A mesoscale phytoplankton bloom in the polar Southern Ocean stimulated by iron fertilization

Changes in iron supply to oceanic plankton are thought to have a significant effect on concentrations of atmospheric carbon dioxide by altering rates of carbon sequestration, a theory known as the 'iron hypothesis'. For this reason, it is important to understand the response of pelagic biota to increased iron supply. Here we report the results of a mesoscale iron fertilization experiment in the polar Southern Ocean, where the potential to sequester iron-elevated algal carbon is probably greatest. Increased iron supply led to elevated phytoplankton biomass and rates of photosynthesis in surface waters, causing a large drawdown of carbon dioxide and macronutrients, and elevated dimethyl sulphide levels after 13 days. This drawdown was mostly due to the proliferation of diatom stocks. But downward export of biogenic carbon was not increased. Moreover, satellite observations of this massive bloom 30 days later, suggest that a sufficient proportion of the added iron was retained in surface waters. Our findings demonstrate that iron supply controls phytoplankton growth and community composition during summer in these polar Southern Ocean waters, but the fate of algal carbon remains unknown and depends on the interplay between the processes controlling export, remineralisation and timescales of water mass subduction.

Impaired wound healing in embryonic and adult mice lacking vimentin

It is generally assumed that the vimentin intermediate filament network present in most mesenchymally-derived cells is in part responsible for the strength and integrity of these cells, and necessary for any tissue movements that require the generation of significant tractional forces. Surprisingly, we have shown that transgenic KO mice deficient for vimentin are apparently able to undergo embryonic development absolutely normally and go onto develop into adulthood and breed without showing any obvious phenotype. However, fibroblasts derived from these mice are mechanically weak and severely disabled in their capacity to migrate and to contract a 3-D collagen network. To assess whether these functions are necessary for more challenging tissue movements such as those driving in vivo tissue repair processes, we have analysed wound healing ability in wild-type versus vimentin-deficient embryos and adult mice. Wounds in vimentin-deficient adult animals showed delayed migration of fibroblasts into the wound site and subsequently retarded contraction that correlated with a delayed appearance of myofibroblasts at the wound site. Wounds made to vimentin-deficient embryos also failed to heal during the 24 hour culture period it takes for wild-type embryos to fully heal an equivalent wound. By DiI marking the wound mesenchyme and following its fate during the healing process we showed that this impaired healing is almost entirely due to a failure of mesenchymal contraction at the embryonic wound site. These observations reveal an in vivo phenotype for the vimentin-deficient mouse, and challenge the dogma that key morphogenetic events occurring during development require generation of significant tractional forces by mesenchymal cells.

Wound healing in embryos: a review

Skin wounds in young embryos heal rapidly, efficiently and perfectly without scar formation, an ability that is lost as developmental proceeds. The tissue movements of repair (re-epithelialisation and connective tissue contraction) are the same in embryos as adults, but the means by which these movements are achieved are very different. Whilst adult wound front epidermal cells crawl forwards over the exposed substratum to close a defect, a gap in the embryonic epidermis is closed by contraction of a rapidly assembled actin purse-string. In the adult wound situation connective tissue contraction is brought about by specialist contractile myofibroblasts, but in the embryo standard embryonic fibroblast exert similar tractional forces to bring the wound margins together. We review what is known of the cellular sources and the levels of various growth factor signals that might activate wound closure movements during embryonic and adult repair and how this knowledge might help in designing therapeutic strategies to enhance adult healing. Finally we discuss how studies of the tissue movements of embryonic wound healing may guide our understanding of more natural tissue movements that occur during embryogenesis, such as gastrulation and neurulation.