Nitrogen excretion and acid-base regulation in invertebrate systems: Mechanisms, regulation and the specific role of Rh-proteins and AMTs.

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Ammonia is a highly toxic waste product derived from protein metabolism. In ammoniotelic animals (e.g. aquatic invertebrates and fish), where ammonia is excreted directly and not transformed into less toxic molecules such as urea or uric acid, an effective excretion system is essential to keep cellular and body fluid ammonia levels within a tolerable range. Although ca. 98% (= 30 phyla) of all animals are invertebrates, our knowledge of nitrogen excretion strategies and mechanisms in this dominant group is rather diminutive. The goal of the proposed research is to gain first and detailed insights of ammonia excretion pathways and mechanisms in a wide variety of different invertebrate groups, including annelids (freshwater leeches), nematodes (soil living roundworms), mollusks (freshwater mussels), arachnids (seawater horseshoe crabs) and crustaceans (fresh-, brackish-, and seawater crabs). The proposed research consists of three main objectives, investigating: (1) the actual excretion mechanisms in aquatic invertebrates to explore common ancestral excretion strategies; (2) the evolution and function of the phylogenetic ancient bacterial and plant "AMTs" and the distantly related animal "Rh-proteins" in the soil nematode Caenorhabditis elegans, a transitional animal that expresses both types of ammonia transporters; (3) the relation of ammonia excretion and acid-base balance in crustaceans and the effects of predicted increases in global aquatic pCO2 levels and ocean acidification on the pH-regulation in euryhaline crabs. The three objectives will provide novel insights of the evolution of ammonia excretion mechanisms and acid-base regulation in phylogentic ancient invertebrates, including the roles of Rh-proteins and AMTs as NH3, CO2 or H+ -channels. It is expected that fundamental and general nitrogen excretion and pH regulatory strategies will be discovered. Understanding the ammonia excretion mechanisms as well as the animal's responses towards elevated environmental ammonia levels are crucial for conservation programs, which are intended to protect animal communities living in aquatic habitats threatened by ammonia-rich agricultural run-off and also to manage aquaculture facilities.