Endosymbiotic associations within protists

The establishment of an endosymbiotic relationship typically seems to be driven through complementation of the host's limited metabolic capabilities by the biochemical versatility of the endosymbiont. The most significant examples of endosymbiosis are represented by the endosymbiotic acquisition of plastids and mitochondria, introducing photosynthesis and respiration to eukaryotes. However, there are numerous other endosymbioses that evolved more recently and repeatedly across the tree of life. Recent advances in genome sequencing technology have led to a better understanding of the physiological basis of many endosymbiotic associations. This review focuses on endosymbionts in protists (unicellular eukaryotes). Selected examples illustrate the incorporation of various new biochemical functions, such as photosynthesis, nitrogen fixation and recycling, and methanogenesis, into protist hosts by prokaryotic endosymbionts. Furthermore, photosynthetic eukaryotic endosymbionts display a great diversity of modes of integration into different protist hosts

Food webs in Mediterranean rivers

River food webs are subject to two regimes of longitudinally varying ecological control: productivity and disturbance. Light-limited productivity increases as channels widen downstream. Time windows for growth, however, shrink as discharge increases, substrate particle size decreases, and the frequency of flood-driven bed mobilization increases downstream. Mediterranean rivers are periodically reset by hydrologic events with somewhat predictable timing. Typically, a rainy winter with high river discharge is followed by summer drought with little or no rainfall and slowly declining river flow. The magnitude and timing of winter floods and severity of subsequent summer drought can vary considerably from year to year, however. Episodic scouring floods or prolonged periods of drought are experienced as disturbances, stressors, or opportunities by river biota. The timing, duration, and intensity of these hydrologic controls affect performances of individuals, distribution and abundances of populations, and outcomes and consequences of species interactions. These interactions in turn determine how river food webs will assemble, develop, and reconfigure after disturbance. We discuss how spatial variation in solar radiation and spatial and temporal variations in disturbance affects river food webs under Mediterranean climate seasonality, focusing primarily on long-term observations in the Eel River of northwestern California, USA