Thick-shelled, grazer-protected diatoms decouple ocean carbon and silicon cycles in the iron-limited Antarctic Circumpolar Current
Thick-shelled, grazer-protected diatoms decouple ocean carbon and silicon cycles in the iron-limited Antarctic Circumpolar Current
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DOI：
10.1073/pnas.1309345110
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发表时间：
2013-12-17
影响因子：
11.1
通讯作者：
Wolf-Gladrow, Dieter
中科院分区：
文献类型：
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作者：
Assmy, Philipp;Smetacek, Victor;Wolf-Gladrow, Dieter
富含铁的大陆边缘和北大西洋的硅藻是有机碳的主要出口者。相比之下，铁有限的南极绕极流中的硅藻在底层深海和沉积物中螯合硅，但相对较少的碳。由于南大洋是海洋营养物分布的主要枢纽，那里的选择性硅固存限制了其他地方的硅藻水华，从而限制了整个海洋的生物碳固存潜力。我们调查了这一矛盾在原位铁施肥实验通过比较积累和下沉的硅藻种群内外的铁施肥补丁超过5周。尽管存在大型食草动物种群，但由各种薄壳和厚壳硅藻物种组成的水华在斑块内发展。第三周后，大部分的薄壳硅藻物种经历了大规模死亡，形成大，粘液聚集，沉出的碳沉子。相比之下，厚壳的物种，特别是Fragilariopsis kerguelensis，坚持在表层，主要是空壳不断下沉，并减少硅酸盐浓度的内部和外部的补丁（硅下沉）类似的水平。这些模式意味着，厚壳，因此放牧保护，硅藻物种的发展，以应对沉重的桡足类放牧压力中存在的丰富的硅酸盐供应。这些硅沉降物种的生态使铁有限的南大洋中的硅和碳循环脱钩，而碳沉降物种在铁施肥的刺激下，每硅输出更多的碳。我们的研究结果表明，富含硅酸盐的南大洋的大规模铁施肥不会改变硅的封存，但会增加碳的下沉硅通量。
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.