Effects of nutrient limitations on the sinking velocity of Thalassiosira weissflogii
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Abstract: The sinking of diatoms is critical to the formation of oceanic biological pumps and coastal hypoxic zones. However, little is known about the effects of different nutrient restrictions on diatom sinking. In this study, we measured the sinking velocity (SV) of Thalassiosira weissflogii using a new phytoplankton video observation instrument and analyzed major biochemical components under varying nutrient conditions. Our results showed that the SV of T. weissflogii under different nutrient limitation conditions varied substantially. The highest SV of (1.77 ± 0.02) m/d was obtained under nitrate limitation, significantly surpassing that under phosphate limitation at (0.98 ± 0.13) m/d. As the nutrient limitation was released, the SV steadily decreased to (0.32 ± 0.03) m/d and (0.15 ± 0.05) m/d, respectively. Notably, under conditions with limited nitrate and phosphate concentrations, the SV values of T. weissflogii significantly positively correlated with the lipid content (P < 0.001), with R2 values of 0.86 and 0.69, respectively. The change of the phytoplankton SV was primarily related to the intracellular composition, which is controlled by nutrient conditions but did not significantly correlate with transparent extracellular polymer and biosilica contents. The results of this study help to understand the regulation of the vertical sinking process of diatoms by nutrient restriction and provide new insights into phytoplankton dynamics and their relationship with the marine nutrient structure.
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Figure 1. Cell abundance and growth rate data for the phosphate depletion-spike experiment (a) and nitrate depletion-spike experiment (b). PR, PL, PD, and PS represent phosphate repletion, phosphate limitation, phosphate depletion, and phosphate spike, respectively. NR, NL, ND, and NS represent nitrate repletion, nitrate limitation, nitrate depletion, and nitrate spike, respectively. R represents the post-recovery time, and five-time points 2 h, 6 h, 12 h, 24 h, and 48 h were recorded after the addition of limiting nutrients. These abbreviations apply to all figures.
Figure 2. Chlorophyll a concentrations and the changes in the optimal photochemical efficiency of photosystem Ⅱ (Fv/Fm) throughout the phosphate depletion-spike experiment (a) and nitrate depletion-spike experiment (b).
Figure 3. Limited nutrient concentrations and sinking velocity data for the phosphate depletion-spike experiment (a) and nitrate depletion-spike experiment (b).
Figure 4. Intracellular contents (protein, glucose-based carbohydrate, and lipid) data for the phosphate depletion-spike experiment (a) and nitrate depletion-spike experiment (b). The pie chart shows the proportion of each component in four different stages.
Figure 5. TEP concentrations in the nitrate depletion-spike experiment (orange pillars) and phosphate depletion-spike experiment (green pillars).
Figure 6. BSi concentrations in the nitrate depletion-spike experiment (purple pillars) and phosphate depletion-spike experiment (yellow pillars).
Figure 8. Correlation between SV and physiological-biochemical parameters for the phosphate depletion-spike experiment (a) and nitrate depletion-spike experiment (b). *: P < 0.05; **: P < 0.01; ***: P < 0.001.
Figure 9. Conceptual model to simulate the effect of nutrient limitation on the sinking velocity of Thalassiosira weissflogii in the Changjiang River Estuary. Owing to the input of diluted water from the Changjiang River, there is an excess of nitrogen and phosphorus nutrient salts. The nearshore waters of the Changjiang River Estuary are in the stage of nutrient repletion without nutrient limitation; however, with the increase in the spreading distance of the freshwater from the Changjiang River, nutrient concentration gradually decreases, and the phenomena of phosphate limitation and nitrate limitation occur successively. Small yellow circles represent nitrate concentrations, red circles represent phosphate concentrations, green triangles represent lipid content and blue shades represent extracellular products.
Table 1. Changes in Fv/Fm, surface area of a single cell, sinking velocity (SV), and the macromolecular composition, transparent extracellular polymeric particle (TEP) concentrations in Thalassiosira weissflogii for the nitrate depletion-spike and phosphate depletion-spike experiments
Nutrient
stageSV/
(m·d−1)Growth
rate/d−1Fv/Fm Surface area/
μm2Protein/
(mg·mg−1)Glucose-based carbohydrate/
(μg·mg−1)Lipid/
(mg·mg−1)TEP/
(μg·L−1) (Xeq)NR 0.21 ± 0.05 0.55 ± 0.03 0.69 ± 0.01 61.45 ± 14.46 0.34 ± 0.06 2.03 ± 0.52 0.12 ± 0.04 229.38 ± 53.11 NL 1.09 ± 0.13 0.10 ± 0.02 0.59 ± 0.03 73.43 ± 21.17 0.15 ± 0.03 3.58 ± 0.31 0.53 ± 0.07 261.96 ± 87.91 ND 0.35 ± 0.04 0.02 ± 0.05 0.34 ± 0.03 68.17 ± 23.01 0.37 ± 0.10 5.15 ± 0.61 0.28 ± 0.07 336.00 ± 46.30 NS 0.49 ± 0.07 0.04 ± 0.05 0.39 ± 0.02 83.01 ± 20.37 0.20 ± 0.05 6.07 ± 1.33 0.37 ± 0.05 316.50 ± 63.68 PR 0.17 ± 0.08 0.48 ± 0.06 0.69 ± 0.01 81.34 ± 19.04 0.32 ± 0.06 2.41 ± 0.65 0.10 ± 0.02 184.44 ± 62.55 PL 0.45 ± 0.06 0.12 ± 0.04 0.63 ± 0.01 77.77 ± 21.65 0.19 ± 0.05 2.52 ± 0.50 0.31 ± 0.03 344.78 ± 75.24 PD 0.35 ± 0.07 0.06 ± 0.08 0.42 ± 0.02 74.34 ± 24.13 0.38 ± 0.07 4.44 ± 1.25 0.33 ± 0.05 349.88 ± 48.56 PS 0.41 ± 0.07 0.10 ± 0.07 0.40 ± 0.02 95.59 ± 21.54 0.43 ± 0.09 5.55 ± 0.67 0.31 ± 0.03 313.59 ± 110.49 
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