COLLOIDAL AND DISSOLVED ORGANIC-MATTER EXCRETED BY A MIXOTROPHIC FLAGELLATE DURING BACTERIVORY AND AUTOTROPHY

Excretion of dissolved and colloidal organic carbon by a mixotrophic f lagellate, the chrysophyte Poterioochromonas malhamensis, was studied. Flagellates were incubated either with C-14-labeled bacteria or with inorganic C-14, in, order to compare organic exudates originating from primary production with exudates originating from ingested bacteria. Colloids of >0.02 mu m constituted a larger fraction of the exudates o riginating from ingested bacteria, compared with exudates derived from primary production. Flagellate feeding on bacteria specifically label ed in different cell components was compared. Cell wall components gav e rise to less colloidal organic carbon than did other cell constituen ts. To investigate the degradability of flagellate C-14-exudates, they were added to lake water and mineralization to (CO2)-C-14 was monitor ed. Bacterially derived exudates were more recalcitrant than exudates originating from photosynthesis. The results support the hypothesis th at bacterial utilization of labile organic compounds, followed by flag ellate bacterivory and exudation, results in a transformation of labil e organic matter into more recalcitrant forms.