSM-5 zeolites[END_REF] studied the catalytic fast pyrolysis (CFP) of cellulose and low-density polyethylene (LDPE), polypropylene (PP) and polystyrene (PS) employing HZSM-5 as catalyst. Among the plastics studied, LPDE showed a higher synergistic effect, with a significant increase in aromatic yield and a reduction in coke formation. These effects were ascribed to interactions between volatiles from plastic and biomass, as discussed previously. However, when the reaction was performed with PP, the synergistic effect was smaller than that observed with LDPE. The explanation relies on the higher concentration of branched olefins obtained with PP when compared to LDPE. The electronic phenomena and steric hindrance of their branching prevented the Diels-Alder reactions essential to the formation of aromatic compounds. Finally, the copyrolysis with PS showed the lowest synergistic effect, with insignificant interactions between PS and cellulose; however, an increase in aromatics yield was observed, mainly due to the aromatic nature of PS itself that depolymerizes into styrene and polyaromatic species.

Ghorbannezhad et al. [START_REF] Ghorbannezhad | Co-pyrolysis of biomass and plastic waste over zeolite-and sodium-based catalysts for enhanced yields of hydrocarbon products[END_REF] evaluated the co-pyrolysis of sugarcane bagasse pith and polyethylene terephthalate (PET) in a tandem micro-reactor at 700 °C using HZSM-5 and sodium carbonate/galumina as catalysts. The main results were an increase in aromatics formation (especially benzene, toluene, xylene, and ethylbenzene -BTXE) and a reduction in coke deposition in the presence of the plastic. The hypothesis proposed for these observations was the occurrence of H-transfer from PET-derived aliphatics towards lignin-derived oxygenated compounds, contributing to the hydrocarbon pool pathway and avoiding cross-linking reactions that result in char formation.

However, the decomposition of PET also resulted in the formation of benzoic acid, which is detrimental to bio-oil quality [START_REF] Diaz-Silvarrey | Benzoic acid recovery via waste poly(ethylene terephthalate) (PET) catalytic pyrolysis using sulphated zirconia catalyst[END_REF].

As our current situation stands, the concentrations of polyamide-based plastics in municipal solid waste (MSW) are expected to increase in the next years due to the COVID-19 pandemic and the massive utilization of personal protective equipment, notably disposable masks that are made from woven polyamide tissues. Therefore, this study focuses on co-pyrolysis of beech wood (BW) and polyamide-6 (PA6) at concentrations between 5 and 20 wt.% (close to those found in municipal solid waste [START_REF] Areeprasert | Municipal Plastic Waste Composition Study at Transfer Station of Bangkok and Possibility of its Energy Recovery by Pyrolysis[END_REF][START_REF] Silva | An urgent call to think globally and act locally on landfill disposable plastics under and after covid-19 pandemic: Pollution prevention and technological (Bio) remediation solutions[END_REF], aiming at understanding the synergistic effects between them and the chemical processes involved. All co-pyrolysis experiments were carried out using HZSM-5 as catalyst for ex-situ treatment of the pyrolysis vapors. In combination with a previous publication on this topic [START_REF] De Rezende Locatel | Copyrolysis of beech wood and polyamide-6: Effect of HZSM-5 catalyst on the properties of pyrolysis oils[END_REF] that assessed more specifically the role of the HZSM-5 catalyst, the present study covers the most important aspects related to the co-pyrolysis of lignocellulosic biomass and polyamide, which are scarce in the literature. In addition, this work presents an extensive range of characterizations of the liquid, gaseous and solid products obtained by co-pyrolysis, as well as the characterization of fresh and spent catalysts.

Experimental section

Feedstocks

The lignocellulosic biomass used consists of beech wood chips supplied by J. Rettenmaier & Söhne; they were sieved to obtain particle sizes I, the range from 0.75 to 1.7 mm. Polyamide-6 (BASF -Ultramid® B) was supplied as pellets, which were crushed and sieved before use to obtain particle sizes in the same range as the wood chips. BW chips were mechanically mixed with 5, 10 and 20 wt.% of PA6 (on dry basis). All feedstock samples were dried in an oven at 110°C for 1h before each pyrolysis experiment and stored in airtight flasks.

Catalyst

Zeolite HZSM-5 with a molar Si/Al ratio of 40 was supplied by Céramiques Techniques Industrielles (CTI) Salindres, France. The catalyst characterization has been described previously [START_REF] De Rezende Locatel | Can Paints and Varnish Impair the Physicochemical Properties of Wood Pyrolysis Oils?[END_REF] and its main properties are summarized in Table 1.

Table 1 -Properties of HZSM-5 catalyst

Property Value

Particle size (µm) 100-300

Water content a (% wt.) 3.9

Si/Al atomic ratio b (SAR) 40.5

Total SSA c (m 2 g -1 )

Microporous surface (m 2 g -1 )

External surface (m

Pyrolysis set-up

Co-pyrolysis of beech wood and PA6 was carried-out in a lab-scale pyrolytic bench already described [START_REF] De Rezende Locatel | Ex-situ catalytic upgrading of pyrolysis vapors using mixed metal oxides[END_REF], with a few modifications to adapt the system to the mixed wood/plastic feedstocks (Figure 1). The jacketed reactor has an internal zone where the thermal pyrolysis is carried out and a bed of catalyst can be placed (or not) just below to treat the vapors which come out of the pyrolysis zone. The BW chips or BW+PA6 blends were previously dried at 110°C for 1.5 h before being placed in the gas-tight hopper and kept under N2 atmosphere.

Unlike the previous description [START_REF] De Rezende Locatel | Ex-situ catalytic upgrading of pyrolysis vapors using mixed metal oxides[END_REF], the new inner reactor (pyrolysis zone) was a straight quartz tube, in which the solid chips were fed semi-continuously from the top for 1 h, by small increments of ≈0.045 g every 10 s, and fell down by gravity onto a quartz porous frit at the bottom of this section, where the chars and solid residues were retained. The total mass of solid chips fed in the reactor was ≈16 g in 1 h of experiment. The temperature in the pyrolysis zone was 450-500°C along the tube and 480°C on the porous frit where the wood chips were stopped. The pyrolysis vapors (formed by thermal pyrolysis) were entrained downstream by a nitrogen flow of 260 smL/min (flow rate standardized at 0°C and 1 atm) and passed through the catalyst bed placed just below the pyrolysis zone on a second quartz frit. All pyrolysis experiments were carried out with ex-situ catalytic treatment of the vapors using the HZSM-5 catalyst described previously. The temperature of the catalyst bed was 453 ± 2°C. In this external part of the reactor, an additional gas flow composed of 240 smL/min N2 and 20 smL/min He (used as internal standard for GC analysis) was added to prevent retro-diffusion of the vapors and to adjust the total gas flow rate.

The residence time of the vapors in the pyrolysis zone was estimated at 41 s, while the contact time with the catalyst (when present) was around 2 s. The pressure in the reactor was continuously monitored by a pressure sensor connected to a safety device which cuts off the experiment automatically if the pressure exceeds 1.3 bars. The pressure measured during the experiments was around 1.1 bar.

Figure 1 -Pyrolysis setup scheme

The vapors were condensed downstream the reactor and collected in a series of traps including a condenser (1) and a round bottom flask (2) both cooled at -5°C, an electrostatic precipitator (3), a trap filled with glass beads at -5°C (4) and a trap at ambient temperature filled with silica gel beads [START_REF] Li | Enhancing the production of renewable petrochemicals by co-feeding of biomass with plastics in catalytic fast pyrolysis with ZSM-5 zeolites[END_REF], that prevented any condensable compound from being introduced in the micro-GC for online gas phase analysis. The total mass of bio-oils formed in the experiments was assessed by mass difference, by weighing the different glassware parts before and after the experiments.

Prior to starting the reaction, the reactor was loaded with 2 g of catalyst and heated to 500 °C under a N2 flow for 2 h to ensure fully stabilized temperatures in the whole system (reactor and cold traps) and to desorb the water adsorbed on the catalyst. 7

Products recovery

Non-condensable gaseous products (hereinafter called "gas phase") were analyzed and quantified online using a micro-GC.

The fractions called "aqueous phase" and "organic phase" were recovered directly in the round bottom flask and simply separated by centrifugation; thus, they constituted the most representative share of the bio-oil produced in the process. Other fractions of bio-oils were also deposited in the downstream traps and remained stuck on the glassware parts. In order to obtain a detailed mass balance, these fractions were recovered by washing the glassware with methanol, followed by evaporation of the methanol at 50°C under a pressure of 400 mbar, yielding the "recovered organic phase". However, the evaporation step was found to modify the composition of this bio-oil fraction due to simultaneous evaporation of volatile components. In addition, 13 C-NMR analysis showed the presence of residual methanol in this fraction. Therefore, it was decided not to mix the "recovered organic phase" with the native "organic phase". Finally, a part of the bio-oil could not be recovered because it was adsorbed on the silicagel beads or lost during the different steps of recovering the other fractions. This fraction was called "organic not recovered" and was quantified by weighing the glassware parts before and after experiment.

The chars were recovered from the first quartz frit, weighed and are hereinafter called "solid phase".

Product characterization

Gaseous products were analyzed online using a micro gas chromatograph Inficon 3000 (SRA instruments) equipped with three analytical modules and TCD detectors, calibrated with appropriate compound mixtures. A MolSieve column equipped with a Plot U pre-column was used to analyze He, H2 and CO; a Plot U column was used to analyze CO2, C2H4 and C2H6; an alumina column was used to analyze the remaining hydrocarbons (C3-C5). The duration of each analysis was 6 minutes and the analyses were repeated over 90 minutes. Helium was used as internal standard to account for the variations of total gas flow rates at the reactor inlet and outlet.

Elemental compositions (CHNS) were obtained with a Flash2000 (Thermo Scientific) equipment. For each analysis, a known mass of sample (1-3 mg) was placed in a tin crucible and each measurement was repeated three times to determine the measurement uncertainty.

The HHV was calculated from the elemental analysis using the equation proposed by Channiwala and Parikh [START_REF] Channiwala | A unified correlation for estimating HHV of solid, liquid and gaseous fuels[END_REF].

HHV = 0.3491[C] + 1.1783[H] -0.1034[O] -0.0151[N]
In this equation the HHV is expressed in MJ/kg and [C], [H], [N] and [O] in mass percentages on dry basis.

The water content of bio-oils was determined by Karl-Fischer (KF) titration using a Metrohm Titrando 852 analyzer. The samples were diluted in THF at a concentration of 10 wt.% and the volumetric method was employed. To avoid side reactions of aldehydes and ketones with traditional KF reagents, which produce water, specific reagents for samples containing these organic compounds were chosen: Karl Fischer ROTI hydroquant C5K (5 mg of water/mL of solution) and Karl Fischer ROTI hydroquant working medium K. The calibration was carried out using a solution of 10 wt.% of H2O in THF.

Molar mass distributions were obtained by gel permeation chromatography (GPC) using an Agilent equipment (1200 series) equipped with two PL gel columns with different pore sizes (500 and 50 Å) using THF as the mobile phase (1 mL/min). Two detectors were used, a refractive index detector (RID) and diode-array detector (DAD), collecting data each 0.4 s in the range of 200-450 nm with a resolution of 2 nm. The samples were prepared with concentration of 1 wt.% in THF and filtered before analysis (PTFE -0.45µm). The measurements were carried out at 35°C and the calibration was performed using hydrocarbons standards (molecular weight between 86 and 1000 g/mol). 13 C nuclear magnetic resonance ( 13 C-NMR) spectra were recorded using a Bruker AVANCE 400 MHz spectrometer and the spectra were analyzed with the TopSpin 3.0 software. The analyses were carried out at room temperature with an accumulation of 4500 scans. Trioxane was used as internal standard, diluted in DMSO-D6 in a coaxial tube configuration that prevented the contact of the solvent with the bio-oil samples.

Gas chromatography coupled with a mass spectrometer was performed with a GCMS-QP2010 SE apparatus (Shimadzu) equipped with a Zebron ZB Wax-Plus column (length 60 m, internal diameter 0.25 mm, film thickness 0.25 µm). The samples were diluted in methanol (20 wt.% for organic phase and 30 wt.% for aqueous phase) and filtered before the analysis (Cellulose acetate -0.45µm). The oven temperature program started at 70 °C (held for 10 min), then was heated at a rate of 5°C/min up to 150 °C. At this point the heating rate changed to 3°C/min up to 250 °C (held for 15 min).

Two-dimensional gas chromatography coupled with mass spectrometry (GCxGC-MS) was carried out with an Agilent GC 6890N apparatus. The first separation was performed by a polar column VF-1701 MS (I.D 0.25 mm; film thickness 0.25 µm; length 30 m) followed by a second apolar column DB1 (I.D 0.25 mm; film thickness 0.25µm; length 1.5m) for the second dimension of separation. The modulation was performed by a cryogenic modulator (Zoex Corporation USA) each 12s. The compounds were identified by the coupled MS detector (5975B). The use of a first polar column followed by a less polar column favored the separation of oxygen and nitrogencontaining compounds, widely present in bio-oils [START_REF] Omais | Investigating comprehensive two-dimensional gas chromatography conditions to optimize the separation of oxygenated compounds in a direct coal liquefaction middle distillate[END_REF]. The temperature program was set as follows: first column from at 50°C to 300 °C with a ramp of 1.75 °C/min, then to 320°C with a ramp of 1.8 °C/min. The second column went from 50 °C to 320°C at 1.8 °C/min. The samples concentration was the same as described for GC-MS analyses.

The spent catalyst (recovered after reaction) was analyzed by thermogravimetric analysis (TGA) under air (40 mL/min) using a TGA/DSC-1-Star e system (Mettler Toledo) equipped with a gas controller GC200. The temperature was initially held at 25°C for 5 minutes, then increased up to 900°C with a heat-up ramp of 10°C/min.

Results and discussion

Co-pyrolysis of BW and PA6 and products characterization

The selectivity for gas, liquid and solid products is displayed in Figure 2. The mass balance obtained for all reactions was ≥ 95% and generally close to 100%, and the liquid products were the main fraction collected, including when pure PA6 was used. The increase in PA6 concentration led to a decrease in the production of gaseous products associated with an increase in the selectivity in liquid products, essentially the organic phase. With pure PA6 there was no phase separation of the liquid fraction, which consists only of an organic phase. Since PA6 does not contain any water or hydroxyl groups its degradation is less likely to yield water than that of biomass. PA6 also yielded more solid products than BW+PA6 blends, but the solid residues included a significant part of white polymer particles, collected downstream the reactor, probably formed by re-condensation of PA6-derived monomers or oligomers.

Figure 3 shows the cumulative production of all non-condensable products, normalized by unit mass of feedstock. Increasing the concentration of PA6 in BW+PA6 blends led to a decrease in the production of gases, and the pyrolysis of pure PA6 produced the fewest gaseous products, indicating that, unlike wood, PA6 and its pyrolysis products cracked only marginally into light compounds. The composition of the gas phase is detailed in Figure 4. The increase in plastic concentration led to a decrease in COx and C2-C3 olefins production, and to a small but significant increase in light paraffins production (C2-6 paraf ), while less olefins (ethylene and propene) are formed. The changes in CH4 and H2 production from BW and BW+PA6 were statistically insignificant, but pure PA6 produced almost no methane or hydrogen, which suggests that they are essentially produced from the wood. Finally, H2 was a minor product in all experiments in terms of mass, but also in terms of moles. The increase in PA6 concentration resulted in an increase in the water content in the organic phase, and to the opposite in the aqueous phase (not present with pure PA6). This could be related to the fact that caprolactam, which is the main product of PA6 degradation as will be shown later, is hygroscopic [START_REF] Fisher | Caprolactam[END_REF]: when the content of PA6 increased, there was more caprolactam in the organic phase, which could promote water solubility in this phase. The organic phase obtained from pure BW contained ≈0.5 wt.% N and the aqueous phase ≈2 wt.%. Using BW/PA6 blends, the N-content in both phases increased regularly with PA6 concentration, which suggests that N-containing compounds derived from PA6 are shared out in both organic and aqueous phases. PA6 also contains oxygen in the form of a carbonyl group and oxygen accounts for ≈14 wt.% of its molar mass, while beech wood contains ≈44 wt.% O. Therefore, the oxygen content in the liquid fractions originated predominantly from the biomass. Accordingly, reducing the BW fraction in the feedstock led to a lower oxygen content in the organic phase (Table 2), the HHV increased with the percentage of PA6 and the organic phase obtained from pure PA6 showed the highest HHV.

A number of studies reviewed by Ansari et al. [START_REF] Ansari | Co-pyrolysis of biomass and plastic wastes: A review on reactants synergy, catalyst impact, process parameter, hydrocarbon fuel potential, COVID-19[END_REF] reported that the co-pyrolysis of biomass with various types of plastics results in bio-oils with higher H/C ratios, the plastic behaving as hydrogen-donor for intermolecular H-transfer. In the present study, the Van Krevelen diagram (Figure 5) indeed showed a slight increase in the H/C ratio for organic phases produced from BW+PA6 blends, but the difference was within the margin of error.

Figure 5 -Van Krevelen diagrams of liquid organic phases compared to solid beech wood

The N/C ratio increased strongly with PA6 concentration, but this increase was not proportional to the concentration of PA6 in the BW+PA6 blends. This trend will be discussed in paragraph 3.3. 36%). These compounds are related to biological functions and metabolic processes in biomass. Adding 5 wt.% of PA6 to BW led to 13 % of N-containing compounds (relative to the total peaks area), and within these 55% was caprolactam, 35% 5-cyano-1-pentene and the remaining 9% other N-containing compounds. The complete list of analyzed compounds can be found in the supplementary information. In all experiments in the presence of PA6, the main N-products were caprolactam and 5-cyano-1-pentene. However, the 5-cyano-1-pentene/caprolactam ratio decreased non-linearly with the PA6 concentration, suggesting a competition between the reaction pathways leading to their formation.

GPC analyses with RI detector provided the molar mass distribution of the compounds present in the different organic phases (Figure 7). At low PA6 concentrations (5 and 10 wt.%), the GPC profiles were very similar to the one obtained with pure BW, with a maximum intensity at ≈140 g/mol, and the MWs below 300 g/mol represented ≈70% of the total area. The organic phase obtained from pure PA6 exhibited a very different profile, with a maximum intensity found at 185 g/mol and the area in the 160-500 g/mol range representing 86% of the total area. With the BW+ 20% PA6 blend, the GPC profile was a combination of the two previous profiles and the formation of heavier compounds was clearly revealed. These modifications are translated in the average molar mass variations reported in Table 3. GPC analyses were also carried out using UV/DAD in order to obtain specific information regarding the unsaturated compounds present in the bio-oils (Figure 8). For comparison, only the results obtained with 100% BW, with BW +20 % PA6 and with 100 % PA6 are shown. In all cases, the profiles were centered at MWs in the range of 150-200 g/mol and a wavelength close to 267 nm. Absorption in this region is assigned to compounds with conjugated double bond systems such as aromatics, and the peak shifts towards larger wavelengths when the size of the conjugated system increases (e.g. benzene -peak at 255 nm; naphthalene -286 nm; anthracene -375 nm) [START_REF]UV talk latter[END_REF]. The pyrolysis of lignocellulosic biomass produces many aromatic compounds such as guaiacols, syringols, phenols and aromatic ethers. This production was marginal using pure PA6, as evidenced by the intensity close to zero. It was also strongly reduced using the BW+20% PA6 blend, but not proportionally to the concentration of PA6 since the maximum peak intensity decreased by more than 50%.

13 C-NMR analyses depicted in Figure 9 show the different functional groups present in the organic phases. The content of aromatic compounds decreased when the concentration of PA6 increased, in agreement with the GPC-DAD analyses shown previously. It should be mentioned that the chemical shift assigned to nitriles (range 110-130 ppm, e.g.: 5-cyano-1-pentene) is similar to that assigned to aromatic C-H (dark yellow). Therefore, the reduction in aromatic groups was probably underestimated in this analysis. Another case of superposition of chemical shifts is related to the carboxylic acid and amide groups (green with squares): in this case, this concentration growth should be associated mainly to the presence of caprolactam, which was the main degradation product of PA6. Consistently, the amount of aliphatic carbon units increased with the concentration of PA6. More detailed information about the composition of the organic phases is given by the GC-MS analyses presented in Figure 10.

Figure 10 -Relative areas of compounds families identified by GC-MS for the different organic phases

As expected, the concentration of nitrogen-containing compounds grew with the increase in plastic concentration, from 4% with pure beech wood to 96% with pure plastic. Concurrently, there was a progressive reduction in the production of aromatic compounds originating from the biomass, such as guaiacol, syringol and phenols. Moreover, the presence of PA6, even at low concentration (5 wt.%), resulted in a significant drop in the amount of deoxygenated aromatic compounds (i.e. PAH and aromatics). These results confirmed that the catalytic activity was impaired in its oxygen-removal function in the presence of PA6. This was corroborated by the sharp increase in sugar concentration together with a decrease in furans (product of sugars dehydration) when the plastic concentration reached 20 %. Figure 11 shows the group of compounds obtained with GCxGC-MS analyses. Figure 11 -Relative blob volumes of compound families identified by GCxGC-MS in the organic phases

These results fully support the trends observed with the previous techniques. The selectivity for nitrogen-containing compounds increased proportionally with the concentration of plastic, whereas that for aromatic oxygenated compounds (i.e. guaiacols, syringols, phenols…) decreased.

Finally, the groups of fully deoxygenated compounds (non-oxygenated aromatics and polycyclics)

were strongly reduced in the presence of plastic. The evolution of peaks assigned to caprolactam and 5-cyano-1-pentene were evidenced in the 3D projections of chromatograms shown in Figure 12.

Characterization of spent catalysts

The spent catalysts were analyzed to determine the coke composition (CHN) and the amounts of deposited coke were quantified using thermogravimetric analysis (TGA) performed under air to burn the carbon deposits. The results are presented in Figure 13. For experiments performed in the presence of BW and BW+PA6 blends, an initial mass loss around 1.5 wt.% was observed between 25 and 200°C. It can be attributed to the desorption of water or light compounds adsorbed in the catalyst pores, since the solids were not pre-heated before TGA. The catalyst obtained after the pyrolysis of pure PA6 showed a more significant mass loss in this region (2.5%). This could indicate that N-containing compounds were also adsorbed on the catalytic sites, corroborating the hypothesis of a partial deactivation of HZSM-5. This is also consistent with the increase in liquid products and decrease in gas products with the amount of PA6, shown in Fig. 2, which could indicate a lower cracking activity of the catalyst. The deactivation of acid catalytic sites by nitrogen poisoning is well documented [START_REF] Lucas | Improving deactivation behaviour of HZSM-5 catalysts[END_REF][START_REF] Takahashi | Effects of acid strength and micro pore size on εcaprolactam selectivity and catalyst deactivation in vapor phase Beckmann rearrangement over acid solid catalysts[END_REF][START_REF] Van Grieken | Nitrogen and sulphur poisoning in alkene oligomerization over mesostructured aluminosilicates (Al-MTS, Al-MCM-41) and nanocrystalline n-HZM-5[END_REF] and even at low concentration (<0.1%), the presence of N-containing compounds may cause a deactivation of zeolites, as reported by Caeiro et al. [START_REF] Caeiro | A FT-IR study of deactivation phenomena during methylcyclohexane transformation on H-USY zeolites: Nitrogen poisoning, coke formation, and acidity-activity correlations[END_REF].

The mass loss between 300 and 900°C was assigned to coke combustion, which took place in a simple one-step profile at similar temperatures, indicating that the carbon deposits were rather homogeneous in their structure. The amount of coke deposited on the catalyst decreased when the PA6 concentration increased; however, the experiment using pure PA6 yielded almost 9 wt.% of coke on the catalyst. The increase in the PA6 amount in the blend also led to a steady rise in the N content in the spent catalyst, although not proportional to PA6 concentration.

Several studies reported the beneficial effect of co-pyrolysis with some plastics (notably olefinderived plastics such as polyethylene and polypropylene) in reducing the catalyst deactivation by coke [START_REF] Ghorbannezhad | Co-pyrolysis of biomass and plastic waste over zeolite-and sodium-based catalysts for enhanced yields of hydrocarbon products[END_REF][START_REF] Lucas | Improving deactivation behaviour of HZSM-5 catalysts[END_REF][START_REF] Takahashi | Effects of acid strength and micro pore size on εcaprolactam selectivity and catalyst deactivation in vapor phase Beckmann rearrangement over acid solid catalysts[END_REF][START_REF] Van Grieken | Nitrogen and sulphur poisoning in alkene oligomerization over mesostructured aluminosilicates (Al-MTS, Al-MCM-41) and nanocrystalline n-HZM-5[END_REF][START_REF] Caeiro | A FT-IR study of deactivation phenomena during methylcyclohexane transformation on H-USY zeolites: Nitrogen poisoning, coke formation, and acidity-activity correlations[END_REF][START_REF] Kim | Catalytic co-pyrolysis of torrefied yellow poplar and high-density polyethylene using microporous HZSM-5 and mesoporous Al-MCM-41 catalysts[END_REF]. The explanation relies on the high H content in the plastics that promotes the formation of liquid products, and on the low amounts of aromatic compounds formed during their degradation, which limits the condensation/polymerization reactions of these species. The effect of plastic, however, should be assessed by comparing the effective coke yields with the calculated arithmetic sum of coke yields expected on the basis of a proportional contribution of coke originating from BW and PA6, respectively. This will be described in the next section.

Synergistic effects

The effect of adding PA6 to BW on the pyrolysis products yields and properties has been assessed by comparing the calculated value of each property, based on an arithmetic contribution of the values obtained using pure BW and pure PA6 (in wt. %), with the value of this property effectively measured, using the following equation [START_REF] Wang | Co-pyrolysis of waste plastic and solid biomass for synergistic production of biofuels and chemicals-A review[END_REF]:

𝑌 !"#$%&#!'()*+,-./0) = 𝛼𝑌 !23()*) + (1 -𝛼)𝑌 !23(-.0) (1)
where 𝑌 !23()*) is the experimental value of the parameter evaluated (e.g. phase selectivity, N concentration, coke content…) obtained with pure BW, 𝑌 !23(-.0) is the experimental value obtained with pure PA6 and 𝛼 is the concentration of plastic in the blend.

The synergistic impact (∆𝑌) was evaluated using the following equation [START_REF] Lu | Synergistic effects on char and oil produced by the co-pyrolysis of pine wood, polyethylene and polyvinyl chloride[END_REF]:

∆𝑌 = 4 !"#(%&'()*+) /4 !-./01.!2(%&'()*+) 4 !-./01.!2(%&'()*+) × 100% (2)
where 𝑌 !23()*+,-.0) is the experimental value obtained from the co-pyrolysis of feedstock containing 𝛼% of plastic and 𝑌 !"#$%&#!'()*+,-.0) was calculated using Eq.( 1). The evaluation of the synergistic impact for different parameters of the pyrolysis process is presented in Table 4. The co-pyrolysis of PA6 and BW resulted in a gas phase selectivity below what was expected.

According to the literature [START_REF] Alam | Catalytic co-pyrolysis of wet-torrefied bamboo sawdust and plastic over the zeolite H-ZSM-5: Synergistic effects and kinetics[END_REF][START_REF] Xue | Catalytic co-pyrolysis of biomass and polyethylene in a tandem micropyrolyzer[END_REF], the melting temperature of PA6 is between 170°C and 260°C.

At the inlet of the heated zone, the plastic starts to melt and it may partially cover the wood particles, which could hinder the evolution of gaseous products from the wood. Besides, it can modify the heat conductivity of wood chips and delay the evolution of gases [START_REF] Liu | On deconvolution for understanding synergistic effects in co-pyrolysis of pinewood and polypropylene[END_REF].

Conversely, the production of liquid products benefited from a positive synergistic effect, with the experimental selectivity for condensable products higher than the calculated value. The blend of BW with PA6 chips could have prevented the premature agglomeration of the melted polymer, as observed in the experiment with pure PA6, and which interferes with heat transfer and with the cracking of the polymeric chain. Another evidence of this agglomeration was provided by the aspect and consistency of the chars obtained after pyrolysis: using pure BW, the char consisted of blackened but well separated wood particles, while the chars obtained from co-pyrolysis of BW+PA6 blends consisted of aggregated particles, with a certain mechanical resistance. The same phenomenon also explains the synergy effect on the solid product selectivity, BW contributing to improve the cracking of PA6, which led to lesser amounts of solid products.

The nitrogen content in the organic phase also exhibited a strong positive synergistic effect, with an experimental value 85% higher than the calculated value. This could be due to the promotion of PA6 cracking by water, present at high concentration, and by oxygen-containing radicals originating from the biomass, which could attack the polymer chain [START_REF] Liu | Towards enhanced understanding of synergistic effects in co-pyrolysis of pinewood and polycarbonate[END_REF].

The impact of PA6 on the H/C atomic ratio of bio-oils was modest (+6%). In contrast, the BW+20 wt.% PA6 blend showed a synergistic effect of -19% on the O/C atomic ratio of the organic phase, indicating that the overall deoxygenation was improved in the presence of PA6, although fewer totally deoxygenated compounds were formed, as shown previously. During the initial stages of co-pyrolysis, the degradation of biomass released highly reactive oxygencontaining species. These species may attack the polymer, subtracting hydrogen from its structure and favoring the chain scission, as mentioned before. In return, this hydrogen transfer from the polymer contributed to the stabilization of the biomass-derived compounds, reducing the coke formation through polymerization and condensation reactions [START_REF] Lu | Synergistic effects on char and oil produced by the co-pyrolysis of pine wood, polyethylene and polyvinyl chloride[END_REF][START_REF] Bockhorn | Kinetic study on the noncatalysed and catalysed degradation of polyamide 6 with isothermal and dynamic methods[END_REF]. This effect on coke formation was confirmed by the synergistic impact that showed a coke deposition 5% lower than the calculated value.

The reactions leading to deoxygenation (dehydration, decarbonylation, decarboxylation) occur mainly inside the zeolite pores and, as regards HZSM-5 (pore diameter -0.55 nm), many compounds were too large to be able to enter the pores. For example, levoglucosan first needs to be converted into furan, by dehydration on acid sites of the external catalyst surface, which can then diffuse into the pores to complete the deoxygenation [START_REF] Kumar | Catalytic Co-pyrolysis of Biomass and Plastics (Polypropylene and Polystyrene) Using Spent FCC Catalyst[END_REF]. Therefore, coke deposition at the entrance or inside the pores reduces the deoxygenation activity. As co-pyrolysis with PA6 decreased the rate of coke deposition, the deoxygenation activity was maintained for a longer period, contributing to the reduction in the O/C ratio.

Conclusion

The co-pyrolysis of beech wood (BW) and polyamide-6 (PA6) combined with ex situ catalytic treatment of the pyrolysis vapors was investigated at different BW/PA6 ratios. The presence of PA6 resulted in a decrease in the gaseous product selectivity and in the coke deposition on the catalyst. N-containing compounds derived from PA6 were found in the liquid phase (mainly as caprolactam) and in the solid products, but no N-containing gaseous product was formed. The production of aromatic compounds derived from the wood was also reduced in the presence of PA6.

This work showed that the properties of the co-pyrolysis products were not the simple arithmetic addition of the properties of the products obtained by pyrolysis of pure BW or pure PA6: positive or negative synergistic effects were observed depending on the property considered.

The most important synergistic effect was observed on the concentration of nitrogen in the organic phase (+87 %). It was attributed to a higher depolymerization of PA6 promoted by reactive oxygenated species originating from the wood, and probably associated to the negative synergistic effects observed on coke deposition (-5 %) and on the O/C ratio (-19 %) of the co-pyrolysis oil.

Figure 2 -

 2 Figure 2 -Product selectivity in experiments with different concentrations of PA6

Figure 3 -

 3 Figure 3 -Total mass flow of gaseous products normalized by the initial mass of feedstock

Figure 4 -

 4 Figure 4 -Cumulative mass of non-condensable gases, normalized by unit mass of feedstock

Figure 6

 6 Figure 6 specifically shows the distribution of N-containing compounds identified by GC-MS in both organic and aqueous phases. The percentages are expressed relative to the total peaks area.

Figure 7 -

 7 Figure 7 -MW distribution of the organic phase normalized by unit mass of dry bio-oil

Figure 8 -

 8 Figure 8 -GPC-DAD chromatograms of organic fraction obtained from (a) BW, (b) BW+ 20% PA6 and (c) PA6

Figure 9 -

 9 Figure 9 -13 C-NMR -Functional group distribution in the different organic phases

N

  i t r o g e n -c o n t a i n i n g G u a i a c o l / s y r i n g o l A c i d / e s t e r P h e n o l s A l d e h y d e / k e t o n e / e t h e r A r o m a t i c e t h e r N o n i d e n t i f i e d p e a

  Figure 12 -3D projections of chromatograms obtained with GCxGC-MS for the organic phases. a) BW; b) BW + PA6 (5%); c) BW + PA6 (10%); d) BW + PA6 (20%); e) PA6.

Figure 13 -

 13 Figure 13 -Mass loss profiles for catalysts after reaction (a); amounts of coke removed from spent catalysts and N content of the coke (b)

Table 2

 2 shows the water content, the ultimate analysis and the calculated higher heating value (HHV) of the two liquid phases obtained in the experiments. The CHNO analyses were calculated on dry basis, therefore they represent the composition of what is not water, in both types of liquid phases.

	(g/g)	0.15			0.015 (g/g)
	feedstock	0.12			0.012 feedstock
	of				of
	mass	0.09			0.009 mass
	mass/Initial	0.06			0.006 mass/Initial
	Accumulated	0.03			0.003 Accumulated
		0.00	CO 2	CO	0.000	CH 4	C 2-6 paraf	C 2 H 4	C 3 H 6

Table 2 -

 2 Water content, ultimate analysis and HHV of the liquid fractions.

	Feedstock	%H2O	C	Ultimate analysis (wt.% on dry basis) H N	O*	HHV (MJ/kg)
			Organic phase
	Beech wood	11.9±1.8 70.6±1.2 6.4±0.6	0.5±0.6 22.4±0.6 29.9±0.8
	BW+PA6 (5%) 12.5±1.1 68.3±1.3 7.0±0.3	2.4±0.8 22.3±1.7 29.8±0.2
	BW+PA6 (10%) 13.4±0.8 70.5±0.3 7.11±0.02 3.8±0.7 18.6±1.0 31.0±0.2
	BW+PA6 (20%) 13.8±1.4 71.6±2.4 7.5±0.5	6.1±0.4 14.8±2.2 32.2±0.7
	PA6	15.2±1.0 74.8±0.7 9.7±0.1 14.3±0.2 1.2±0.5 37.2±0.2
			Aqueous phase
	Beech wood	78.9±1.3 61.3±1.6 3.9±3.2	2.1±1.3 32.7±2.2 23.5±2.6
	BW+PA6 (5%) 74.3±0.7 62.0±3.3 6.2±0.4	3.6±0.7 28.2±2.9 26.0±1.9
	BW+PA6 (10%) 76.2±6.2 66.4±2.5 4.6±1.6	5.8±2.2 23.3±5.0 26.1±2.7
	BW+PA6 (20%) 65.8±2.3 54.5±1.8 6.2±7.2	8.7±1.8 30.6±5.3 23.1±8.9
	* by difference.

Table 3 -

 3 Average molar mass of organic fractions

	Feedstock	MW (g/mol, in eq. HC)
	Beech wood	191
	BW+5 % PA6	179

Table 4 -

 4 Synergistic impact between BW and PA6 for different parameters

	Parameter		Experimental		Calculated	DY
		100% BW	BW+20%PA6	100% PA6	BW+20%PA6
	Gas products (%)	22	16	2	18	-11%
	Liquid products (%)	57	64	70	60	+8%
	Solid products (%)	20	19	29	22	-14%
	Coke on catalyst (wt.%)	13.43	11.83	8.81	12.50	-5%
	N (wt.%) in organic phase	0.5	6.1	14.3	3.3	+87%
	H/C organic phase	1.1	1.3	1.6	1.2	+6%
	O/C organic phase	0.24	0.16	0.01	0.19	-19%

ACKNOWLEDGEMENTS

This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement N° 818120 (Waste2Road

). The authors also thank Véronique Bounor-Légaré (CNRS and University of Lyon) for providing different samples of plastics.


Loïck Kléparski

Grégory Beaugrand

Richard R Kirby

How do

Keywords: plankton paradox, niche theory, biodiversity, pelagic environment, principle of competitive exclusion

HAL is

Marine plankton is composed of both uni-and multicellular organisms covering a wide range of size, from less than 0.2 µm (virioplankton) to a few meters (e.g. the giant jellyfish Nemopilema nomurai) [START_REF] Kirby | Ocean drifters: a secret world beneath the waves[END_REF].

They are at the basis of the food web and play a key role in carbon dioxide regulation, influencing the composition of the Earth's atmosphere [START_REF] Kirby | Ocean drifters: a secret world beneath the waves[END_REF][START_REF] Platt | Spring algal bloom and larval fish survival[END_REF][START_REF] Cushing | Plankton Production and Year-class Strength in Fish Populations: an Update of the Match/Mismatch Hypothesis[END_REF][START_REF] Passow | The biological pump in a high CO2 world[END_REF]. The great diversity they exhibit has fascinated marine biologists for a long time [START_REF] Hardy | The open sea: the world of plankton[END_REF][START_REF] Hardy | A new method of plankton research[END_REF][START_REF] Haeckel | Kunstformen der Natur[END_REF] and it is now apparent that there is even more diversity than originally realised [START_REF] Goetze | Cryptic speciation on the high seas; global phylogenetics of the copepod family Eucalanidae[END_REF][START_REF] De Vargas | Eukaryotic plankton diversity in the sunlit ocean[END_REF]. The mechanisms sustaining the plankton biodiversity remain debated, however, because it is assumed that plankton species coexist in an apparently homogeneous environment with few niches (sensu Hutchinson realised niche i.e. the set of environmental conditions modulated by biotic interactions, enabling a species to growth and reproduce [START_REF] Hutchinson | Concluding remarks[END_REF]) thought to be available [START_REF] Hutchinson | The paradox of the plankton[END_REF][START_REF] Behrenfeld | Phytoplankton biodiversity and the inverted paradox[END_REF].

Although it has been suggested that plankton species may share niches and coexist [START_REF] Ghilarov | The Paradox of the Plankton Reconsidered; Or, Why Do Species Coexist?[END_REF], many theoretical and laboratory studies have tried to resolve the "paradox of the plankton" [START_REF] Hutchinson | The paradox of the plankton[END_REF] and have shown that the number of coexisting plankton species at equilibrium in a community cannot exceed the number of limiting resources [START_REF] Levin | Community Equilibria and Stability, and an Extension of the Competitive Exclusion Principle[END_REF][START_REF] Arthur | Competition, habitat selection and character displacement in a patchy environment[END_REF][START_REF] Rothhaupt | Mechanistic resource competition theory applied to laboratory experiments with zooplankton[END_REF][START_REF] Tilman | Resource competition between planktonic algae: an experimental and theoretical approach[END_REF]. Consequently, coexistence of a large number of species on a small number of limiting resources is only possible when other mechanisms are involved such as (i) temporal heterogeneity (i.e. a continuously changing pelagic environment due to the seasonal changes in hydro-meteorological forcing) and its influence on the structure of the water column [START_REF] Hutchinson | The paradox of the plankton[END_REF][START_REF] Bracco | Mesoscale vortices and the paradox of the plankton[END_REF][START_REF] Richerson | Contemporaneous Disequilibrium, a New Hypothesis to Explain the 'Paradox of the Plankton[END_REF][START_REF] Levins | Coexistence in a Variable Environment[END_REF], (ii) spatial heterogeneity in the relative resource supply rate [START_REF] Tilman | Resources: A Graphical-Mechanistic Approach to Competition and Predation[END_REF]), (iii) spatial subdivision [START_REF] Tilman | Competition and biodiversity in spatially structured habitats[END_REF], (iv) chaotic variations in species abundance induced by competition [START_REF] Huisman | Biodiversity of plankton by species oscillations and chaos[END_REF] or (v) difference in the array of resources limiting species growth [START_REF] Tilman | Resource competition between planktonic algae: an experimental and theoretical approach[END_REF][START_REF] Petersen | The Paradox of the Plankton: An Equilibrium Hypothesis[END_REF]. The tenet that underlies the paradox of the plankton -that the plankton environment is unstructured and so different plankton share the same niche, and therefore the question of whether niche separation might promote plankton coexistence-has received little attention, however.

In this article, we examine how niche separation can enable plankton coexistence. Niche separation, which is often estimated by the degree of overlapping (i.e. the opposite of niche separation) [START_REF] Beaugrand | Towards an understanding of the pattern of biodiversity in the oceans: The pattern of biodiversity in the oceans[END_REF], results from the principle of competitive exclusion [START_REF] Gause | The struggle for existence[END_REF][START_REF] Hutchinson | An introduction to population ecology[END_REF]. By allowing a better exploitation of the available resources, this mechanism has been shown to enable terrestrial plant species coexistence [START_REF] Tilman | The ecological consequences of changes in biodiversity: a search for general principles[END_REF][START_REF] Silvertown | Plant coexistence and the niche[END_REF]. Here, we examine the two postulates on which the paradox of the plankton is based, i.e. (i) do plankton species share niches and (ii) is the oceanic environment unstructured. To do so, we estimate the degree of niche overlapping of 117 plankton species belonging to three taxonomic groups (diatoms, dinoflagellates and calanoid copepods). We show that species' niches of the three taxonomic groups are well separated and that the environment is much more structured in space and time than Hutchinson envisioned [START_REF] Hutchinson | The paradox of the plankton[END_REF]. We conclude that these two mechanisms jointly ensure plankton species coexistence.

Materials and Methods

(a) Biological data

Plankton abundance data came from the Continuous Plankton Recorder (CPR) survey (Supplementary Text S1) [START_REF] Batten | CPR sampling: the technical background, materials and methods, consistency and comparability[END_REF]. It is a long-term plankton monitoring programme using a high-speed plankton recorder sampling at a depth of ~7-10m. Started in 1931, the CPR programme has since sampled plankton on a monthly basis in the North Atlantic Ocean and its adjacent seas [START_REF] Hays | Consistency of Towing Speed and Sampling Depth for the Continuous Plankton Recorder[END_REF][START_REF] Warner | Sampling by the continuous plankton recorder survey[END_REF]. We used the data collected between 1998 and 2018 in the North Atlantic Ocean and its adjacent seas [START_REF] Helaouët | Marine Biological Association of the UK (MBA)[END_REF]. This time period was chosen because it corresponds to the period covered by the environmental datasets we used (see Sections b and c). Species that were found in less than 100 CPR samples were removed from the analysis so that the total number of species used was 117; this reflected abundance data for 71 phytoplankton (i.e. 45 diatoms and 26 dinoflagellates) and 46 calanoid copepod taxa.

(b) Environmental data

We used a set of nine different environmental variables to characterise the niches of the 117 species.

The chosen variables were: bathymetry (in m), nitrate, phosphate and silicate concentrations (mmol.m - 3 ), Mixed Layer Depth (MLD, m), temperature (°C), Photosynthetically Active Radiations (PAR, E.m - 2 .day -1 ), salinity (no unit) and chlorophyll-a concentration (mg.m -3 ). Detail descriptions and references for each dataset are provided in Supplementary Text S2.

(c) Data pre-processing We used the environmental data collected between 1998 and 2018 to work on a common time period with respect to all datasets. All data were subsequently arranged on a grid covering the North Atlantic Ocean (100°W-10°E and 35°N-65°N). By means of nearest-neighbour interpolation [START_REF] Wackernagel | Multivariate geostatistics. An introduction with applications[END_REF], we attributed to each CPR sample a value for each of the nine chosen environmental variables at a depth of 8 m (except for bathymetry), a value included in the range of sampling depth of the CPR instrument [START_REF] Batten | CPR sampling: the technical background, materials and methods, consistency and comparability[END_REF][START_REF] Hays | Consistency of Towing Speed and Sampling Depth for the Continuous Plankton Recorder[END_REF].

All the samples with a missing value along a single dimension were discarded prior to the analyses. We therefore used a total of 90,527 CPR samples.

(d) Niche separation

To characterise and compare the multidimensional niches of the 117 planktonic species, we used the recently proposed "species chromatogram" method [START_REF] Kléparski | Plankton biogeography in the North Atlantic Ocean and its adjacent seas: Species assemblages and environmental signatures[END_REF][START_REF] Kléparski | The species chromatogram, a new graphical method to represent, characterise and compare the ecological niches of different species[END_REF]; this technique uses a chromatogram to display the multidimensional niche of a species in a two-dimensional space. To do so, p-standardised environmental gradients (one per environmental variable, e.g. temperature or nutrients), representing the p-niche environmental dimensions, are defined between 0 (the lowest value taken by an environmental variable) and 1 (the highest). Each gradient is divided into α equidistant categories (here α=50 for all chromatograms). For each category along each dimension (i.e. gradient), the mean species abundance is estimated by averaging the values of the k% of the CPR samples (associated to that category by means of the nearest-neighbour interpolation, see the previous section) with the highest abundance values (here k= 5%), if at least m samples are present in that category (here m=20 CPR samples). The mean abundances along each dimension are then standardised between 0 and 1, and smoothed by means of a second-order simple moving average to alleviate the noise associated with the CPR data. Finally, the niche is displayed as an array of coloured cells, which is called a species chromatogram (Supplementary Figure S1-12) [START_REF] Kléparski | The species chromatogram, a new graphical method to represent, characterise and compare the ecological niches of different species[END_REF].

As each environmental gradient is globally standardised between 0 and 1, the niches of different species can be compared and a degree of niche overlapping (called index D) can be estimated among species [START_REF] Kléparski | The species chromatogram, a new graphical method to represent, characterise and compare the ecological niches of different species[END_REF]. This index is estimated by considering the categories with an abundance greater than or equal to a threshold T along each dimension, with 0<T≤1. They are then used to calculate the niche hypervolume, the shape of the niche being assimilated to a hyperrectangle. D is finally assessed for each pair of species by calculating the ratio of the hypervolume of the niche common to the two species on the total volume filled by the two species. The method is fully described in Kléparski & Beaugrand [START_REF] Kléparski | The species chromatogram, a new graphical method to represent, characterise and compare the ecological niches of different species[END_REF].

Here, D was estimated for each pair of plankton species and for each combination of 1 to penvironmental dimensions (i.e. 1 to p-dimensions considered simultaneously) and five thresholds T: 0, 0.1, 0.25, 0.5 and 0.75. For phytoplankton (i.e. diatoms and dinoflagellates), we chose eight environmental dimensions (p=8): bathymetry, nitrate, phosphate and silicate concentrations, MLD, temperature, PAR and salinity (Figure 1 and Supplementary Figure S1-8). For copepods, we used six dimensions (p=6): bathymetry, MLD, temperature, PAR, salinity and chlorophyll-a concentration (Figure 1 and Supplementary Figure S9-12). D was calculated among species belonging to the same taxonomic group, i.e. among diatoms (45 species), dinoflagellates (26 species), phytoplankton (45 diatoms + 26 dinoflagellates = 71 species) or calanoid copepods (46 species).

All species chromatograms on which index D is based are displayed in Supplementary Figures S1-12.

We have synthetized the information of all species chromatograms by means of a community chromatogram [START_REF] Kléparski | Plankton biogeography in the North Atlantic Ocean and its adjacent seas: Species assemblages and environmental signatures[END_REF][START_REF] Kléparski | The species chromatogram, a new graphical method to represent, characterise and compare the ecological niches of different species[END_REF]. To do so, species' abundance was first replaced by 1 when its standardised abundance was higher than or equal to 0.5 in a category [START_REF] Kléparski | Plankton biogeography in the North Atlantic Ocean and its adjacent seas: Species assemblages and environmental signatures[END_REF](see Figure 1a-d). Then, we estimated the number of co-occurring species in each category of the chromatogram and we converted it into percentage. Therefore, a community chromatogram displays the percentage of co-occurring species in each environmental category.

(e) Habitat diversity

Habitat diversity of the pelagic environment was estimated with respect to each environmental variable (but bathymetry that is constant) by means of the Shannon entropy [START_REF] Magurran | Measuring biological diversity[END_REF]. In other words, we quantified the diversity of values that is taken by an environmental variable for each day or in each geographical cell. To do so, eight environmental matrices (121 latitudes x 441 longitudes x 24 depth levels (except for MLD) x 7670 days) were used and standardised between 0 (i.e. the lowest value of an environmental variable at the scale of the North Atlantic) and 1 (i.e. the highest). The environmental values taken by each matrix were then divided into 50 categories and the habitat diversity 𝐻(𝑆) (expressed in bit) was calculated as follows:

𝐻(𝑆) = -∑ 𝑆 𝑖 × 𝑙𝑜𝑔 2 (𝑆 𝑖 ) 50 𝑖=1 (1)
With 𝑆 𝑖 the probability that the value of the 𝑖 𝑡ℎ category was observed, i.e. its relative frequency. The estimate of the Shannon entropy was performed in two ways for each year between 1998 and 2018:

(i) at a daily scale using all geographical cells and depth levels between 0 and 20 m and (ii) at a spatial scale using all the days and depth levels between 0 and 20m. The resulting annual cycles were then averaged (1998-2018).

Results

(a) Niche separation

Species chromatograms are all displayed in Supplementary Figure S1-12. Their examination revealed that each species has a unique chromatogram and therefore a specific niche, although many overlaps were observed along all niche dimensions. The species chromatograms were summarised by taxonomic group under the form of a community chromatogram, where species abundance was replaced by the percentage of co-occurring species in an environmental category (Figure 1a-d). At this organisational level, niche separation occurred between diatoms and dinoflagellates along five niche dimensions (bathymetry, silicate, temperature, PAR and salinity; Figure 1b-c). Similarly, a clear niche separation can be seen between phytoplankton and copepods with respect to salinity, although the two groups have overlapping chromatograms for bathymetry, temperature and PAR (Figure 1a andd).

Hence, each group seems to have distinct environmental requirements, although niche overlaps are the rule.

The calculation of the degree of niche overlapping (i.e. Index D) confirmed that species niches were rapidly separated when the number of environmental dimensions of the niche rose (Figure 1e-h). Niche overlapping diminished rapidly below 30% for the four taxonomic groups (Figure 1e-h). We therefore investigated whether T (i.e. the threshold of abundance between 0 and 1) influenced our perception of niche separation. When T increased from 0 to 0.75, overlapping rapidly reached a value below 20% and even close to 0% when T becomes higher than 0.25. Consequently, the niches of the four groups were almost totally separated towards the optimal part of their niche (i.e. the part of the niche where a species has its highest abundance [START_REF] Brown | On the relationship between abundance and distribution of species[END_REF][START_REF] Helaouët | Physiology, Ecological Niches and Species Distribution[END_REF]), although some overlapping was still observed in the suboptimal parts (i.e. the parts where a species has low abundance, i.e. when T<0.25) (Figure 1e-h), as might be the expected outcome of competition and natural selection [START_REF] Begon | Ecology: From Individuals to Ecosystems, 4th Edition[END_REF]. Categories where no CPR samples are available are shown in white. Panels e-f represent the changes in the degree of niche overlapping for (e) phytoplankton, (f) diatoms, (g) dinoflagellates and (h) calanoid copepods.

Niche overlapping has been estimated by means of a species chromatogram (i.e. index D, M&M section).

Overlapping was assessed by considering simultaneously an increasing number of environmental dimensions, from 1 to 8 for phytoplankton, diatoms and dinoflagellates, and from 1 to 6 for calanoid copepods. Overlapping among species was based on five thresholds of abundance T=0 (blue lines with diamond), 0.1 (blue lines with dot), 0.25 (red lines with diamond), 0.5 (red lines with dot) and 0.75 (black lines with dot).

(b) Habitat diversity

We quantified habitat diversity in the North Atlantic Ocean by means of the Shannon entropy. The analysis was performed in two ways. First, based on different depth levels (0-20 m) for all geographical cells of the North Atlantic, we found two different daily patterns in habitat diversity: the first pattern revealed higher diversity with respect to nutrients, MLD and chlorophyll-a in Spring (plots in Figure 2a, b, e, f and h) and the second pattern revealed high diversity in Summer for temperature, salinity and PAR (plots in Figure 2c, d andg). Second, based on depth levels (0-20m) and days, we identified spatial patterns in habitat diversity. High diversity was found in the northern parts of the North Atlantic Ocean for nutrients, MLD and chlorophyll-a (maps in Figure 2 a, b, e, f and h) whereas high habitat diversity was found along continental shelves and in the southern part of the North Atlantic Ocean for temperature and salinity (maps in Figure 2c-d). PAR showed high habitat diversity everywhere in the North Atlantic (Figure 2g). Our results therefore confirm that not only the environment is diverse in space and time but that it is also highly impermanent at the same scales (Figure 2).

Discussion

It is known that diatoms and dinoflagellates have distinct environmental requirements [START_REF] Irwin | Phytoplankton niches estimated from field data[END_REF]. Here, our results are going farther and show that the niches of both phyto and zooplankton species rapidly separate when the number of niche dimensions rises (Figure 1). They also demonstrate that the pelagic environment is not only diverse but also impermanent in space and time (Figure 2), a result that has been shown by many studies since the original paper of Hutchinson [START_REF] Longhurst | Ecological geography of the sea[END_REF][START_REF] Beaugrand | An ecological partition of the Atlantic Ocean and its adjacent seas[END_REF][START_REF] Keerthi | Intermittency in phytoplankton bloom triggered by modulations in vertical stability[END_REF]. Therefore, the two implicit postulates on which the Hutchinson's paradox was based are incorrect. High niche differentiation leads species to have a unique combination of environmental conditions (i.e. a unique niche) where their performances in terms of growth and reproduction are optimal [START_REF] Tilman | The ecological consequences of changes in biodiversity: a search for general principles[END_REF]. Therefore, each species covers a part of the multidimensional environmental space and the segregation along various environmental niche dimensions enables them to coexist in space and time, a mechanism that has been suggested to enable terrestrial plant coexistence [START_REF] Tilman | The ecological consequences of changes in biodiversity: a search for general principles[END_REF][START_REF] Silvertown | Plant coexistence and the niche[END_REF]. Furthermore, we have shown that niche separation can occur along a single unique dimension in a multivariable space. For example, two phytoplankton species may still coexist if they share the same niche for a nutrient but utilise a different light wavelengths [START_REF] Burson | Stable coexistence of equivalent nutrient competitors through niche differentiation in the light spectrum[END_REF]. In other words, to simply resolve the plankton paradox we just need to fully understand marine environmental heterogeneity as it applies to the plankton.

Because of the duality between the niche space and the real physical space (i.e. Hutchinson's duality [START_REF] Colwell | Hutchinson's duality: The once and future niche[END_REF]), niche separation (Figure 1) can be related with the diversity of habitats we have observed (Figure 2). Environmental variability is an important mechanism known to promote species coexistence because of a mechanism known as the "storage effect" [START_REF] Adler | Climate variability has a stabilizing effect on the coexistnce of prairie grasses[END_REF][START_REF] Descamps-Julien | Stable coexistence in a fluctuating environment: an experimental demonstration[END_REF]. This mechanism has three requirements: (i) the species of a given community have to develop strategies to survive during unfavourable environmental conditions, (ii) these species have distinct responses to environmental variability (i.e. distinct niches) and (iii) this variability affects competition, meaning that the nicheenvironment interaction modulates competition (i.e. a species is more resistant to competition when its environment is more in adequation with its niche and inversely) [START_REF] Chesson | Mechanisms of Maintenance of Species Diversity[END_REF][START_REF] Beaugrand | Quasi-deterministic responses of marine species to climate change[END_REF].

Expectedly, niche separation is maximal at the optimal part of a species' niche (Figure 1e-h). Therefore, this separation alleviates competition and prevents competitive exclusion. Niche separation takes place through evolution via an alteration in life history strategies. Plankton have developed various strategies, like the development of pigments, cysts, spores, resting eggs or lipidic bags, that enable them to survive during adverse environmental conditions [START_REF] Mcminn | Dark survival in a warming world[END_REF][START_REF] Mcquoid | Diaom resting stages[END_REF][START_REF] Stomp | Adaptive divergence in pigment composition promotes phytoplankton biodiversity[END_REF][START_REF] Jónasdóttir | Seasonal copepod lipid pump promotes carbon sequestration in the deep North Atlantic[END_REF][START_REF] Miller | Biological Oceanography[END_REF]. Some strategies (e.g. diatom morphological traits) even explain phenology and annual plankton succession [START_REF] Kléparski | Morphological traits, niche-environment interaction and temporal changes in diatoms[END_REF][START_REF] Breton | Seasonal Variations in the Biodiversity, Ecological Strategy, and Specialization of Diatoms and Copepods in a Coastal System With Phaeocystis Blooms: The Key Role of Trait Trade-Offs[END_REF]. Our study therefore shows that the niche theory, and its corollary the principle of competitive exclusion, applies for the plankton as for other groups, be they terrestrial or marine.

Figure 1 .

 1 Figure 1. Community chromatograms and changes in the degree of niche overlapping for four groups of plankton. Community chromatogram of (a) phytoplankton, (b) diatoms, (c) dinoflagellates and (d) calanoid copepods. In a-d each column represents an environmental dimension. The Y axis corresponds to the 50 environmental categories standardised between 0 and 1, i.e. it represents all the values taken by an environmental variable from the lowest (bottom category) to the highest (top category). Colours denote the percentage of co-occurring species in each category. High co-occurrence values are in red and low values in blue.

Figure 2 .

 2 Figure 2. Habitat diversity in the North Atlantic Ocean. Habitat diversity for (a) nitrate, (b) silicate, (c) temperature, (d) salinity, (e) phosphate, (f) MLD, (g) PAR and (h) chlorophyll-a. Changes in temporal diversity are shown by the plots and changes in spatial diversity by the maps below the plots. Habitat diversity was assessed by means of the Shannon entropy estimated on the annual changes of each environmental variable (see M&M section). High values of Shannon entropy (red colour in the maps) mean that habitats diversity is high and low values (blue colour in the maps) that habitat diversity is small.

Funding statements

LK is funded by the Region Hauts-de-France, the Marine Biological Association of the UK and the University of Littoral Côte d'Opale. GB is funded by the French National Center for Scientific Research (CNRS).

Data availability

All the data used in this article are already freely available (see the M&M section and Supplementary Text S2).

Conflict of interest

The authors declare no conflict of interest.