Ecological and evolutionary processes have shaped current biodiversity patterns. For brackish-water ecosystems, Remane's Artenminimum ('species minimum') concept argues that taxonomic diversity in organisms is lowest within the horohalinicum, which occurs at salinity 5 to 8. This concept developed from macrozoobenthos data; it originated from, and is still applied to, the geologically young Baltic Sea, the world's largest semi-enclosed, brackish water body with a unique permanent salinity gradient. We re-assessed pelagic biodiversity in the Baltic Sea, which had long remained underestimated. We show that phyto-and zooplankton in Baltic waters exhibit unexpectedly high diversity (> 4000 taxa), with dominance by protists. Protists in the Baltic Sea follow a binomial distribution mode, while metazooplankton diversity decreases exponentially with higher salinity; however, species richness of both groups peaks in the horohalinicum. Drifting within large water masses, planktonic organisms are affected by only moderate salinity fluctuations (compared to benthic species), and thus prosper in brackish environments. The present study challenges Remane's concept for large water bodies with relatively stable salinity gradients and substantiates a novel ecological perspective of the previously overlooked high protistan diversity in brackish waters. We infer that the pronounced adaptability and advanced osmoregulation strategies of protists are the result of large-scale ecological and evolutionary processes. The novel brackish-water biodiversity pattern underpins the proposed protistan species-maximum concept, which refines Remane's model by discriminating between the effects of the horohalinicum on the biodiversity of small motile versus large sessile organisms.

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Microsporidia are obligatory intracellular parasites, most species of which live in the host cell cytosol. They synthesize and then transport secretory proteins from the endoplasmic reticulum to the plasma membrane for formation of the spore wall and the polar tube for cell invasion. However, microsporidia do not have a typical Golgi complex. Here, using quick-freezing cryosubstitution and chemical fixation, we demonstrate that the Golgi analogs of the microsporidia Paranosema (Antonospora) grylli and Paranosema locustae appear as 300-nm networks of thin (25- to 40-nm diameter), branching or varicose tubules that display histochemical features of a Golgi, but that do not have vesicles. Vesicles are not formed even if membrane fusion is inhibited. These tubular networks are connected to the endoplasmic reticulum, the plasma membrane and the forming polar tube, and are positive for Sec13, γCOP and analogs of giantin and GM130. The spore-wall and polar-tube proteins are transported from the endoplasmic reticulum to the target membranes through these tubular networks, within which they undergo concentration and glycosylation. We suggest that the intracellular transport of secreted proteins in microsporidia occurs by a progression mechanism that does not involve the participation of vesicles generated by coat proteins I and II.

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