Nitrogen Waste or Nitrogen Source? Urate Degradation in the Renal Sac of Molgulid Tunicates

Two urate-producing ascidians, Molgula manhattensis and M. occidentalis, were tested for urate oxidase activity. Microradioassays were carried out on the wall and lumen fluid of the urate-containing, molgulid renal sac, on the renal sac endosymbiont Nephromyces, and on non-renal sac molgulid tissue. These assays indicate that urate is degraded enzymatically in the renal sac. However, this uricolytic activity is concentrated in Nephromyces, rather than in host renal sac tissue.Thus, the urate "waste" of Nephromyces-infected Molgula is not stored permanently in the concretions in the renal sac lumen. Since molgulids are universally infected by Nephromyces in nature, renal sac function should be reassessed with attention to urate as a nitrogen source as well as a nitrogenous catabolite.