Silicon extracted and precipitated by siliceous algae indicate that the dissolved silicon levels of the Rio Negro, Brazil, are in part controlled by biological activity. Diatoms are the most prolific and adhesive eucaryotic microorganisms found in the study area; wood, leaves, and rocks serve as both solid substrates for the adhesive microbes and as nutrient sources. Scanning electron micrographs of the wood samples revealed a siliceous "gel" precipitated on both the outer surface and within the submerged wood. A biomineralization process was occurring, leading to the silicification of the wood sample. On a regional scale, this process may have important implications for the freshwater silica cycle.