Local communities can be structured by both local interactions (competition, predation, environmental variables, etc.) and by regional interactions (dispersal of individuals between habitats). Using data from a three‐year study on 34 neighboring ponds in an interconnected pond system, we tested whether zooplankton communities show a metacommunity structure, how much variation in zooplankton community structure is related to local environmental factors vs. spatial configuration (taking into account the dispersal pathways), and what environmental variables are the locally structuring forces. In three different years, we found evidence for a metacommunity structure. We also found that spatial and environmental components act independently of each other due to the small geographic area and the high dispersal rates in this system. Despite these homogenizing forces, local environmental variables (associated with alternative equilibria in shallow lakes) were strongly related with zooplankton community structure in the studied years. This suggests that, even in this system of highly interconnected ponds, local environmental constraints can be strong enough to structure local communities. Corresponding Editor: P. Legendre.

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Iron oxide nanoparticle internalization exerts detrimental effects on cell physiology for a variety of particles, but little is known about the mechanism involved. The effects of high intracellular levels of four types of iron oxide particles (Resovist, Endorem, very small organic particles, and magnetoliposomes (MLs)) on the viability and physiology of murine C17.2 neural progenitor cells and human blood outgrowth endothelial cells are reported. The particles diminish cellular proliferation and affect the actin cytoskeleton and microtubule network architectures as well as focal adhesion formation and maturation. The extent of the effects correlates with the intracellular concentration (= iron mass) of the particles, with the biggest effects for Resovist and MLs at the highest concentration (1000 microg Fe mL(-1)). Similarly, the expression of focal adhesion kinase (FAK) and the amount of activated kinase (pY397-FAK) are affected. The data suggest that high levels of perinuclear localized iron oxide nanoparticles diminish the efficiency of protein expression and sterically hinder the mature actin fibers, and could have detrimental effects on cell migration and differentiation.

Iron oxide nanoparticles (NPs) are frequently employed in biomedical research as magnetic resonance (MR) contrast agents where high intracellular levels are required to clearly depict signal alterations. To date, the toxicity and applicability of these particles have not been completely unraveled. Here, we show that endosomal localization of different iron oxide particles results in their degradation and in reduced MR contrast, the rate of which is governed mainly by the stability of the coating. The release of ferric iron generates reactive species, which greatly affect cell functionality. Lipid-coated NPs display the highest stability and furthermore exhibit intracellular clustering, which significantly enhances their MR properties and intracellular persistence. These findings are of considerable importance because, depending on the nature of the coating, particles can be rapidly degraded, thus completely annihilating their MR contrast to levels not detectable when compared to controls and greatly impeding cell functionality, thereby hindering their application in functional in vivo studies.

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