Norwegian Polar Institute, 9296 Tromsø, Norway.
Proc Natl Acad Sci U S A. 2013 Dec 17;110(51):20633-8. doi: 10.1073/pnas.1309345110. Epub 2013 Nov 18.
Diatoms of the iron-replete continental margins and North Atlantic are key exporters of organic carbon. In contrast, diatoms of the iron-limited Antarctic Circumpolar Current sequester silicon, but comparatively little carbon, in the underlying deep ocean and sediments. Because the Southern Ocean is the major hub of oceanic nutrient distribution, selective silicon sequestration there limits diatom blooms elsewhere and consequently the biotic carbon sequestration potential of the entire ocean. We investigated this paradox in an in situ iron fertilization experiment by comparing accumulation and sinking of diatom populations inside and outside the iron-fertilized patch over 5 wk. A bloom comprising various thin- and thick-shelled diatom species developed inside the patch despite the presence of large grazer populations. After the third week, most of the thinner-shelled diatom species underwent mass mortality, formed large, mucous aggregates, and sank out en masse (carbon sinkers). In contrast, thicker-shelled species, in particular Fragilariopsis kerguelensis, persisted in the surface layers, sank mainly empty shells continuously, and reduced silicate concentrations to similar levels both inside and outside the patch (silica sinkers). These patterns imply that thick-shelled, hence grazer-protected, diatom species evolved in response to heavy copepod grazing pressure in the presence of an abundant silicate supply. The ecology of these silica-sinking species decouples silicon and carbon cycles in the iron-limited Southern Ocean, whereas carbon-sinking species, when stimulated by iron fertilization, export more carbon per silicon. Our results suggest that large-scale iron fertilization of the silicate-rich Southern Ocean will not change silicon sequestration but will add carbon to the sinking silica flux.
富含铁的大陆边缘和北大西洋的硅藻是有机碳的主要输出者。相比之下，铁限制的南极绕极流中的硅藻将硅而不是相对较少的碳固定在底层深海和沉积物中。由于南大洋是海洋营养物质分布的主要枢纽，因此那里的选择性硅固定限制了其他地方的硅藻大量繁殖，并限制了整个海洋的生物碳固存潜力。我们通过比较铁施肥斑块内外的硅藻种群在 5 周内的积累和下沉，来研究这种悖论。尽管存在大量的大型食草动物，但在斑块内仍形成了由各种薄壳和厚壳硅藻物种组成的大量繁殖。第三周后，大多数薄壳硅藻物种发生大规模死亡，形成了大的粘液聚集体，并大量下沉（碳汇）。相比之下，厚壳物种，特别是 Fragilariopsis kerguelensis，在表层持续存在，连续下沉主要是空壳，并将硅酸盐浓度降低到斑块内外相似的水平（硅汇）。这些模式表明，厚壳硅藻，即受食草动物保护的硅藻物种，是在富含硅酸盐的情况下，为应对大量桡足类动物的摄食压力而进化而来的。这些硅沉降物种的生态学使铁限制的南大洋中的硅和碳循环脱钩，而当受到铁施肥刺激时，碳汇物种每硅输出更多的碳。我们的研究结果表明，对富含硅酸盐的南大洋进行大规模铁施肥不会改变硅的固定，但会增加下沉硅通量中的碳。
