Direct use of high molecular weight polysaccharide by heterotrophic flagellates

Abstract

In aquatic ecology it is assumed that the heterotrophic (non-photosynthetic) protozoa that are abundant in marine and fresh water cannot compete with smaller-sized bacteria for labile dissolved substrates of low relative molecular mass (M_r), such as simple sugars and amino acids, at the low concentrations found in situ^1,2 (typically 1–100 μg litre⁻¹) because of surface to volume considerations. Consequently dissolved organic matter (DOM) is not considered to be a source of nutrition for heterotrophic protozoa, which are thought to be predominantly phagotrophic on bacteria and other small planktonic cells². But there is no information as to whether protozoa might be able to use DOM of higher M_r. Here I report that heterotrophic flagellates in a salt marsh estuary and in a freshwater pond were able to ingest molecules of the polysaccharide dextran of M_r> 500,000 (500K). Mixed species cultures of estuarine and pond flagellates also showed enhanced growth in the presence of 2,000K, but not 40K, dextran. Compounds with high-M_r are often a large fraction of total DOM in natural waters^3,4, and the concentrations (dry weight litre⁻¹) of some types of labile high-M^r compounds in natural waters are equal to, or greater than, those of bacterioplankton^5,6. Thus DOM of high M_r may be an alternative food resource for aquatic flagellates. Direct ingestion of high M_r compounds by heterotrophic flagellates would represent a more efficient pathway for returning a portion of DOM to aquatic food webs compared with the DOM-bacteria-flagellate-ciliate microbial loop⁷.