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 (Mr), such as simple sugars and amino acids, at the low concentrations found in situ1,2 (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 Mr. 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 Mr> 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-Mr are often a large fraction of total DOM in natural waters3,4, and the concentrations (dry weight litre−1) of some types of labile high-Mr compounds in natural waters are equal to, or greater than, those of bacterioplankton5,6. Thus DOM of high Mr may be an alternative food resource for aquatic flagellates. Direct ingestion of high Mr 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.