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Degradation of diatom carbohydrates: A case study with N- and Si-stressed Thalassiosira weissflogii

Abstract : Diatoms are a key phytoplanktonic group that affects the carbon cycle in the ocean. Although the effect of nutrient limitation on the primary productivity of diatoms is well-studied, the effect of such a limitation on the organic matter quality of diatoms and their vertical transport through the water column remains unclear. In this study, the diatomThalassiosira weissflogii (TW)was grown under twodifferent nutrient conditions, ‘N-stress’ and ‘Si-stress’, and was compared against healthy TW cells (Nutrient-replete). Biodegradation experiments of TW were performed for all of the above conditions, and the particulate fraction was monitored over time (~one month) in terms of the organic carbon (POC), nitrogen (PON), and sugars (PCHO), including prokaryotic counting. Using these results, we estimated the degradation rate constants for POC, PON, PCHO, and the individual carbohydrate monomers (monosaccharides) for the TW cells. Our results indicated that the N- and Si-limitations increase the organic carbon content of the TW cells with a concomitant decrease in the silicon content (bSiO2), suggesting a modification of the TW cells. The PCHO content increased by a factor of 2.6 and 3.8 in the N-stress and Si-stress cells, respectively. At the beginning of the experiment (T0), the N-stress and Si-stress cellswere characterized by higheramounts of glucose (5–32mol%) and xylose (13–19mol%), compared with the Nutrient-replete cells, which were dominated by ribose (~22 mol%), indicating differences in the physiological status of the TW cells and/or in the synthesis of storage/structural polysaccharides. The first order degradation rate constants (k1) for the POC were similar in all of the experiments (k1 = 0.096–0.113 d−1), which was not the case for the PON in which the highest values (by a factor of ~2.5) were observed for the nutrient-replete experiment. This result indicates a different behavior in the utilization and/or accessibility to prokaryotes of carbon and nitrogen during the biodegradation experiment. Moreover, ribose, glucose, and galactose exhibited the highest degradation rate constants in all of the experiments, which further reflects the differences in their initial macromolecular origin (e.g., storage vs structural carbohydrates) and highlights the changes in the organic matter quality during growth under nutrient-limited conditions and degradation. These results suggest that the ‘nutrient-stress’ diatoms may affect the export of carbon (particularly carbohydrates) relative to nitrogen in the ocean interior compared with the diatoms grown under optimal conditions.
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Submitted on : Friday, May 15, 2015 - 6:51:44 PM
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Maxime Suroy, Christos Panagiotopoulos, Julia Boutorh, Madeleine Goutx, Brivaëla Moriceau. Degradation of diatom carbohydrates: A case study with N- and Si-stressed Thalassiosira weissflogii. Journal of Experimental Marine Biology and Ecology, Elsevier, 2015, 470, pp.1-11. ⟨10.1016/j.jembe.2015.04.018⟩. ⟨hal-01152313⟩



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