UPTAKE AND UTILIZATION OF COLLOIDAL DOM BY HETEROTROPHIC FLAGELLATES IN SEAWATER

A variety of macromolecules labeled with FITC (fluorescein isothiocyanate), including carbohydrates (dextran) and proteins (ferritin, casein, albumin, concavalin A), spanning a range of molecular weights from 55 to 2000 kD, were actively ingested by marine heterotrophic flagellates at substrate concentrations of 1 to 10 mg l-1. The uptake capacity for the macromolecules was positively correlated to clearance rate for bacterial-sized prey. Ingested FITC appeared in the food vacuoles of the cells. The biomass of a cultured bacterivorous flagellate species increased in cultures amended with 10 mg of 2000 kD dextran l-1 to an extent that could not be explained by bacterivory. However, 40 kD dextran did not stimulate flagellate growth. On the basis of these results, we suggest that protists have the capacity to ingest colloids of widely different sizes, spanning the size range of bacteria and larger organisms to macromolecules, possibly including the recently discovered abundant non-living submicrometer colloids in sed water.