Production of extracellular organic carbon in the total primary production by freshwater benthic algae at the littoral zone and inflow river of Lake Biwa

摘要

In general, a high proportion of benthic algae flora consists of diatoms belonging to the pennates (Round et al. 1990). One of the conspicuous characteristic of diatoms is the production of an adhesive extracellular secretion. This mucilaginous substance is secreted from the raphe and/or mucilage pores of diatoms to the outside of the cell, covering it in various forms such as stalks, tubes, apical pads, adhering films and fibrils (Hoagland et al. 1993). These secretions are highly hydrated, carbohydrate-rich substances containing extracellular polymeric substances (EPS) consisting mainly of acidic polysac-charides that may be carboxylated and/or sulfated to varying degrees. Various functions have been proposed for benthic algae secretions in aquatic ecosystems, including protection against rapid changes in enhabited environments, sessile adhesion, motility, stabilization of sediments, and adsorption of dissolved organic matter and metals (Decho 1990, Hoagland et al. 1993, Miller et al. 1996). Moreover, from the viewpoint of food webs in inshore ecosystems, the extracellular organic carbon produced by benthic algae plays an important role as an organic carbon source for het-erotrophic organisms (Decho & Lopez 1993, Kawa-mura et al. 1998, Goto et al. 2001).