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Monospecific diatom cultures (Thalassiosira punctigera and Skeletonema costatum) were incubated in rotating cylinders together with clay suspensions, present in a range of concentrations (5-100 mg kaolinite L Ϫ1) and as different minerals (50 mg L Ϫ1 kaolinite, smectite, illite, and clay-sized quartz powder). The addition of lithogenic suspensions to diatom cultures accelerated the formation of visible aggregates in the roller tanks by a factor of Ͼ3. Aggregate size decreased and density increased proportionally to the amount of kaolinite added to the diatom cultures. In the presence of kaolinite and illite, aggregate sizes were smaller and sinking rates lower than in the presence of smectite and quartz. The influence of lithogenic matter on the sinking velocities of aggregates was ambiguous. Compound aggregates sank faster with increasing amounts of lithogenic matter present in cultures of T. punctigera until a certain ratio between lithogenic and biogenic material was reached; further increasing the amount of lithogenic matter did not increase sinking rates significantly. In contrast, increasing the concentration of kaolinite added to cultures of S. costatum could decrease sinking velocities of the evolving compound aggregates. This nonlinear behavior is argued to be primarily a function of aggregate composition on aggregate sizes and excess densities. Although the possibility of a mutual acceleration of vertical flux of algae and clay is confirmed, the results show that the presence of lithogenic material could also decrease the downward flux of phytoplankton biomass.

ABSTRACT-Colonies of the prymnesiophyte marine microalga Phaeocystis globosa were tested for mechanical properties, permeability and biochemical composition using the rnicropipette aspiration technique. We found that the Phaeocystis colony is enclosed by a thin, yet very strong, semi-permeable skin (pore size between 1 and 4.4 nm diameter) with plastic and to a limited extent also elastic properties. Qualitative staining of single colonies with selective fluorescent dyes indicated absence of lipophilic compounds and chitin but presence of amino groups in the colony skin. Individual cells in the colony appear to be weakly connected with one another and attached to a very dilute, peripheral gel. Suction apphed to the colony resulted in volume loss due to expulsion of water and squeezing together of the cells within the skin into a tight pouch; the presence of any firm gelatinous matter within the colony was not discernible. On increasing suction pressure, the skin eventually ruptured and the cells were sucked out of the hole leaving the empty skin behind. We propose that the skin effectively protects the colony cells from grazing and infection by viruses and other pathogens. The unsuspected presence of a skin is probably the main reason why Phaeocystis colonies have reduced mortality relative to solitary cells and form large blooms in many regions of the world's ocean. Our findings indicate that the colonies should be viewed as 'bags of water' rather than 'balls of jelly'

Multilayer Laue lenses are volume diffraction elements for the efficient focusing of X-rays. With a new manufacturing technique that we introduced, it is possible to fabricate lenses of sufficiently high numerical aperture (NA) to achieve focal spot sizes below 10 nm. The alternating layers of the materials that form the lens must span a broad range of thicknesses on the nanometer scale to achieve the necessary range of X-ray deflection angles required to achieve a high NA. This poses a challenge to both the accuracy of the deposition process and the control of the materials properties, which often vary with layer thickness. We introduced a new pair of materials—tungsten carbide and silicon carbide—to prepare layered structures with smooth and sharp interfaces and with no material phase transitions that hampered the manufacture of previous lenses. Using a pair of multilayer Laue lenses (MLLs) fabricated from this system, we achieved a two-dimensional focus of 8.4 × 6.8 nm2 at a photon energy of 16.3 keV with high diffraction efficiency and demonstrated scanning-based imaging of samples with a resolution well below 10 nm. The high NA also allowed projection holographic imaging with strong phase contrast over a large range of magnifications. An error analysis indicates the possibility of achieving 1 nm focusing.

This paper discusses structure and function of the Phaeocystis colony skin, and relates them to the specific impact of Phaeocystis colonies on ecology and biogeochemistry. The potential advantage of the recently discovered tough skin around the colonies of Phaeocystis globosa is discussed in context with the metabolic costs of this structure, and compared to potential functions of structures around other phytoplankton. It is further proposed that mainly small, fast-growing pathogens and predators will be deterred by the colony skin. It will be shown that these theoretical predictions are consistent with available data from the literature, and can explain the dominance of the colonial form in Phaeocystis blooms. Finally, the peculiar biogeochemistry of Phaeocystis colonies, especially the sedimentation of Phaeocystis-derived organic matter, is argued to be a function of the susceptibility of Phaeocystis colonies to certain grazers, which in turn is strongly determined by the architecture and function of the colony skin. During the exponential phase of the bloom, Phaeocystis-derived organic matter can efficiently sink in faecal material of large zooplankton, which actively feed on the colonies. However, the integrity of the colony skin, and consequently its protection for the cells therein, seems to be closely coupled to the phase of active growth. Accordingly, the cells are massively affected by small grazers and pathogens and thus rapidly disintegrate after the culmination of the bloom, so that sedimentation of Phaeocystis-derived organic matter becomes probably restricted to the more refractory extracellular components of the colonies. ᭧

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Diatoms are unicellular algae with silicified cell walls, which exhibit a high degree of symmetry and complexity. Their diversity is extraordinarily high; estimates suggest that about 10(5) marine and limnic species may exist. Recently, it was shown that diatom frustules are mechanically resilient, statically sophisticated structures made of a tough glass-like composite. Consequently, to break the frustules, predators have to generate large forces and invest large amounts of energy. In addition, they need feeding tools (e.g., mandibles or gastric mills) which are hard, tough, and resilient enough to resist high stress and wear, which are bound to occur when they feed on biomineralized objects such as diatoms or other biomineralized protists. Indeed, many copepods feeding on diatoms possess, in analogy to the enamelcoated teeth of mammals, amazingly complex, silica-laced mandibles. The highly developed adaptations both to protect and to break diatoms indicate that selection pressure is high to optimize material properties and the geometry of the shells to achieve mechanical strength of the overall structure. This paper discusses the mechanical challenges which force the development of mechanical defenses, and the structural components of the diatom frustules which indicate that evolutionary optimization has led to mechanically sophisticated structures. Understanding the diatom frustule from the nanometer scale up to the whole shell will provide new insights to advanced combinations of nanostructured composite ceramic materials and lightweight architecture for technological applications.

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