|Degradation of diatom carbohydrates: A case study with N- and Si-stressed Thalassiosira weissflogii |
(Article) Publié: Journal Of Experimental Marine Biology And Ecology, vol. 470 p.1-11 (2015)
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Diatoms are a key phytoplanktonic group that affects the carbon cycle in the ocean. Although the effect ofnutrient limitation on the primary productivity of diatoms is well-studied, the effect of such a limitation on theorganic matter quality of diatoms and their vertical transport through the water column remains unclear. Inthis 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 ofTW 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 prokaryoticcounting. Using these results, we estimated the degradation rate constants for POC, PON, PCHO, and theindividual carbohydrate monomers (monosaccharides) for the TW cells. Our results indicated that the N- andSi-limitations increase the organic carbon content of the TW cells with a concomitant decrease in the siliconcontent (bSiO2), suggesting a modification of the TW cells. The PCHO content increased by a factor of 2.6 and3.8 in the N-stress and Si-stress cells, respectively. At the beginning of the experiment (T0), the N-stress andSi-stress cellswere characterized by higheramounts of glucose (5–32mol%) and xylose (13–19mol%), comparedwith the Nutrient-replete cells, which were dominated by ribose (~22 mol%), indicating differences in the physiologicalstatus of the TW cells and/or in the synthesis of storage/structural polysaccharides. The first orderdegradation 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 thenutrient-replete experiment. This result indicates a different behavior in the utilization and/or accessibility toprokaryotes of carbon and nitrogen during the biodegradation experiment. Moreover, ribose, glucose, andgalactose exhibited the highest degradation rate constants in all of the experiments, which further reflects thedifferences in their initial macromolecular origin (e.g., storage vs structural carbohydrates) and highlights thechanges in the organic matter quality during growth under nutrient-limited conditions and degradation. Theseresults 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.