Süleyman Gülcemal

Agnès Labande
email: agnes.labande@lcc-toulouse.fr

Jean-Claude Daran

Bekir Çetinkaya

Rinaldo Poli

Keywords: N-heterocyclic carbenes, benzimidazole, ferrocene, Rhodium, hydrosilylation

Rhodium(I) complexes of new ferrocenyl benzimidazol-2-ylidene ligands:

The importance of the chelating effect for ketone hydrosilylation catalysis.

Introduction

The recent developments in the field of functionalised Nheterocyclic carbene (NHC) ligands have proved their great potential in transition metal catalysis. [1][2][3] Indeed, the presence of an NHC and a second, structurally different donating group on the metal can radically alter the catalytic properties. On the other hand, the chelating nature of these ligands results in the production of highly stable complexes. In our search for more efficient catalysts, we have shown that very small structural variations on bifunctional ligands could change dramatically the reactivity of rhodium(I) catalysts for the hydrosilylation of ketones. [4,5] Although NHC ligands bearing a benzimidazole backbone offer a vast potential for catalytic applications, their coordination chemistry has remained relatively unexplored. [6][7][8][9][10][11][12] The -donating properties of benzimidazol-2-ylidene ligands stand between those of unsaturated imidazol-2-ylidenes and of saturated imidazolin-2ylidenes. Thus, although they have the topology of unsaturated NHCs, they show a reactivity typical of saturated carbenes. [11,13] We have tried to improve the catalytic activity of rhodium(I) complexes bearing ferrocenylphosphanyl-NHC ligands. Indeed, ferrocenyl-substituted NHCs are a class of ligands that has been little developed and has a great potential in both asymmetric and non-asymmetric catalysis. [2,14] We thus prepared new ferrocenylphosphane ligand precursors bearing a benzimidazolium moiety, and the corresponding rhodium(I) complexes. Although benzimidazolium salts with a ferrocenyl substituent are already known, [15] we report here the first example of a ferrocenylphosphane bearing a benzimidazolium unit. In order to evaluate the contribution of the phosphanyl group to catalysis, rhodium(I) complexes bearing monodentate ferrocenyl NHC ligands were also prepared.

Results and Discussion

(a) Synthesis and characterisation of ligand precursors 5 and 6

The benzimidazolium salts 4a and 4b were obtained in very good yields from ferrocenyl alcohol 1 and, respectively, substituted benzimidazoles 2a and 2b [7], [16] (Figure 1, Scheme 1), following the procedure developed previously in our group for related imidazolium salts. [4] The phosphane group was deprotected by desulfurisation with Raney nickel to afford the ligand precursors 5a and 5b. Two related systems, either with (b) or without (a) methyl groups at positions 5 and 6 of the benzimidazole moiety were developed, in order to check whether subtle variations could induce electronic changes on the metal centre or modify the physical properties of the metal complexes. [12] N N X X In parallel, compounds 6a and 6b were prepared by reaction of N-(ferrocenylmethyl) trimethylammonium iodide (3) with substituted benzimidazoles in refluxing acetonitrile (Scheme 2), following the literature procedure. [START_REF] Demirhan | [END_REF] These systems will allow a performance comparison of the benzimidazol-2-ylidene/phopshine ligand systems either in the presence of absence of a chelating effect. All compounds were fully characterised by standard analytical methods. The 1 H NMR spectra of compounds 5a and 5b showed signals at respectively 9.10 and 8.86 ppm, characteristic of the acidic C2 proton of the benzimidazolium group. Comparatively, the signal of the same proton in compounds 6a and 6b is shifted downfield (10.60 and 10.21 ppm respectively), due to the stronger C-H•••X -bond polarisation when X = I. [3] The crystal structures of compounds 5a and 6b have been resolved by X-ray diffraction analysis (Figure 2). The C-C bond of the imidazolium motif is significantly longer in the case of benzimidazolium salts 5a and 6b (1.395(3) Å and 1.399(6) Å, respectively), compared to a 1,1'ferrocenylphosphane imidazolium salt previously described by us [1.330(6) Å]. This denotes the delocalization of the C-C bond  electron density into the condensed benzene ring (the C-C bond lengths within the ring are in the 1.372-1.402 Å range for 5a and in the 1.354-1.424 Å range for 6b) and could be responsible for the different reactivity of the resulting carbenes. The two structures show hydrogen bonds, as is typically observed for imidazolium salts, between a fluorine atom of the BF4 -anion and the C62 proton (F3•••H62-C62 = 2.312 Å) for 5a, and between the iodide anion and the C1 proton (I1•••H1-C1 = 2.962 Å) for 6b. The H-bond is significantly shorter in the case of benzimidazolium salt 5a, possessing a weakly-coordinating anion, certainly because the F atom is much smaller than the I atom, although the NMR study (vide supra) indicates a stronger H bond for the iodide salt.

(b) Synthesis and characterisation of rhodium(I) complexes 7 and 8

Addition of compounds 5a or 5b to [Rh(COD)(OtBu)]2prepared in situ from [Rh(COD)Cl]2 and tBuOK [18] in THF led to rhodium(I) complexes 7a and 7b. [Rh(OMe)(COD)]2 reacted with benzimidazolium salts 6a and 6b in dichloromethane to give complexes 8a and 8b, respectively (Scheme 3). The reaction with ligand precursors 5a and 5b led directly to cationic complexes, whereas the neutral form was obtained in the case of precursors 6a and 6b, with iodide coordination.

The 13 C NMR signals of the carbene C atoms of complexes 7 appear at 192.5 ppm (7a) and 189.9 ppm (7b), within the expected range for cationic NHC-Rh(I) complexes with a benzimidazol-2-ylidene moiety. [10,19] These signals are found at slightly lower fields for complexes 8a (196.4 ppm) and 8b (194.5 ppm). [8,18] Complexes 7 and 8 gave X-ray quality crystals by slow diffusion of ether into a dichloromethane solution (Figures 3 and4). Selected bond distances and angles are listed in Table 1, and compared to those of previously described complex 9a (Figure 5). [4] The squareplanar coordination environment in complexes 7-8 was confirmed by X-ray diffraction studies (Figures 3 and4). The cis-NHC-Rh-P angles in complexes 7a, 7b (92.06 (18)° and 93.78(12)°, respectively) and the cis-NHC-Rh-I angles in complexes 8a, 8b (89.57(6)° and 86.1(3)°, respectively) are close to the expected 90° value. The structures of complexes 7a and 7b reveal slightly shorter Rh1-C(NHC) bond lengths than in imidazol-2ylidene/phosphane complex 9a, as well as in a related complex described by Field et al. [2.064(2) Å] [20] . However, this difference is not observed with neutral complexes 8a and 8b: the Rh1-C(NHC) bond lengths are in the typical range observed for NHC-Rh complexes with iodide coordination. [8,9,21] There is no evidence for a structural effect of the methyl groups in positions 5 and 6 of the benzene ring (complexes 7b and 8b), as we can not observe any significant variation in the Rh1-C(NHC) bond lengths related to this parameter. The mesityl group is found roughly orthogonal to the benzimidazole backbone, with dihedral angles ranging from 86 to 90°, to limit steric interactions. Figure 5. Previously described Rh I complexes with an imidazol-2-ylidene moiety. [4]

(c) Catalytic hydrosilylation of methyl aryl ketones

The catalytic activity of NHC complexes 7 and 8 was evaluated in the hydrosilylation of carbonyl compounds. [22,23] The reactions were typically carried out with 2 mol% of catalyst, in THF at room temperature (Scheme 4, Table 2). The conversion of acetophenone was followed by TLC and the crude mixture was analysed by 1 H NMR. In our first attempts, we were pleased to note that all complexes were active (entries 1-2 and 6-7), although a reaction time of 20 h was needed to obtain reasonable yields. The yields of alcohol were much higher in the case of bifunctional, cationic complexes 7a,b.

As observed in some of our previous work, the concentration of acetophenone in the medium can alter dramatically the reaction rate. [5] We thus carried out the next catalytic tests with a more concentrated medium (2 M instead of 1 M). The results were very encouraging, since we obtained a nearly quantitative yield of alcohol in 2 h with complex 7b (entry 4). 20 h were still needed with complex 7a to reach 80% conversion of the ketone (entry 3), probably due to the poor solubility of the catalyst in THF. In comparison, our previously described complexes 9a and 9b were poorly active, [4] even at this concentration, as the reactions were not complete after 5 days at room temperature (entries 14 and 15). However, increasing the concentration did not improve the yield in the case of neutral complexes 8a,b (entries 8 and 9). In order to have a better comparison with cationic complexes 7a and b, one equivalent of AgBF4 was added to compounds 8a and b to give, respectively, cationic complexes 10a and 10b (Scheme 5), where the Rh(I) centre is probably coordinated by one THF molecule in order to achieve the preferred square planar configuration. These complexes were not isolated but used directly for catalytic reactions. Both complexes were completely soluble in THF, in contrast to their neutral precursors. It has been reported in some other cases that the use of AgX (X = OSO2CF3, BF4, PF6) was required in order to improve the catalytic activity, [23][24][25] and this proved to be the case also with our systems. Indeed, the conversion of acetophenone was complete after only 2 h, as shown by the 1 H NMR spectra of the reaction mixture before hydrolysis of the silyl ether (entries 10 and 11). However, the NMR yields calculated after hydrolysis were rather low and hardly reproducible, showing a lack of selectivity of these systems for the formation of the expected alcohol. The 1 H NMR spectra of the reaction mixtures before hydrolysis suggest the formation of a silyl enol ether as by-product (Scheme 6), which would then be reconverted to acetophenone by the hydrolysis step. Indeed, the 1 H NMR spectra before hydrolysis show two doublets at 4.65 and 5.05 ppm (=CH2, J = 2.4 Hz), characteristic of the silyl enol ether (the doublet at 5.05 ppm, however, is often hidden by the Si-H singlet of Ph2SiH2, showing at 5.06 ppm). These signals are absent from the 1 H spectra of the mixtures after hydrolysis, whereas two new signals, typical of acetophenone, appear at 2.6 ppm (CH3, s) and 8.0 ppm (Ar-H, d, J = 7.5 Hz). This process is frequently observed in ketone hydrosilylation. [26] In order to see whether the phosphane was necessary to maintain a good selectivity, we added 1 equivalent of triphenylphosphane to complexes 10a and 10b prior to the reaction (entries 12 and 13). The reaction was complete after 2 h with complex 10b, but needed 20 h with complex 10a, as the addition of PPh3 decreased the solubility of the complex in THF. The analysis of the reaction mixture before and after hydrolysis showed a selectivity increase in both cases, which confirmed our hypothesis. However, the selectivity was lower than that reached with complexes 7a and 7b, showing clearly that the chelating effect is beneficial for the reaction selectivity. Encouraged by the results obtained with complex 7b, we decided to lower the catalyst loading to 1 mol% (entry 5), and were pleased to observe that the activity was preserved, with a total conversion and 95% yield in only 2 h. Catalytic tests with acetophenone derivatives allowed us to determine the efficiency of complexes 7a and 7b (2 mol%) with a variation of electronic or steric properties (Scheme 4, Table 3). Whereas marked differences were observed with complex 7a, catalyst 7b showed a very high activity whatever the substituents of acetophenone. Indeed, even the presence of an ortho substituent did not lower the yield (entry 6). The higher catalytic activity observed at higher ketone concentration agrees with previous reports of a saturation effect: the reaction was found first order in ketone at low concentrations, but eventually became [ketone] independent at high concentrations. [27][28][29] The mechanism of this catalytic process has been addressed by many authors. Although a few variations have been discussed, the most commonly accepted mechanism remains the originally proposed one, [27,[30][31][32] which is based on a rate determining Si-H oxidative addition process, yielding a Rh III silyl hydride intermediate. This step would be followed by ketone coordination, insertion into the metal-silyl bond, and final reductive elimination. Interestingly, the observation by Kolb and Hetflejs of a saturation effect (while working with a cationic system) was interpreted as indicating reversible complexation of the ketone prior to the rate determining silane addition, [28] but Giering later discarded this variant when he could properly model, on the basis of the other mechanism, kinetic results that he obtained with a neutral system, which also included a saturation effect in [ketone]. [27] However, the data fittings were described as unsatisfactory unless additional processes of catalyst decomposition were included at various places in the catalytic cycle. We believe that the interpretation offered by Kolb and Hetflejs is reasonable and probably applies to our system, for the following reasons. The COD ligand is probably lost rapidly from the coordination sphere by hydrosilylation or hydrogenation processes, as also observed or proposed in other cases. [27,[32][33][34] The resulting [(NHCFcPPh2)Rh] + system is then likely coordinated by two solvent molecules, but the lability of these should easily allow ketone coordination, faster than the rate-determining silane oxidative addition. Thus, the rate-determining silane oxidative addition may preferentially occur on the ketone substituted complex, as shown in Scheme 7, when the ketone concentration is high, whereas the order of events may be inverted (pathway indicated with dashed arrows in Scheme 7) at low ketone concentrations.

Table 2. Hydrosilylation of acetophenone (R=H). [a] Entry Catalyst (mol%)

Additive Concentration (M) t (h) Conversion (%) [b] Ratio silyl ether/silyl enol ether [b] Yield of 1phenylethanol (%) [c] 1 7a [e] 62 ; 46 ; 60 [e] 11 10b (2) -2 2 100 n/c ; 69:31 ; 58:42 [e] 83 ; 66 ; 58 [e] 12 10a (2) PPh3 (1 eq.) 2 20 100 78:22 76 [d] 13 10b (2) PPh3 (1 eq.) 2 2 100 87:13 84 14 f 9a (2) -2 5 days 63 >99:1 63 [d] 15 f 9b (2) -2 5 days 90 99:1 87 [d] [a] Ketone (1 eq.), diphenylsilane (1.1 eq.), room temperature, THF, then hydrolysis with MeOH/HCl. [b] Determined by 1 H NMR signal integration of the crude reaction mixture before hydrolysis, n/c = not calculated. [c] Determined by 1 H NMR signal integration of the crude reaction mixture after hydrolysis. [d] Catalyst only partially soluble in THF. [e] Results of three runs. [f] Data from ref. [4] .

Finally, the formation of the unsaturated silyl ether can be easily rationalized on the basis of a competition between the reductive elimination of the hydrosilylation product and a -H elimination step from a common intermediate ( Scheme 7). Although the mechanism of ketone hydrosilylation is not as well established as that of olefin hydrosilylation, [35] there is good evidence supporting a ketone insertion into the Rh-Si bond followed by C-H reductive elimination, rather than the alternative insertion into the Rh-H bond followed by Si-C reductive elimination. [24,36] Supposedly, the absence of the phosphane ligand renders the -H elimination pathway more easily accessible through the availability of an additional open coordination site.

Conclusions

In conclusion, the benzimidazol-2-ylidene unit gives highly active hydrosilylation rhodium catalysts, proving superior to the imidazol-2-ylidene. Moreover, bidentate NHC ligands bearing a phosphanyl group ensure a much better selectivity in the desired product than monodentate NHC ligands. The introduction of methyl substituents on the benzene ring of the benzimidazol-2ylidene ligand enhanced the solubility of the rhodium complex in the reaction medium. This work was carried out with achiral ligands, and we are now planning to prepare planar chiral 1,2disubstituted ferrocenyl ligands, using the methodology developed here, for applications to asymmetric hydrosilylation.

Experimental Section

All reactions were carried out under a dry argon atmosphere using Schlenk glassware and vacuum line techniques. Solvents for syntheses were dried and degassed by standard methods before use. Spectra were recorded on Brüker ARX250, AV300 or DPX300 spectrometers. All spectra were recorded in CDCl3, unless otherwise stated. Mass spectra were obtained from acetonitrile solutions on a TSQ7000 instrument from ThermoElectron.

(1'-diphenylthiophosphinoferrocen-1-yl)methanol 1 was prepared according to our previsouly published procedure. [37] [Rh(OMe)(COD)]2 [38] , 2a and 2b [7] and 3 [39] were prepared according to literature procedures. Benzimidazolium salt 4a: To a solution of ferrocenyl alcohol 1 (100 mg, 0.23 mmol) in degassed dichloromethane (5 mL) was quickly added HBF4 (35 L, 54 wt% in Et2O), immediately followed by N-(2,4,6trimethylbenzyl) benzimidazole 2a (86 mg, 0.35 mmol). The mixture was washed with 2 M aq. HCl, water, satd. aq. NaHCO3 and water again. The organic phase was dried (MgSO4), filtered and concentrated in vacuo. The residue was purified by column chromatography on silica gel (eluent: CH2Cl2/acetone 9:1) to give a yellow-orange solid (121 mg, 70% yield). C40H38BF4FeN2PS (752.1): Calcd: C 63.82, H 5.05, N 3.72; found: C 63.60, H 5.32, N 3.64. 1 H NMR (300 MHz, CDCl3, 25 ºC): δ = 9.30 (s, 1H, NCHN + ), 7.98 (d, JH,H = 8.4 Hz, 1H, benzimidazole), 7.29 (d, JH,H = 8.4 Hz, 1H, benzimidazole) 7.43-7.67 (m, 12H, benzimidazole + ArH), 6.98 (s, 2H, Mes), 5.61 (s, 2H, CH2-Mes), 5.50 (s, 2H, CH2-Fc), 4.75, 4.71, 4.48, 4.08 (t, JH,H =.1.8 Hz, 8H, Cp), 2.33 (s, 3H, p-CH3 Mes) 2.26 (s, 6H, o-CH3 Mes). 13 C{ 1 H} NMR (75.5 MHz, CDCl3, 25 ºC): δ = 141.32 (NCN + ), 140. 20, 137.93, 134.55, 133.39, 131.69, 131.55, 131.49, 131.41, 128.49, 128.33, 127.26, 124.32, 113.89, 113.41 (Ar-C), 130.31 (Mes), 80.73, 74.31, 74.15, 73.47, 73.33, 72.06, 70.91 (Cp), 47.01 (CH2-Mes), 46.98 (CH2-Fc), 21.09 (p-CH3 Mes), 19.84 (o-CH3 Mes). 31 P{ 1 H} NMR (121.5 MHz, CDCl3, 25 ºC): δ = 41.6. Benzimidazolium salt 4b: To a solution of ferrocenyl alcohol 1 (100 mg, 0.23 mmol) in degassed dichloromethane (5 mL) was quickly added HBF4 (35 L, 54 wt% in Et2O), immediately followed by N-(2,4,6trimethylbenzyl)-5,6-dimethyl benzimidazole 2b (96 mg, 0.35 mmol). The mixture was washed with 2 M aq. HCl, water, satd. aq. NaHCO3 and water again. The organic phase was dried (MgSO4), filtered and concentrated in vacuo. The residue was purified by column chromatography on silica gel (eluent: CH2Cl2/acetone 9:1) to give a yellow-orange solid (108 mg, 60% yield). C42H42BF4FeN2PS ( 780 13 C{ 1 H} NMR (75.5 MHz, CDCl3, 25 ºC): δ = 139.71 (NCN + ), 137.92, 137.87, 137.54, 137.42, 133.46, 131.69, 131.54, 131.40, 131.36, 128.48, 128.36, 128.32, 128.20, 124.49, 120.31, 113.51, 112.89, 109.77 (Ar-C), 130.22 (Mes), 81.01, 74.28, 74.11, 73.46, 73.33, 72.18, 72.05, 71.93, 70.84, 69.29, 69.22 138.87, 137.97, 137.33, 137.25, 133.53, 128.73, 128.31, 113.16, 112.97 (Ar-C), 130.19 (Mes), 79.82, 74.10, 72.45, 70.60, 70.37, 65.85 (Cp) 140.03, 137.98, 131.35 (d, JC,H = 4.5 Hz), 127.22 (d, JC,H = 10.6 Hz), 124.65, 113.67 (d, JC,H = 12.8 Hz), (Ar-C), 130.32 (Mes), 79.20, 69.77, 69.64, 69.55, 69.48, 69.35, 69.30 (Cp) 139.86, 138.72, 137.99, 137.90, 137.31, 137.26, 132.00, 130.29, 130.12, 129.97, 129.62, 127.37, 124.82, 120.35, 113.18, 113.04, 109.78 20.31, 20.42 (CH3 benzimidazole), 20.75, 21.92 (o-CH3 Mes).

Rhodium complex 7a: t-BuOK (23.3 mg, 0.21 mmol) and [Rh(COD)Cl]2 (47.6 mg, 0.097 mmol) were placed in a Schlenk tube, the solids were degassed and THF (10 mL) was added. The solution was stirred for 30 min at room temperature, then slowly added to 5a (139 mg, 0.194 mmol), which was precooled to -78 ºC. The mixture was stirred at -78 ºC for 15 min then 2 h at room temperature. The solvent was evaporated and the residue taken up into degassed CH2Cl2 (10 mL). AgBF4 (56.7 mg, 0.291 mmol) was added, the mixture was stirred for 30 min at room temperature then filtered on Celite ® . The solvent was evaporated in vacuo and the residue was filtered on silica gel (eluent: CH2Cl2/acetone 95:5) to give 7a as an orange solid (154 mg, 86% yield 134.11, 131.96, 131.65, 130.92, 129.96, 128.71, 126.10, 123.65, 111.38, 110.93 (Ar-C), 129.82 (Mes), 71.26, 69.98, 69.31, 68.44 (Cp), 99.13, 96.50, 73.46 Rhodium complex 7b: t-BuOK (19 mg, 0.171 mmol) and [Rh(COD)Cl]2 (38.5 mg, 0.078 mmol) were placed in a Schlenk tube, the solids were degassed and THF (10 mL) was added. The solution was stirred for 30 min at room temperature, then slowly added to 5b (117 mg, 0.156 mmol), which was precooled to -78 ºC. The mixture was stirred at -78 ºC for 15 min then 2 h at room temperature. The solvent was evaporated and the residue taken up into degassed CH2Cl2 (10 mL). AgBF4 (45.6 mg, 0.234 mmol) was added, the mixture was stirred for 30 min at room temperature and then filtered on Celite ® . The solvent was evaporated in vacuo and the residue was filtered on silica gel (eluent: CH2Cl2/acetone 95:5) to give 7b as an orange solid (126 mg, 84% yield (d, JRh,C = 50.7 Hz, Ccarbene), 139.69, 138.68, 134.27, 134.10, 133.87, 132.80, 132.67, 131.64, 130.86, 129.33, 129.19, 128.75, 128.62, 126.40, 111.87, 110.87 (Ar-C), 129.60 (Mes), 71.31, 69.91, 69.24, 68.43 (Cp), 98.89, 96.47, 96.33, 73.58 (COD-CH), 30.93, 30.43, 22.34 (COD-CH2) (d, JRh,C = 50.6 Hz, Ccarbene), 138.82, 138.53, 135.86, 134.69, 128.20, 122.05, 121.47, 110.71, 110.60 (Ar-C), 129.51 (Mes), 82.08, 71.15, 69.50, 69.22, 68.91, 67.76 (Cp), 97.74, 97.63, 97.55, 97.44, 72.50, 72.28, 72.11, 71.89 (COD-CH), 32.73, 32.08, 30.16, 29. (d, JRh,C = 50.8 Hz, Ccarbene), 138.83, 138.37, 134.51, 133.32, 130.61, 130.26, 129.35, 128.56, 111.15 (Ar-C), 129.47 (Mes), 82.43, 71.07, 69.38, 68.76, 67.69 (Cp), 97.36, 97.27, 97.16, 97.08, 72.35, 72.16, 71.98, 71.80 (COD-CH), 32.70, 32.10, 30.11, 29. Representative procedure for Rh I -catalyzed hydrosilylation of ketones.

In a flame-dried Schlenk flask was placed the Rh I complex (10 -6 mol, 2 mol%) under argon. Anhydrous THF (0.25 mL) and then acetophenone (56 L, 4.8 10 -4 mol) were added by syringe. Diphenylsilane (100 L, 5.4 10 -4 mol) was added slowly, the reaction mixture was stirred at room temperature and the conversion of acetophenone followed by TLC analysis.

After the reaction was finished, the solvent was evaporated in vacuo and the residue analyzed by 1 H NMR (CDCl3). The NMR sample was taken up into CH2Cl2, MeOH (1 mL) was added, the reaction mixture stirred for 1h at room temperature. 1M HCl (1 mL) was then added, and the reaction mixture stirred for 1 additional hour at room temperature. The phases were separated, the aqueous phase extracted with CH2Cl2, the organic phases were dried (MgSO4), filtered and concentrated in vacuo. The residue was analyzed by 1 H NMR.

X-ray diffraction studies.

A single crystal of each compound was mounted under inert perfluoropolyether at the tip of glass fibre and cooled in the cryostream of either an Oxford-Diffraction XCALIBUR CCD diffractometer for 6b , 7b, 7a and 8a, a Stoe IPDS diffractometer for 8b or a Bruker APEXII CCD diffractometer for 5a. Data were collected using the monochromatic MoK radiation (= 0.71073). The structures were solved by direct methods (SIR97) [40] and refined by least-squares procedures on F 2 using SHELXL-97. [41] All H atoms attached to carbon were introduced in calculation in idealised positions and treated as riding models. In compound 6b, some residual electron density were difficult to model and therefore, the SQUEEZE function of PLATON [42] was used to eliminate the contribution of the electron density in the solvent region from the intensity data, and the solvent-free model was employed for the final refinement. There are two cavities of about 173 Å 3 per unit cell. PLATON estimated that each cavity contains 12 electrons which may correspond to half a molecule of acetonitrile as suggested by chemical analyses. In compound 7b and 8a, the SQUEEZE function has been also used. In 7b, there are two cavities of 470 Å 3 per unit cell. PLATON estimated that each cavity contains 190 electrons which may correspond to roughly a mixture of four CH2Cl2 and four (C2H5)2O molecules within the cell as suggested by the use of these solvents during the crystallization procedure. In compound 8a, there are two cavities of 172 Å 3 per unit cell, which may roughly correspond to a mixture of two CH2Cl2 and two (C2H5)2O molecules within the cell. The drawing of the molecules was realised with the help of ORTEP32. [43] Crystal data and refinement parameters are shown in Tables 4 and5

Figure 1 .

 1 Figure 1. Substituted benzimidazoles.

  Scheme 1. Synthesis of 1,1'-disbustituted ferrocenyl benzimidazolium salts.

Figure 2 .

 2 Figure 2. ORTEP views of 5a (top) and 6b (bottom). Ellipsoids are shown at the 30% probability level. All hydrogen atoms except H(62) (top) and H(1) (bottom) are omitted for clarity. Selected bond lengths (Å) and angles (°), 5a: C(1)-P(1) 1.811 (2), C(62)-N(1) 1.325 (3), C(62)-N(2) 1.331 (3), C(63)-C(68) 1.395 (3), N(1)-C(62)-N(2) 110.5 (2) ; 6b: C(1)-N(11) 1.321 (6), C(1)-N(12) 1.321 (6), C(112)-C(113) 1.399 (6), N(11)(C(1)-N(12) 110.4 (6).

3 .

 3 Scheme 3. Synthesis of rhodium complexes.

Figure 3 .

 3 Figure 3. ORTEP views of 7a (top) and 7b (bottom). Ellipsoids are shown at the 50% (7a) or 30% (7b) probability level. All hydrogen atoms are omitted for clarity.

Figure 4 .

 4 Figure 4. ORTEP views of 8a (top) and 8b (bottom). Ellipsoids are shown at the 50% (8a) or 30% (8b) probability level.All hydrogen atoms are omitted for clarity.

4 .

 4 Scheme 4. Hydrosilylation of acetophenone and its derivatives with Rh I complexes.

FeScheme 5 .

 5 Scheme 5. Generation of cationic Rh I complexes bearing monodentate NHC ligands.

  Scheme 6. Side-products generated during hydrosilylation reactions.

  (COD-CH),30.90, 30.42 (COD-CH2), 50.80 (CH2-Fc), 47.12 (CH2-Mes), 21.12 (o-CH3 Mes), 19.89 (p-CH3 Mes). 31 P{ 1 H} NMR (121.5 MHz, CDCl3, 25 ºC): δ = 18.4(d, JP,Rh = 154.3 Hz) ppm.

  -heterocyclic carbenes / benzimidazole / ferrocene / Rhodium / hydrosilylation

Table 1 .

 1 Selected bond lengths (Å) and bond angles (°) for complexes 7

, 8 and 9a. 7a 7b 9a [a] 8a 8b

	Bond lengths (Å)
	Rh1-C(NHC)		2.018 (7)		2.031 (5)	2.047 (3)	2.012 (2)	1.993 (10)
	Rh1-P1		2.3138 (18)	2.3233 (12)	2.3345 (8)	-	-
	Rh1-I1		-			-	-	2.6848 (3)	2.6784 (11)
			2.180 (7)		2.209 (5)	2.215 (3)	2.143 (2)	2.100 (10)
			2.181 (7)		2.209 (5)	2.231 (3)	2.109 (2)	2.091 (11)
	Rh1-C(COD)		2.209 (7)		2.239 (4)	2.220 (3)	2.234 (2)	2.187 (10)
			2.248 (7)		2.229 (4)	2.217 (3)	2.204 (2)	2.226 (10)
	N1-C(NHC)		1.355 (8)		1.367 (5)	1.359 (4)	1.366 (3)	1.364 (11)
	N2-C(NHC)		1.362 (8)		1.373 (6)	1.357 (4)	1.360 (3)	1.382 (11)
	Bond angles (°)
	N1-C(NHC)-N2		105.5 (6)		105.2 (4)	104.5 (2)	106.02 (18)	104.7 (8)
	C(NHC)-Rh1-P1		92.06 (18)	93.78 (12)	90.83 (8)	-	-
	C(NHC)-Rh1-I1		-			-	-	89.57 (6)	86.1 (3)
	Fe	N Ph 2 P Rh	N	R	BF 4
	9
	a: R = Me
	b: R = 2,4,6-trimethylphenyl

  JH,H = 2.1 Hz, 2H, Cp), 2.43(d, JH,H = 10.2 Hz, 6H, o-CH3-Mes), 2.34 (d, JH,H = 8.1 Hz, 6H, CH3 benzimidazole), 2.27 (s, 3H, p-CH3-Mes).13 C{ 1 H} NMR (75.5 MHz, CDCl3, 25 ºC): δ = 140.95 (NCN + ), 140.00,

	benzimidazole), 6.98 (s, 2H, Mes), 5.64 (d, JH,H = 8.7 Hz, 2H, CH2-Mes),
	5.22 (s, 2H, CH2-Fc), 4.52, (t, JH,H = 2.1 Hz, 2H, Cp), 4.30, (s, 5H, Cp),
	4.22 (t,
	, 48.09 (CH2-Mes), 47.12 (CH2-Fc),
	21.13 (p-CH3 Mes), 20.44 (d, JC,H = 4.5 Hz, o-CH3 Mes).
	Benzimidazolium salt 6b: (Ferrocenylmethyl)trimethylammonium iodide
	3 (465 mg, 1.2 mmol) was slowly added to a solution of N-(2,4,6-

trimethylbenzyl)-5,6-dimethyl benzimidazole 2b (361 mg, 1.3 mmol) in acetonitrile

(10 mL)

. The mixture was heated to reflux for 48 h then the solvent was removed in vacuo. The residue was purified by column chromatography on silica gel (eluent: CH2Cl2/acetone 8:2) to give an orange solid (590 mg, 81% yield). C30H33FeIN2 (606.8): Calcd: C 59.

33, H 5.44, N 4.61; found: C 61.01, H 6.08, N 4.80. 1

H NMR (300 MHz, CDCl3, 25 ºC): δ = 10.21 (s, 1H, NCHN + ),

7.11, 7.27, 7.32, 7.50, 7.59 (s, 2H,

Table 4 .

 4 Crystal data and structure refinement parameters for benzimidazolium salts.

	.

Table 5 .

 5 Crystal data and structure refinement parameters for rhodium complexes.Entry for theTable of Contents ((Please choose one layout.))

	Compound	7a	7b		8a	8b
	Empirical formula	C46H45FeN2PRh, BF4	C50H51FeN2PRh, BF4	C36H40FeIN2Rh	C38H44FeN2Rh, I
	Formula weight	891.57	956.47		786.36	814.41
	Temperature, K	180(2)	180(2) K		180(2) K	296(2)
	Wavelength, Å	0.71073	0.71073		0.71073	0.71073
	Crystal system	Monoclinic	Monoclinic		Tetragonal	Monoclinic
	Space group	P 21/c	P 21/n		I-4	P 21/n
	a, Å	10.4291(4)	14.6210(15)		28.3187(10)	12.5094(14)
	b, Å	27.7380(12)	13.7691(11)		28.3187(10)	15.0659(12)
	c, Å	14.1065(6)	24.724(2)		8.0375(6)	18.3417(19)
		90.0	90.0		90.0	90.0
		91.019	91.663(3)		90.0	98.151(13)
		90.0	90.0		90.0	90.0
	Volume, Å3	4080.1(3)	4975.3(8)		6445.7(6)	3421.9(6)
	Z	4	4		8	4
	Density (calculated), Mg/m 3	1.515	1.277		1.621	1.581
	Absorption coefficient, mm -1	0.855	0.703		1.947	1.837
	F(000)	1912	1968		3152	1640
	Crystal size, mm 3	0.336 x 0.125 x 0.080	0.293 x 0.085 x 0.039	0.44 x 0.08 x 0.071	0.40 x 0.40 x 0.16
	Theta range, °	2.82 to 26.37	1.64 to 22.12		2.27 to 33.22	1.76 to 24.20
	Reflections collected	30948	79794		128931	27214
	Independent reflections (Rint)	8322 (0.0756)	6134 (0.1124)		12376 (0.0463)	5458 (0.1564)
	Completeness, %	99.9	98.8		99.9	99.0
	Absorption correction	Multi-scan	Semi-empirical	from	Semi-empirical from	Multi-scan
			equivalents		equivalents
	Max. and min. transmission	1.0 and 0.970	1.0 and 0.784		1.0 and 0.881	0.5312 and 0.4354
	Refinement method	F 2	F 2		F 2	F 2
	Data / restraints / parameters	8322 / 51 / 526	6134 / 0 / 546		12376 / 1 / 373	5458 / 0 / 393
	Goodness-of-fit on F2	1.243	1.076		1.035	0.790
	R1, wR2 [I>2(I)]	0.0792, 0.1418	0.0406, 0.0978		0.0294, 0.0674	0.0544, 0.1175
	R1, wR2 (all data)	0.1146, 0.1500	0.0598, 0.1051		0.0336, 0.0689	0.1360, 0.1409
	Residual density, e.Å-3	1.068 / -1.517	0.508 / -0.403		0.600 / -0.359	1.200 / -1.130

Ferrocenyl benzimidazol-2-ylidene Rh I complexes

	Precursors	of	new	ferrocenyl
	phosphane-benzimidazol-2-ylidene
	bifunctional ligands, and monodentate
	ferrocenyl	benzimidazol-2-ylidene
	ligands, have been prepared. Cationic
	and neutral rhodium(I) complexes
	showed good activities for the catalytic
	hydrosilylation of ketones. The use of
	bidentate ligands proved essential to
	obtain good selectivities in the desired
	alcohol.

Acknowledgment

We thank the Centre National de la Recherche Scientifique (CNRS) and the Institut Universitaire de France (IUF) for support of this work. ____________


Nadia Frontier
email: frontier.nadia@gmail.com

Florian De Bettignies

Andy Foggo

Dominique Davoult

Sustained productivity and respiration of degrading kelp detritus in

Keywords: Conceptualisation, Methodology, Data curation, Formal analysis, Writing - 29A Climate change, Detritus, North East Atlantic, Oxygen production, PAM fluorescence, Respiration, Trophic transfer

come

Sustained productivity and respiration of degrading kelp detritus in the shallow benthos: Detached or broken, but not dead

INTRODUCTION

Within the temperate to sub-polar regions of the globe, kelps are of paramount importance to marine ecosystem functioning. They contribute to the functional integrity of coastal ecosystems as habitat forming species (HFS) via their biogenic structure, through wave energy dissipation and via cycling of carbon [START_REF] Christie | Species distribution and habitat exploitation of fauna associated with kelp (Laminaria hyperborea) along the Norwegian coast[END_REF][START_REF] Laffoley | The management of natural coastal carbon sinks[END_REF][START_REF] Teagle | The role of kelp species as biogenic habitat formers in coastal marine ecosystems[END_REF]. As highly productive primary producers, kelps fix carbon dioxide to produce organic matter via photosynthesis [START_REF] Bartsch | The genus Laminaria sensu lato: Recent insights and developments[END_REF]. The accumulated biomass is eventually released or eroded and exported as detritus in a variety of forms into recipient benthic ecosystems [START_REF] Duggins | Magnification of secondary production by kelp detritus in coastal marine ecosystems[END_REF]Krumhansl and Scheibling 2012;Pessarrodona et al. 2018a) . This transfer of carbon helps to sustain adjacent food webs. The breakdown of detrital material by erosion and grazing activity provides a source of particulate organic matter (POM) available to filter-feeding invertebrates [START_REF] Renaud | Macroalgal detritus and food-web subsidies along an Arctic fjord depth-gradient[END_REF]de Bettignies et al. 2020a;[START_REF] Vilas | Kelp-carbon uptake by Arctic deep-sea food webs plays a noticeable role in maintaining ecosystem structural and functional traits[END_REF] which in turn can play a major role in mediating carbon fluxes along the sediment-water interface [START_REF] Queirós | A snap shot of the short-term response of crustaceans to macrophyte detritus in the deep Oslofjord[END_REF][START_REF] Pedersen | Detrital carbon production and export in high latitude kelp forests[END_REF].

A significant proportion of primary production from marine primary autotrophs is exported as detritus rather than entering the consumer food web [START_REF] Moore | Detritus, trophic dynamics and biodiversity[END_REF]. In a global context, J o u r n a l P r e -p r o o f as much as 82% of annual kelp biomass may be transferred as detrital subsidies (Krumhansl and Scheibling 2012;[START_REF] De Bettignies | Contrasting mechanisms of dislodgement and erosion contribute to production of kelp detritus[END_REF]. Knowing the fate of such detritus is fundamental in understanding the dynamics, connectivity and functioning of coastal ecosystems [START_REF] Polis | Toward an integration of landscape and food webs ecology: the dynamics of spatially subsidized food webs[END_REF]. Indeed, detritus enhances both primary and secondary production [START_REF] Polis | Toward an integration of landscape and food webs ecology: the dynamics of spatially subsidized food webs[END_REF][START_REF] Marczak | Meta-analysis: Trophic level, habitat, and productivity shape the food web effects of resource subsidies[END_REF]) whilst contributing to the structure and stability of food webs in recipient ecosystems [START_REF] Huxel | Effects of partitioning allochthonous and autochthonous resources on food web stability[END_REF]. Furthermore, accumulation of detritus can physically modify the habitat structure [START_REF] Arroyo | The Role of Drifting Algae for Marine Biodiversity[END_REF].

However, the influences upon recipient ecosystems are dependent upon the timing, frequency and residence time of the detrital deposition [START_REF] Yang | What can we learn from resource J o u r n a l P r e -p r o o f[END_REF].

Macroalgae represent an as yet under-estimated contributor to the oceanic carbon cycle and in particular to carbon sequestration and long-term storage. Their contribution to global carbon assimilation and sequestration is currently not considered by the IPCC [START_REF] Ciais | Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth[END_REF] , and whilst the Global Carbon Project (GCP) identifies coastal habitats as carbon stocks [START_REF] J O U R N A L P R E -P R O O F Friedlingstein | Global carbon budget 2019[END_REF], their full contribution to the global marine carbon sink remains neglected. Unlike seagrasses or phytoplankton, macroalgae are not routinely considered as contributors to the 'Blue Carbon' budget [START_REF] Mcleod | A blueprint for blue carbon: Toward an improved understanding of the role of vegetated coastal habitats in sequestering CO2[END_REF][START_REF] Howard | Clarifying the role of coastal and marine systems in climate mitigation[END_REF]. Marine macrophytes collectively fix around 1.8 Gt C yr -1 (of which macroalgae contribute ~1.5 Gt C yr -1 ) [START_REF] Duarte | The role of coastal plant communities for climate change mitigation and adaptation[END_REF][START_REF] Duarte | Export from seagrass meadows contributes to marine carbon sequestration[END_REF][START_REF] Krause-Jensen | Substantial role of macroalgae in marine carbon sequestration[END_REF] in contrast to the ~58 Gt C yr -1 of phytoplankton production [START_REF] Buitenhuis | Combined constraints on global ocean primary production using observations and models[END_REF][START_REF] Middelburg | Marine carbon biogeochemistry: a primer for earth system scientists[END_REF]. However, the lability of phytoplankton carbon and the high rate of remineralisation through consumption and degradation mean that only in the region of 0.23 Gt C yr -1 (0.4%) of phytoplankton production becomes sequestered into carbon stores or sinks, whilst the lower rate of consumption and refractory chemical nature of macroalgal detritus mean that ~0.17 Gt C yr -1 (11.4%) is sequestered. Thus, including J o u r n a l P r e -p r o o f seagrasses, marine macrophytes potentially contribute a comparable mass to direct global carbon sequestration to that of phytoplankton. Species of the order Laminariales are particularly important contributors to coastal primary production [START_REF] Kirk | Light and photosynthesis in aquatic ecosystems, 2nd edn[END_REF]) and form key constituents of shallow subtidal kelp forests across four continents [START_REF] Steneck | Kelp forest ecosystems: biodiversity, stability, resilience and future[END_REF][START_REF] Teagle | The role of kelp species as biogenic habitat formers in coastal marine ecosystems[END_REF]. Along North East Atlantic coastlines, the stipitate canopy-forming Laminaria hyperborea dominates shallow subtidal rocky ecosystems [START_REF] Smale | Variability in kelp forest structure along a latitudinal gradient in ocean temperature[END_REF]. Detrital production by L. hyperborea is unique because the majority of the previous-season's post-meristematic growth (the 'growth collar') is shed, often intact, between March and May [START_REF] Bartsch | The genus Laminaria sensu lato: Recent insights and developments[END_REF]; the release of this pulse of biomass contributes significantly to the total detritus production (Pessarrodona et al. 2018a).

L. hyperborea's detritus is a trophic resource connecting habitats which becomes increasingly accessible to consumers as it degrades [START_REF] Norderhaug | Trophic importance of Laminaria hyperborea to kelp forest consumers and the importance of bacterial degradation to food quality[END_REF] and is an especially important dietary subsidy during periods of low plankton productivity (Leclerc et al. 2013a).

As well as accumulating in coastal embayments and on beaches, a significant fraction of the carbon fixed by L. hyperborea flows into seafloor depressions, low-energy habitats and deep subtidal regions [START_REF] Filbee-Dexter | Spatial Patterns and Predictors of Drift Algal Subsidy in Deep Subtidal Environments[END_REF][START_REF] Filbee-Dexter | Movement of pulsed resource subsidies from kelp forests to deep fjords[END_REF]. Research on L. hyperborea has already revealed that its degradation is slow relative to other species and accumulations in shallow subtidal ecosystems are able to maintain photosynthetic activity across several months (de Bettignies et al. 2020b), acting as net producers for a sustained period of time dependent upon the illumination regime.

The conspicuous thermally tolerant congeneric kelp [START_REF] Franco | The 'golden kelp ' Laminaria ochroleuca under global change : Integrating multiple ecophysiological responses with species distribution models[END_REF][START_REF] Hargrave | The effects of warming on the ecophysiology of two co-existing kelp species with contrasting distributions[END_REF] of Lusitanian origin, L. ochroleuca (Bachelot de la Pylaie), has expanded its range northwards from Morocco and across the English Channel in response to the changing J o u r n a l P r e -p r o o f climate over the last century. Today, L. ochroleuca is expanding its leading edge eastwards and northwards at around 5.4 and 2.5 km per year, respectively [START_REF] Straub | The dynamic biogeography of the Anthropocene: The speed of recent range shifts in seaweeds[END_REF]. Its range now overlaps substantially with the native assemblage's dominant kelp species, L. hyperborea [START_REF] Smale | The rise of Laminaria ochroleuca in the Western English Channel (UK) and comparisons with its competitor and assemblage dominant Laminaria hyperborea[END_REF][START_REF] Hargrave | The effects of warming on the ecophysiology of two co-existing kelp species with contrasting distributions[END_REF] which has undergone a ~250 km range contraction at its warm leading-edge since 1970 [START_REF] Assis | Future climate change is predicted to shift long-term persistence zones in the cold-temperate kelp Laminaria hyperborea[END_REF]. In comparison to the pulsed detrital production from L. hyperborea, detrital production by L. ochroleuca is greater in volume, more frequent and degrades faster between May and October (Pessarrodona et al. 2018b). The expanding thermal envelope and subsequent spread of L. ochroleuca is modifying both the taxonomic and functional composition of HFS, with resulting effects on detrital composition. The re-arrangement of HFS has been shown to have marked ecological implications; extending to adjacent communities which rely on these subsidies of allochthonous material [START_REF] Bishop | Cross-habitat impacts of species decline: Response of estuarine sediment communities to changing detrital resources[END_REF][START_REF] Straub | The dynamic biogeography of the Anthropocene: The speed of recent range shifts in seaweeds[END_REF].

The ecological function of detritus varies between species. Recent studies (Pessarrodona et al. 2018b) comparing L. ochroleuca and L. hyperborea have demonstrated interspecific differences in seasonal detrital decomposition rates, with L. ochroleuca exhibiting significantly faster rates (6.5 x) compared to L. hyperborea (Pessarrodona et al. 2018b) in spring. In addition, the species exhibit differences in timing of detrital production: the May cast from L. hyperborea accounting for ~40-60% of annual lamina erosion (Pessarrodona et al. 2018a) compared with the continual release of detritus from the fronds of L. ochroleuca (Pessarrodona et al. 2018b). Such alterations to the supply and cycling of organic matter will likely be amplified as sea surface temperatures increase. In the North East Atlantic, a rise of > 2 o C is forecast in the next 100 years [START_REF] Philippart | Impacts of climate change on European marine ecosystems: Observations, expectations and indicators[END_REF][START_REF] Masson-Delmotte | IPCC Summary for Policymakers[END_REF]), which will likely result in the gradual replacement of the native cool-water L. hyperborea by the warm-water tolerant L. ochroleuca.

J o u r n a l P r e -p r o o f

Degradation is dynamic and testing the response of detritus across different environmental conditions is important to contextualise this process. The depth distribution of photosynthetic kelp-forest communities is strongly influenced by light availability [START_REF] Kirk | Light and photosynthesis in aquatic ecosystems, 2nd edn[END_REF][START_REF] Gorman | Modeling kelp forest distribution and biomass along temperate rocky coastlines[END_REF][START_REF] Bajjouk | Multiapproach mapping to help spatial planning and management of the kelp species L. digitata and L. hyperborea: Case study of the Molène Archipelago, Brittany[END_REF] and the photosynthetic mechanisms of some kelp species have been demonstrated to respond to changes in underwater light (e.g. in Saccharina latissima; and L. digitata) [START_REF] Gevaert | Photo-inhibition and seasonal photosynthetic performance of the seaweed Laminaria saccharina during a simulated tidal cycle: Chlorophyll fluorescence measurements and pigment analysis[END_REF][START_REF] Delebecq | In situ photosynthetic performance of J o u r n a l P r e -p r o o f Laminaria digitata (phaeophyceae) during spring tides in northern Brittany[END_REF]. L. hyperborea and L. ochroleuca forests have similar natural depth range but little is known about the photosynthetic efficiency of these two species within their depth thresholds. Investigating the degradation of kelp detritus across a gradient of depth will further elucidate its role in the marine environment.

The aim of this study therefore was to monitor the break-down dynamics and physiological performance of material from L. hyperborea and L. ochroleuca across a gradient of light attenuation to investigate the influence of depth across time. We examined biomass dynamics, oxygen production, respiration and photosynthetic performance and efficiency of photosystem II of detritus from the two species. We hypothesized that Laminaria species have the capacity to resist degradation, maintaining primary production function depending upon the illumination regime. Following the findings of Pessarrodona et al. (2018a) we hypothesised that the two species should exhibit differential responses with respect to degradation, with a quicker decomposition and breakdown of physiological capacity for L. ochroleuca compared to L. hyperborea. We also speculated that there would be an effect of the amount of light, and for the first time, empirically tested the influence of depth-related light attenuation as a potential driver of variability in the degradation of kelp material.

J o u r n a l P r e -p r o o f

MATERIAL AND METHODS

The study was conducted in the Roscoff Biological Station close to the Bay of Morlaix (France). In May 2018, L. hyperborea was harvested at La Veille (48°42'36.22''N, 3°54'04.81''W) and L. ochroleuca at An Nehou (48°41'34.27''N, 3°56'25.52''W) from ~ 8-10 m below chart datum. Five typical mature canopy-forming individuals of each kelp species were harvested by severing the stipe just above the holdfast. The mean total length of L. hyperborea individuals was 348 cm compared to L. ochroleuca individuals with a mean length of 169 cm. Within two hours of collection, the sporophytes were submerged in the dark in a 500 L flow-through tank of aerated ambient sea water pumped directly from the Bay and left to acclimate for 48 hours.

Experimental design

For each adult individual, three blade fragments of equal length (20 cm) were cut at a standardised distance (20 cm) above the meristem, away from the distal section of the kelp blade which is subject to greater epiphytic colonisation. Fragments were tagged with a unique ID number, their mass recorded and were randomly allocated to a depth (0, 15 and 30 m) treatment. Thus there were five replicates per species within each depth treatment.

To simulate different light intensities across a depth gradient, light attenuation optical filters (LEE Filters, Andover, UK) were constructed to completely cover the aquaria (Fig. 1). A photon flux simulating light availability at 15 and 30 m was recreated using an average light extinction coefficient (k) of 0.14 (from [START_REF] Boutler | Quelques observations sur la pénétration de la J o u r n a l P r e -p r o o f lumière dans les eaux marines au voisinage de Roscoff et ses conséquences écologiques[END_REF]spring The irradiance at depth (t) I (t) depends on the light at the surface I (Z0) , the depth (z) and the extinction coefficient of the water in the column (k) . The 0 m treatment, representing strandline and floating detritus, were submerged in 8 cm of water hence z = 0.08 m. The percentage of light available at 0, 15 and 30 m was 98.9%, 12.3% and 1.5% respectively;

these values were utilised when assembling the relevant filters. We did not, however, account for the change in spectral character as a function of depth [START_REF] Saulquin | Archimer Estimation of the diffuse attenuation coefficient K dPAR using MERIS and application to seabed habitat mapping[END_REF]. The fragments acclimated for a further 48 hours before being transferred to 8 L aquaria corresponding to their respective species by depth treatment where they remained for the J o u r n a l P r e -p r o o f duration of the experiment. Each aquaria was installed with a wide mesh basket which held the fragments in 8 cm of water and raised them 5 cm from the bottom. The baskets facilitated water circulation whilst constant aeration promoted diffusion across the boundary layer [START_REF] Noisette | Abiotic and biotic interactions in the diffusive boundary layer of kelp blades create a potential refuge from ocean acidification[END_REF]. The six aquaria were held in a circulating water bath system supplied with a continual flow of unfiltered seawater (renewal rate of 10 times the aquarium volume per hour) located outside (Fig. 1). The fragments were therefore exposed to ambient temperature and irradiance in order to re-create their natural environment. Two aquaria were fitted with temperature/light loggers (HOBO Pendant Temperature/Light Weatherproof Pendant Data Logger 16K) recording at 10-minute intervals. Data was regularly downloaded to ensure conditions did not deviate significantly from outside conditions. Physiological measurements (see below) began 9 days after the fragments were transferred into the experimental system and observations were repeated at 7-day intervals thereafter across fivetime periods, (T1-T5) with a 19 day gap between T5 and T6, for a total duration of 56 days.

Reproductive tissues and biomass change

Photographs of each fragment were taken at each time point to monitor visual changes in tissue surface (Figure S1), for example, the presence of sorus (reproductive) tissue (Figure S2). Repeated measures of wet biomass were taken on the same fragment at each time point.

Biomass changes were calculated as a percentage of the starting mass remaining at each time point. Completely degraded fragments were removed and their biomass from the preceding time was recorded as mass lost. This aspect of the trial was extended to 108 days and final biomass measurements were recorded after all the fragments from the 30 m treatment had degraded.

J o u r n a l P r e -p r o o f

Production and respiration measurements

Production and respiration rates were measured using a closed bottle respirometry technique [START_REF] Migné | A closed-chamber CO2-flux method for estimating intertidal primary production and respiration under emersed conditions[END_REF][START_REF] Biscéré | High pCO2 promotes coral primary production[END_REF]. Twenty incubation chambers, each comprising of a 1.2 L transparent glass jar closed by a watertight seal, were distributed across four 50 L experimental units. The experimental units were each fitted with optical filters replicating light conditions in the corresponding aquaria and were replenished with the same ambient unfiltered seawater after each set of production and respiration incubations. Fragments were transferred in the dark from their aquaria into the corresponding depth treatment of the experimental unit with each jar containing an individual fragment. Two different incubations were conducted consecutively, measuring both oxygen production and consumption (respiration) respectively. Production was measured during a 60-minute incubation (long enough to allow detection of a change in oxygen concentration whilst avoiding oxygen saturation) under the experimental aquarium conditions. The jars were then opened to allow water exchange for 30 minutes and the containers were covered by dark tarpaulin to halt photosynthetic activity. The jars were subsequently re-sealed and the respiration incubations were conducted on the same fragments for 60 minutes in complete darkness. Dissolved oxygen concentrations were measured before and after both incubations (production and respiration) using a portable multi-meter (HQ40d, Hach®, Loveland, USA) coupled with a luminescent/optical dissolved oxygen probe (Intellical™ LDO101, Hach®, accuracy ± 0.2 mg L -1 ). At the end of the incubations, fragments were retrieved from the jars, gently blotted dry and weighed (WW in g). Production and respiration rates were estimated by calculating the difference between initial and final oxygen O 2 concentrations after being corrected for temperature change (Aminot and Kérouel, 2004). The rates were expressed in mgO 2 kg WW - (GPP) was calculated by adding the dark respiration rate (R) to the NPP to derive the total oxygen produced via photosynthesis. The seawater temperature and incident photosynthetically active radiation (PAR, 400-700 nm, µmol photons m -2 s -1 , Li-Cor QuantumSA-190, LI-COR®, Lincoln, USA), were recorded every minute during the trials.

Photosynthetic efficiency measurement

To reveal to the extent to which fragments' photosynthetic apparatuses were active, the operating efficiency of photosystem II (ΦPSII) and the maximal quantum yield of PSII photochemistry (F v /F m ) were measured using a portable pulse-amplitude-modulated fluorometer (PAM, Heinz Walz, Effeltrich, Germany). In vivo ΦPSII was measured in ambient light whereas F v /F m measurements were obtained from fragments that had been dark adapted for 15 minutes (following [START_REF] Hargrave | The effects of warming on the ecophysiology of two co-existing kelp species with contrasting distributions[END_REF], before being flashed with a 0.8 s saturating white light pulse (2500 μmol photons.m -2 s -1 ). F v /F m values normally range between 0.7-0.8 for Phaeophyceae and values below are indicative of a stress response [START_REF] Bischof | Acclimation of maximal quantum yield of photosynthesis in the brown alga Alaria esculenta under high light and UV radiation[END_REF][START_REF] Hanelt | Photosynthesis assessed by chlorophyll fluorescence[END_REF] 2.5 Statistical analyses

Biomass change

As a consequence of bimodality in the bounded biomass data, a Euclidean distance-based permutational ancova (PERMANCOVA) was used in PRIMER ver 6.1 (Primer-E, Plymouth)

to test effects of species and depth (fixed factors) over time (continuous covariate), with fragment identity (the plant from which the fragment was cut) as a random factor. P values J o u r n a l P r e -p r o o f

were obtained from type III sums of squares. MDS plots were inspected as a way of identifying potential issues relating to heterogeneities of dispersion, though no formal tests were performed due to the complexity of the model.

Production, respiration and photosynthetic parameters

The effects of species and depths across time on net and gross oxygen production (NPP and GPP), dark respiration (R) and combining NPP with 12 h of R to represent a full day's rate of oxygen flux (NPP-R), were tested with linear mixed effect models using Maximum Likelihood (package lme4 in R 3.6.1) [START_REF] Bates | Fitting Linear Mixed-Effects Models Using lme4[END_REF]; R Core Team 2019). To account for variation in abiotic conditions across the experiment, mean PAR was used as a continuous covariate for oxygen production and mean temperature for dark respiration (PAR and temperature were strongly correlated). Fragment identity was included as a random factor and p values were obtained from type III Wald χ 2 tests. Plots of residuals versus fits were used to check assumptions of analyses. Data describing the photosynthetic parameters Fv/Fm and ΦPSII were analysed by similar model structure using generalised least squares (gls) fitting (package nlme, [START_REF] Pinheiro | Package 'nlme': Linear and Nonlinear Mixed Effects Models[END_REF] due to heterogeneities of variance; PAR was again used as a continuous covariate; no random factor could be included in the gls model, betweenplant variability being accounted for in the overall model fit. P values were derived as described above.

Omnibus PERMANCOVA

To examine the in toto physiological response of the two species, a multivariate approach was employed. A z-transformation was used to normalise the physiological variables (GPP, J o u r n a l P r e -p r o o f R, Fv/Fm and ΦPSII), and a Euclidean distance -based multivariate similarity matrix was produced in PRIMER ver 6.1. This was then used as the basis for a mixed model PERMANCOVA with 999 permutations of a reduced model employing the same structure as the linear models described above; p values were obtained from type III sums of squares using a pseudo-F statistic.

RESULTS

Degradation dynamics

The analysis of biomass change with depth indicated strong heterogeneity of response to the environmental factor over time in the different species (3-way interaction pseudo-F 2,144 = 12.857, p <0.001; Fig. 2); no evidence of confounding heterogeneity of dispersion was apparent in the MDS plots. The interspecific differences were most apparent in the 15 and 30 m treatments, in both cases some of the L. hyperborea fragments degraded more quickly than L. ochroleuca. Under surface conditions neither species degraded markedly until after 56 days. No intact fragments of L. ochroleuca remained at the end of the 108 days, while L. hyperborea fragments were generally still intact at this point. With light attenuated to levels at 15 m, L. hyperborea began to degrade after 23 days, whilst L. ochroleuca persisted until the end of the study in the majority of cases. At 30 m most L. hyperborea fragments had completely broken down after 36 d, in contrast to L. ochroleuca where degradation was significantly slower (Fig. 2, Table 1).

Reproductive tissues and biomass change

J o u r n a l P r e -p r o o f

Clear evidence of sorus material was first detected whilst conducting the final physiological measurements, 56 days into the experimental period on two L. ochroleuca fragments, one in the 0 m treatment and one in the 15 m treatment. When the study was concluded 108 days later, two different L. ochroleuca fragments had evidence of sorus material on the blade surface (Figure S2).

Production and respiration measurements

As expected, the availability of light was the dominant factor affecting GPP; both in terms of PAR at the surface (χ 2 1,161 = 15.735, p < 0.001) and as a main effect of photo-attenuation related to depth (χ 2 2,161 = 92.036, p < 0.001). Contrary to expectation, however, across the degradation period GPP did not decline as expected, and whilst the two species responded differently to the passage of time (χ 2 1,161 = 5.122, p = 0.024), it was apparent that overall oxygen evolution was indicative of the metabolic integrity of surviving tissues remaining remarkably intact often beyond 36 days post-detachment (Fig. 3a, Table 2).

Respiration (R) was independent of temperature, but differed strongly between species (χ 2 1, ,161 = 22.555, p < 0.001), and as a function of time (χ 2 1,161 = 8.462, p = 0.004) and depth (χ 2 2,161 = 10.276, p = 0.006), modifying the species effect. Notwithstanding the declining number of fragments persisting to the end of the trial (Fig. 3), it was clear that R did not display a simple response over time; only the data for L. hyperborea under surface light levels are suggestive of a simple linear increase over time (Fig. 3b, Table 2).

Most importantly, Net Primary Production (NPP) during lit periods remained at, or recovered to, positive net production levels in the majority of fragments until the end of the 56 day trial (Fig. 3c, Table 2). As with GPP and R, NPP differed strongly at different depths (χ 2 1,161 = 88.31, p < 0.001). NPP did not differ statistically between the two species, but the nature of this experiment restricts these conclusions to brightly lit daylight hours. Assuming J o u r n a l P r e -p r o o f equinoctial fully light and dark conditions, with NPP and an additional 12 h of R representing a full day's oxygen flux, suggests a significant effect of species with an additional modifying effect of depth upon this (χ 2 2,161 = 6.244, p = 0.044). Under this '24 h' model, a more realistic interpretation of differential GPP responses between the species can be detected (Fig. 3d,

Photosynthetic response 388

PAR was a significant covariate for the ΦPSII response (χ 2 1 = 7.901, p < 0.01). Despite time 389 exerting a non-statistically significant effect on ΦPSII (as it did upon NPP and GPP), the 390 response of detrital fragments clearly declined over the degradation period (Fig. 4a) but 391 became highly variable at increasing depth, swamping any trend. Fig. 4b shows that F v /F m 392 displayed a similarly noisy decline over time. There was no significant difference in the 393 photosynthetic response between the species. Both parameters were significantly affected by 394 depth. No interactions were significant and therefore the photosynthetic behaviour of detrital 395 resources from different species was not affected by time or by depth (Figs 4 a-b, Table 3).

Omnibus permanova

The combined response of GPP, R, ΦPSII and F v /F m variables was strongly affected by the light availability in terms of PAR (χ 2 1 = 11.176, p < 0.001) and Depth (χ 2 1 = 21.585, p < 0.001). A realistic picture of detrital degradation revealed species responded differently to the passage of time (χ 2 1 = 9.327, p < 0.001) but not depth (Table 4).

DISCUSSION

Production and respiration measurements

For the first time, the primary producer functions of algal detritus have been quantified across a simulated depth spectrum. Detritus continued to sustain net production of oxygen across a 56-day period, underscoring its capacity to maintain key functions such as carbon fixation.

Detrital tissues slightly increased in biomass during the early stages of degradation, demonstrating that measured oxygen production is reflective of fresh material being generated. Over time, the respiration of detrital fragments, as a proxy for stress or associated bacterial activity, increased in some contexts [START_REF] Sosik | Isotopic evidence and consequences of the role of microbes in macroalgae detritus-based food webs[END_REF]. Evidence of some degradation was further reinforced by a measured decline in photosynthetic performance (ΦPSII and F v /F m ), in accordance with the initial hypothesis. At the end of the first time period, values between 0.6-0.8 at 15 m and 30 m reflected an optimum performance at depths at which the kelps inhabit [START_REF] Gorman | Modeling kelp forest distribution and biomass along temperate rocky coastlines[END_REF]. Low photosynthetic parameters at 0 m follow reported trends in the literature of decreases in both measurements with increasing irradiances (for Laminaria digitata, [START_REF] Delebecq | In situ photosynthetic performance of J o u r n a l P r e -p r o o f Laminaria digitata (phaeophyceae) during spring tides in northern Brittany[END_REF].

Respiration is a central process in kelp decomposition (de Bettignies et al. 2020b) and measured increases in the rate of oxygen consumption were mirrored by a decline in the J o u r n a l P r e -p r o o f detrital biomass. However, it is important to note that the measurements reflect the holobiont respiration of Laminaria tissue together with its ensemble of microorganisms [START_REF] Baedke | The holobiont concept before Margulis[END_REF]. The results of this study reveal that the respiration response differed between the two species across time. Detritus from L. hyperborea exhibited higher rates of respiration compared to L. ochroleuca over time in the 30 m treatment and only under surface conditions was a linear increase in respiration detected for L. hyperborea. This differential response between the species may be indicative of stress or higher biofilm activity associated with L.

hyperborea fragments. L. hyperborea degraded faster than L. ochroleuca at 15 m and 30 m, suggesting that low light conditions may impede the regeneration of damaged tissue for L.

hyperborea which points to the role of light in determining the detrital fate of some species [START_REF] Hader | Effects on aquatic ecosystems[END_REF][START_REF] Swanson | Altered kelp (Laminariales) phlorotannins and growth under elevated carbon dioxide and ultraviolet-B treatments can influence associated intertidal food webs[END_REF]. Furthermore, in some instances, during the initial stages of degradation at 0 and 15 m, L. ochroleuca positively increased in biomass compared to L. hyperborea (Fig. 2). However, this trend was reversed after 56 days in the 0 m treatment when all L. ochroleuca fragments were completely degraded in sharp contrast to the relatively intact fragments of L. hyperborea. 4.2 Considering the first 40 days of this trial, this result is comparable, yet contrary to the findings of Pessarrodona et al.'s (2018b) in situ study comparing the biomass loss from the same study species. Pessarrodona et al. (2018b) report that L. ochroleuca lost biomass at a faster rate than L. hyperborea in spring, prior to the latter shedding of old fronds. The present study separated the change in mass across regular time intervals and elongated the temporal scale of degradation to 108 days, as well as eliminating physical erosion as a mechanism of degradation, casting light on the purely organic dimension of how kelp matter breaks down. Our results suggest that L. hyperborea has the potential to be a more persistent spatial subsidy and thus may reside in the ecosystem for longer under attenuated illumination. However, hydrodynamics and physical processes J o u r n a l P r e -p r o o f such as abrasion and grazing may further confound elucidation of the decomposition process. [START_REF] Nielsen | Decomposition Of Marine Primary Producers: Consequences For Nutrient Recycling And Retention In Coastal Ecosystems[END_REF][START_REF] Braeckman | Degradation of macroalgal detritus in shallow coastal Antarctic sediments[END_REF].

Contextualizing the findings

These differences in persistence will become increasingly important as L. ochroleuca gradually replaces L. hyperborea, in habitats where the two species coexist, [START_REF] Teagle | Climate-driven substitution of habitat-forming species leads to reduced biodiversity within a temperate marine community[END_REF] and will be further exacerbated by the seasonal modification of detrital production between the species; ultimately affecting the quality and supply of organic kelp derived material entering the detrital pathway [START_REF] Bishop | Cross-habitat impacts of species decline: Response of estuarine sediment communities to changing detrital resources[END_REF]. The extent to which benthic ecosystems in the North East Atlantic are dependent on detritus derived from L.

hyperborea remains unquantified. However, modifications in the turnover of organic material, transport [START_REF] Pedersen | Biomass dynamics of exotic J o u r n a l P r e -p r o o f Sargassum muticum and native Halidrys siliquosa in Limfjorden, Denmark -Implications of species replacements on turnover rates[END_REF], trophic connectivity (Leclerc et al. 2013b), organic matter content [START_REF] Abdullah | The impact of the kelp (Laminaria hyperborea) forest on the organic matter content in sediment of the west coast of Norway[END_REF]) and a homogenization of the nutrient supply from kelp detritus derived from different species will likely have ramifying effects across food webs.

Extending the investigation to consider the degradation process across environmental contexts revealed that other measured physiological processes of kelp detritus (NPP, GPP and ΦPSII and F v /F m ) are not only species specific but also highly dependent on light availability.

Depth significantly affected 24-hour oxygen flux and the Species response to the effect of Depth was significantly heterogeneous in the case of both respiration and the 24-hour model of oxygen flux (Fig. 3b andd). Therefore, considering the dynamic nature of detrital transport, the depth to which detritus is exported significantly determines the carbon fixation, respiration and photosynthetic performance of kelp material (Filbee-Dexter and Scheibling, 2016). Furthermore, detritus was clearly less productive as a function of decreasing irradiance J o u r n a l P r e -p r o o f from 0 m > 15 m > 30 m (Fig. 4a). Photosynthetic capacity was critically impaired at 0 m, probably as a result of photoinhibition [START_REF] Delebecq | In situ photosynthetic performance of J o u r n a l P r e -p r o o f Laminaria digitata (phaeophyceae) during spring tides in northern Brittany[END_REF], but remained mostly invariant between 15 and 30 m. Although the activity of the photosynthetic apparatus was maintained in low light conditions, the decline in oxygen production is indicative that detritus is a less effective primary producer at 30 m. The differing responses of respiration in the two species over time and at different depths highlight that the process of degradation needs to be considered across different environmental conditions with a temporal dimension. Species' respiration fluctuated across time and their response was dependent on light attenuation, which indicates that one species may have the physiological capacity to persist for longer than its congener at a specific depth.

Re-defining kelp detritus

An unexpected result was that L. ochroleuca fragments developed visible sorus tissue at the tissue surface indicating that such fragments remain capable of maintaining reproductive functions (Fig S1). Moreover, this was detected first for two fragments during the final time period (T6) in July.

Visible sorus remained at the blade surface two independent L. ochroleuca fragments when the experiment ended 108 days post fragment preparation in September. This time window is in accordance with the reproductive period for L. ochroleuca [START_REF] Pereira | Life history traits of Laminaria ochroleuca in Portugal: The range-center of its geographical distribution[END_REF] science. Although such tissues may be no longer attached, and as our data suggests, may be in the process of slowly entering the detrital food web, they remain 'alive' and physiologically competent for much longer than would be typical for tissues of land plants.

This 'productive necromass' -physiologically persistent and viable, unattached or fragmented algal biomass -continues to photosynthesize and, in the right conditions, can continue to fix carbon for months after it would otherwise be inaccurately defined as detrital.

Our study has demonstrated that kelp detritus can sustain productivity for long periods of time and therefore detrital contribution to Blue Carbon is likely to be much greater than has been previously accounted for. Accordingly, incorporating productivity from macroalgal detritus could account for important increases to the contribution of macroalgae in the Blue Carbon economy.

The role of kelp detritus in benthic ecosystems

J o u r n a l P r e -p r o o f

The vast majority of kelp production enters the detrital pathway (about 82% according to Krumhansl and Scheibling 2012), therefore fundamental variability in the respiration and photosynthetic performance of kelp material across time and under different environmental conditions are important for understanding the persistence and productivity of detrital resources. The composition of kelp forests and thus their detritus, will likely be modified as climate warming persists [START_REF] Smale | The rise of Laminaria ochroleuca in the Western English Channel (UK) and comparisons with its competitor and assemblage dominant Laminaria hyperborea[END_REF], with the gradual increase of HFS with high thermal affinities and gradual local extinctions of native HFS with cooler thermal affinities [START_REF] Wiens | Climate-related local extinctions are already widespread among plant and animal species[END_REF]). These replacements will likely impact many aspects of the cycling of organic material, from the spatial subsidies exported kelp matter provides, to the fitness of grazers and detritivores feeding upon it, and the rate of carbon sequestration into longer residence forms in the benthos. Pessarrodona et al. (2018b) speculated about an alteration in the functional importance of this detrital material due to a variation in the rate of supply from different kelp species. Here we have shown that the continued biological activity of fragmented kelp tissues also differs between species, across time and environments, further altering the contribution of kelp mass to spatial subsidies (Leclerc et al. 2013a).

This study provides a snapshot of the degradation dynamics of two kelps under controlled conditions. There are numerous potential sources for error; for example a change in detrital respiration could be attributed an upregulation of metabolic processes in L. hyperborea or to an increase in oxygen consuming saprophytic bacteria and fungi [START_REF] Fenchel | Detritus Food Chains of Aquatic Ecosystems: The Role of Bacteria[END_REF][START_REF] Williams | Respiration in aquatic ecosystems[END_REF]). Furthermore, variation in species physiology cannot be generalized across the entirety of benthic ecosystems in the North East Atlantic because despite advances in areas of lowmoderate wave exposure [START_REF] Smale | The rise of Laminaria ochroleuca in the Western English Channel (UK) and comparisons with its competitor and assemblage dominant Laminaria hyperborea[END_REF], L. ochroleuca may not be able to fully replace L.

hyperborea in sites exposed to high wave exposure (Pessarrodona et al. 2018b) and therefore the composition of detritus from such assemblages may remain relatively unchanged. Also, the role of However, we argue that detrital accumulations can exert measurable impacts on ecosystem functioning and deserve enhanced attention.

CONCLUSION

J o u r n a l P r e -p r o o f

  and summer surveys J o u r n a l P r e -p r o o f around Roscoff) incorporated into equation 1, where light availability at a specified depth equates to: exp equation 1.

Fig. 1 .

 1 Fig. 1. Experimental set-up composed of filters for 15 and 30 m aquaria, constant air

Fig. 2 .

 2 Fig. 2. Remaining biomass at three different simulated depths over time in Laminaria

Fig. 3 .

 3 Fig. 3. (a) Gross Primary Production (GPP), (b) Respiration (R), and (c) Net Primary 376

Fig. 4 .

 4 Fig. 4. (a) ΦPSII and (b) F v /F m measurements at three different simulated depths over time

  macrofaunal colonization and consumption(de Bettignies et al. 2020; Ramirez-Llodra et al. 2016) of J o u r n a l P r e -p r o o f the kelps in question could change the degradation dynamics completely.Hargrave et al. (2016) reported lower concentrations of grazer-deterring polyphenolics and flavonoids in L. ochroleuca than in another common NE Atlantic congener: L. digitata, with higher grazing rate by herbivorous gastropods in the former. Meanwhile,Pessarrodona et al. (2018b) found a similar herbivore preference for L. ochroleuca over L. hyperborea, recording a faster degradation of the former. If detritivore preference for the more palatable L. ochroleuca mirrors that of grazers, then the reconciliation of the slower degradation of L. ochroleuca in the earlier stages of the present study with the faster in situ degradation of L. ochroleuca observed byPessarrodona et al. (2018b) could be attributed to mechanical effects, especially of saprovores aiding conversion of frond tissue into coarse particulate organic matter and facilitating ingress by saprophytic micro-organisms. Future study should therefore aim to evaluate the importance of the ecological consequences of environmentally realistic deposits of macroalgal detritus with their attendant bacterial and detritivorous assemblages and also continue to compare the detrital compositions from species at risk of local extinction from a variety of range-expanding or invasive algae[START_REF] Sorte | Marine range shifts and species introductions: Comparative spread rates and community impacts[END_REF], taking into account different environmental contexts in which kelp degradation may occur, testing the degradation response in situ across a gradient of turbulence, and explicitly focusing on detritivores as a driver of tissue fragmentation and a facilitator of micro-organismal colonization. These questions have indeed been addressed in other contexts spanning from shallow shorelines to the deep sea whilst the shallow subtidal has often been considered as a mere transitional deposition area.

Table 1 :

 1 Results of permanova analysis (type III SS) of % biomass remaining in the different

	species and depths (fixed factors) over 108 days (continuous covariate). Significant terms
	(P<0.05) are indicated in bold.
	Source	Df	Pesudo-F	P
	Time	1	119.37	0.001

Table 2

 2

	).

d J o u r n a l P r e -p r o o f

Table 2 :

 2 Results of likelihood ratio tests based upon linear mixed effect models examining 382

	383 384 385 386	responses of (a) Gross Primary Production (GPP), (b) Respiration, (c) Net Primary Production (NPP) and (d) '24 hour' model of oxygen flux (NPP-R) rates across time (56 days), between kelp species and depth (fixed factors) with PAR or Temperature as a covariate. Significant terms (P < 0.05) are indicated in bold. Source df Chisq P (a) GPP PAR 1 15.735 < 0.001 Time (days) 1 0.566 0.452 Species 1 0.000 0.996 Depth 2 92.036 < 0.001 Time: Species 1 5.122 0.024 Species: Depth 2 1.157 0.561 J o u r n a l P r e -p r o o f
		Time: Depth	2	6.316	0.043
		Time:Species:Depth	2	4.058	0.131
		Fragment ID	1	9.664	0.002
		(b) R
		Temp	1	0.098	0.754
		Time (days)	1	6.110	0.013
		Species	1	22.555	< 0.001
		Depth	2	2.234	0.327
		Time:Species	1	8.462	0.004
		Species: Depth	2	10.276	0.006
		Time: Depth	2	1.369	0.504
		Time: Species: Depth	2	0.047	0.977
		Fragment ID	1	3.727	0.054
		(c) NPP
		PAR	1	9.160	0.002

Table 3 :

 3 Results of likelihood ratio testing based upon generalized least squares fitting to examine responses of (a) ΦPSII (b) F v /F m across time (56 days), between kelp species and depths (fixed factors) with PAR as a covariate. Significant terms (P < 0.05) are indicated in bold.

	a		Species	Laminaria hyperborea	Laminaria ochroleuca
			0m			15m	30m
		0.8
		0.6
	m
	v F	0.4
	F
		0.2
		0.0
		9	16 23 30 36 56	9	16 23 30 36 56	9	16 23 30 36 56
						Days
			Species	Laminaria hyperborea	Laminaria ochroleuca
			0m			15m	30m
		0.8
		0.6
	φ PSII	0.4
		0.2
		0.0
		9	16 23 30 36 56	9	16 23 30 36 56	9	16 23 30 36 56
						Days

b

J o u r n a l P r e -p r o o f

Table 4 :

 4 Results of omnibus PERMANOVA analysis of standardized data describing GPP, R, ΦPSII and F v /F m across time (56 days), between kelp species and depth (fixed factors) with PAR as a covariate. Significant terms (P < 0.05) are indicated in bold.

		J o u r n a l P r e -p r o o f
	Source	df	Pesudo-F	P

  , further revealing the metabolic competency of the kelps post-detachment. Further research would be necessary to determine spore viability from such tissues. de Bettignies et al. (2020b) reported a similar phenomenon for L. hyperborea in October after subjecting detrital fragments to 5 months of degradation in situ. In terms of reproductive capacity, some of the energy generated by detrital , then kelp populations structure is likely to be far more fluid than would be indicated by typical spore or gamete dispersal, which is low for individual sporophytes within a kelp forest(~ 5- 200 m, Fredriksen et al.1995). This phenomenon compliments the functional role of herbivores in increasing fertilisation success and dispersing kelp spores which germinate post-digestion(Ruz et al.

	2018).
	Another important outcome of the present study and other recent work on physiological
	viability of macroalgal 'detritus' (de Bettignies et al. 2020b) is a questioning of the rationale
	behind defining this material as 'detritus'. Currently, cast or detached macroalgal fragments
	are considered as 'non-living' organic matter generated by the growth and production of
	living organisms (Hagen et al. 2012): a viewpoint that is based upon terrestrial botanical

photosynthesis is invested into the generation of reproductive tissue which potentially has considerable implications for the dispersal of these species. If current-borne kelp detritus can still J o u r n a l P r e -p r o o f reproduce

hr -1 . The production rate is a measure of Net Primary Production (NPP), which represents the sum of photosynthesis and respiration in tandem. Meanwhile, Gross Primary Production J o u r n a l P r e -p r o o f

ACKNOWLEDGEMENTS

We thank the marine operation staff at Roscoff Biological Station (Service Mer & Observation SBR) L. Levêque, Y. Fontana, W. Thomas, M. Camusat, N. Guidal and F. Le Ven for their help with scuba diving kelp collection and T. de Bettignies from the Natural History Museum of Paris. We also thank Marine Roscoff Aquarium Services (FR2424), especially R. Garnier and G. Schires for their help with the experimental design and set-up. E. Bocher and M. Kervellec for their assistance in the experimental work. NF, FdB and DD conceived the ideas and designed the study. FdB and NF built the experimental system. NF and FdB conducted the experimental manipulation. AF and NF analysed the data. NF and FdB wrote the first draft and AF extensively contributed to subsequent drafts. All authors gave final approval for publication.

FUNDING

This work was supported by the Brittany Regional Council and the French Government through the National Research Agency with regards to the investment expenditure program IDEALG [grant number ANR-10-BTBR-04].

DATA AVAILABILITY

Data will be archived in GitHub repository: https://github.com/nadiafrontier/Thedegradation-of-kelp-detritus.git

Kelp fragments remained physiologically competent for 56 days and possibly reproductively active for up to 108 days after they were cut from the thallus. The fragments' photosynthetic apparatus continued to function adequately to allow short-term (~ 56 days in L. ochroleuca) net gains in biomass which was sustained under ambient light levels. Light availability was the greatest determinant of photosynthetic performance, and net and gross primary production, whilst light and interspecific differences dominated the response of respiration.

The range-expanding L. ochroleuca broke down more slowly under attenuated illumination, challenging previous findings. We believe that a re-evaluation of recently formed macroalgal detritus as 'living material' is due and should certainly be considered when evaluating the functional integrity and spatial subsidies afforded by kelp forests. The predicted expansion of the thermally tolerant species, L. ochroleuca, and replacement of the less tolerant L. hyperborea across extensive areas is likely to alter the cycling of organic matter. Overall, these findings demonstrate the indispensable need to consider detritus as an important autotrophic resource, even after exportation, depending on the environmental conditions.

SUPPLEMENTARY INFORMATION

J o u r n a l P r e -p r o o f

Supplementary tables 1-4 contain abridged statistical tables following permanova, linear mixed effect models and generalised last squares analysis to support the summary statistics quoted in the manuscript. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: