Dissolved organic carbon from "white" waters in Amazon basin (Brazil) and the osmoregulatory responses of tambaqui (Colossoma macropomum) in neutral and acidic environments.

Optical characterization of dissolved organic carbon (DOC) freshly collected from the circumneutral "white water" of the Rio Solimoes revealed that it had lower aromaticity, lower molecular weight, and a greater autochthonous content than DOC from the acidic "black water" of the Rio Negro. The tambaqui (Colossoma macropomum), a characid member of the Serrasalmidae, is a model neotropical fish that migrates annually between the two rivers. We analysed ionoregulatory responses of the tambaqui over 24 h in ion-poor water at pH 7.0 and pH 4.0 in the absence and presence of 10 mg L^-1 Rio Solimoes DOC ("SOL DOC"). Measured parameters included transepithelial potential (TEP) across the gills, net flux rates, and plasma concentrations of Na⁺, Cl^-, ammonia, and urea, and branchial Na⁺, K⁺ -ATPase, H⁺, ATPase, and carbonic anhydrase activities. Results were compared to our earlier study using similar protocols to examine the ionoregulatory effects of 10 mg L^-1 DOC ("SGC DOC") collected from black water at São Gabriel da Cachoeira (SGC) in the upper Rio Negro. At pH 7.0, SOL DOC had no effect on the negative TEP across the gills. Exposure to pH 4.0 caused a marked depolarization of the TEP to positive values that was not ameliorated by the presence of SOL DOC. This contrasts with SGC DOC that drove TEP more negative at pH 7.0 and fully protected against the depolarization at pH 4.0. However, SOL DOC promoted positive balance of Na⁺ and Cl^- at pH 7.0 and helped ameliorate the negative balance of these ions seen at pH 4.0. This again contrasts with SGC DOC that exacerbated ion losses at pH 4.0. The protective effects of SOL DOC on ion balance maybe related to increased v-type H⁺ ATPase activity in the gills, and unrelated to TEP. The very different responses to the two DOCs are discussed with respect to their optical properties, the time that they were in storage prior to testing (SOL <1 month, SGC 2 years), and the life history of the tambaqui in the two rivers.