Uptake and utilization of 'colloidal DOM' by heterotrophic flagellates in seawater

A vanety 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 actlvely ingested by marine heterotrophic flagellates at substrate concentrations of 1 to 10 m g I-' The uptake capacity for the macromolecules was positively correlated to clearance rate for bacterial-s~zed prey Ingested FITC appeared In the food vacuoles of the cells The biomass of a cultured bactenvorous flagellate species Increased in cultures amended with 10 mg of 2000 kD dextran 1.' to an extent that could not be explained by bacterivory However.40 kD devtran did not stimulate flagellate growth O n the basis of these results, w e suggest that protists have the capacity to ~n g e s t colloids of widely different sizes, spanning the slze range of bacteria and larger organisms to macromolecules possibly lncludlng the recently discovered abundant non-hving submicrometer colloids in sea water