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.evolutionary arms race | top-down control | geo-engineering

Diatoms are encased within sophisticated stable lightweight silica cell walls. These frustules have the potential to protect the algal cell against the feeding tools of their most abundant metazoan predators, the copepods. We examined the mechanical strengths of the 3 North Sea diatom species Actinoptychus senarius, Thalassiosira punctigera and Coscinodiscus wailesii and their effect on feeding efficiency of copepods. (1) We determined the stability of the diatoms by means of 'micro-crush-tests' performed in the laboratory with calibrated microneedles. (2) In feeding experiments, we compared the ability and efficiency of the 3 North Sea copepod species Temora longicornis, Centropages hamatus and Acartia clausi to crush frustules. The results showed a remarkable correlation between mechanical properties and size of diatom frustules and feeding success of the copepods. The weakly silicified diatom T. punctigera was the least stable and best fed upon, whilst having the highest growth rate. The diatoms having the most complex frustule, A. senarius, exhibited the greatest stability, whilst being fed upon least. The largest diatom, C. wailesii, was partially protected by its size, but was nonetheless suitable as prey for the large copepods that, in the case of C. hamatus, seem to have developed special feeding techniques to overcome the size-mediated protection.

Key words. Confocal laser scanning microscopy (CLSM), diatom, finite element analysis, photogrammetry lightweight value, scanning electron microscopy (SEM), three-dimensional reconstruction. SummaryExact geometric description, numerical analysis and comparison of microscopic objects such as the frustules of diatoms are of increasing importance in basic research (e.g. functional morphology, taxonomy and biogeochemistry). Similarly, applied research and product development in the fields of lightweight construction and nanotechnology can benefit from machine-readable data of such structures. This paper presents a new method to combine data from scanning electron microscopy and confocal laser scanning microscopy to generate exact three-dimensional models of diatom frustules. We propose a method to obtain a high-quality mesh for subsequent analysis through finite element analysis, for example, for biomechanical research on diatom frustules. A specific lightweight value as a universal tool to describe and compare the biomechanical quality of microscopic objects is introduced.Our approach improves the precision of three-dimensional reconstructions, but the generation of usable finite element meshes from complex three-dimensional data based on microscopic techniques requires either a transformation of grid points into elements or smoothing algorithms. Biomechanical analyses of differently obtained models indicate that more complex three-dimensional reconstructions lead to more realistic results.

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