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Drosophila melanogaster photoreceptors are highly polarized cells and their plasma membrane is organized into distinct domains. Zonula adherens junctions separate a smooth peripheral surface, the equivalent of the basolateral surface in other epithelial cells, from the central surface (cong apical surface). The latter consists of the microvillar rhabdomere and the juxtarhabdomeric domain, a nonmicrovillar area between the rhabdomere and the zonulae adherens. The distribution of Na/K-ATPase over these domains was examined by immunocytochemical, developmental, and genetic approaches. Immunofluorescence and immunogold labeling of adult compound eyes reveal that the distribution of Na/ K-ATPase is concentrated at the peripheral surface in the photoreceptors R1-R6, but extends over the juxtarhabdomeric domain to the rhabdomere in the photoreceptors R7/R8. Developmental analysis demonstrates further that Na/K-ATPase is localized over the entire plasma membrane in all photoreceptors in early pupal eyes. Redistribution of Na/K-ATPase in R1- R6 occurs at about 78% of pupal life, coinciding with the onset of Rh1-rhodopsin expression on the central surface of these cells. Despite the essential role of Rh1 in structural development and intracellular trafficking, Rh1 mutations do not affect the distribution of Na/K-ATPase. These results suggest that Na/K-ATPase and rhodopsin are involved in distinct intracellular localization mechanisms, which are maintained independent of each other.
Water column mixing is known to have a decisive impact on plankton communities. The underlying mechanisms depend on the size and depth of the water body, the nutrient status, and the plankton community structure and they are well understood for shallow polymictic and deep stratified lakes. Two consecutive mixing events of similar intensity under different levels of herbivory were performed in enclosures in a shallow, but periodically stratified, eutrophic lake, in order to investigate the effects of water column mixing on bacteria abundance, phytoplankton abundance and diversity, and rotifer abundance and fecundity. When herbivory by filter-feeding zooplankton was low, water column mixing provoking a substantial nutrient input into the euphotic zone, led to an intense net increase of bacteria and phytoplankton biomass. Phytoplankton diversity was lower in the mixed enclosures than in the undisturbed ones owing to the larger contribution of a few fast-growing species. After the second mixing event at high biomass of filter-feeding crustaceans, the increase of phytoplankton biomass was lower than after the first mixing, and diversity remained unchanged as the enhanced growth of small fast-growing was prevented by zooplankton grazing. Bacteria abundance did not increase after the second mixing, when cladoceran biomass was high. Rotifer fecundity indicated a transmission of the phytoplankton response to the next trophic level. Our results suggest that water column mixing in shallow eutrophic lakes with periodic stratification has a strong effect on the plankton community by enhanced nutrient availability rather than resuspension or reduced light availability. This fuels the basis of the classic and microbial food chain via enhanced phytoplankton and bacteria growth, but the effects on biomass may be dampened by high levels of herbivory.