The response of diazotrophs to nutrient amendment in the South China Sea and western North Pacific
<p>The availability of iron (Fe) and phosphorus (P) has been shown to be a key factor regulating rates of nitrogen fixation in the western subtropical Pacific. However, the relative importance of Fe and P at finer spatial scales between the northern South China Sea (NSCS) and the western bound...
| Main Authors: | , , , , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2022-11-01
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| Series: | Biogeosciences |
| Online Access: | https://bg.copernicus.org/articles/19/5237/2022/bg-19-5237-2022.pdf |
| Summary: | <p>The availability of iron (Fe) and phosphorus (P) has been shown
to be a key factor regulating rates of nitrogen fixation in the western
subtropical Pacific. However, the relative importance of Fe and P at finer spatial
scales between the northern South China Sea (NSCS) and the western boundary
of the North Pacific is poorly constrained. Furthermore, nutrient limitation
of specific diazotroph types has not yet been assessed. Here we investigated
these unknowns by (i) carrying out measurements of finer-scale spatial
variabilities in N<span class="inline-formula"><sub>2</sub></span> fixation rates and diazotroph <i>nifH</i> gene abundances
throughout these regions and (ii) conducting eight additional Fe and
phosphate addition bioassay experiments where both changes in N<span class="inline-formula"><sub>2</sub></span>
fixation rates and the <i>nifH</i> gene abundances of specific diazotrophs were
measured. Overall, nitrogen fixation rates and <i>nifH</i> gene abundances were lower
in the NSCS than around the Luzon Strait and the western North Pacific. The
nutrient addition bioassay experiments demonstrated that N<span class="inline-formula"><sub>2</sub></span> fixation
rates in the central NSCS were co-limited by Fe and P, whereas at the
western boundary of the North Pacific they were P-limited. Changes in the
abundances of <i>nifH</i> in response to nutrient addition varied in how well they
correlated with changes in N<span class="inline-formula"><sub>2</sub></span> fixation rates, and in six out of eight
experiments the largest responses in <i>nifH</i> gene abundances were dominated by
either <i>Trichodesmium</i> or UCYN-B (unicellular diazotrophic cyanobacteria group B). In general, nutrient addition had a relatively restricted
impact on the composition of the six phylotypes that we surveyed apart from
on UCYN-B. This unicellular cyanobacterium group showed increased
contribution to the total <i>nifH</i> gene abundance following P addition at sites
where N<span class="inline-formula"><sub>2</sub></span> fixation rates were P-limited. Our study provides
comprehensive evidence of nutrient controls on N<span class="inline-formula"><sub>2</sub></span> fixation biogeography
in the margin of the western North Pacific. Future research that more
accurately constrains nutrient supply rates to this region would be
beneficial for resolving what controls diazotroph community structure.</p> |
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| ISSN: | 1726-4170 1726-4189 |