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SUMMARY 1. Previous studies of mixotrophy in the flagellate Poterioochromonas malhamensis (Chrysophyceae) were performed on strains that had been in culture for > 30 years. This study aims to compare mixotrophy in a cultured strain with one recently isolated from a mesotrophic lake (Lacawac) in Pennsylvania, U.S.A. 2. P. malhamensis from the lake exhibited a nutritional flexibility similar to that of the culture strain, growing phototrophically but inefficiently in comparison to other nutritional modes (growth rate (m) = 0.015 h ‐1 ). Supplementing an inorganic salts medium with 1 mM glucose resulted in a doubling of m to 0.035 h ‐1 and 0.033 h ‐1 in the light and the dark, respectively. Addition of an algal prey, Nannochloris, to the inorganic salts medium increased growth to rates similar to those observed with glucose. Maximum growth of the lake strain, 0.095 h ‐1 , was achieved when bacteria was supplied as food. During growth on bacteria, cellular chlorophyll a (Chl a) decreased from 140 fg cell ‐1 to 10 fg cell ‐1 over 22 h when cultured either in the light or dark. In illuminated cultures, cell-specific Chl a concentration recovered to 185 fg cell ‐1 after bacteria became limiting. 3. In contrast to the cultured strain, however, the lake isolate exhibited an inverse relationship between light intensity and ingestion rate. Calculated grazing rates, based upon the ingestion of fluorescently labeled bacteria, were 3.2, 5.2 and 9.4 bacteria flagellate ‐1 h ‐1 , for P. malhamensis incubated in high light, low light and darkness, respectively. Phagotrophy is thus influenced by a light regime in this predominately heterotrophic mixotroph.

The toxicity of the chrysophyte flagellate Poterioochromonas malhamensis to the rotifer Brachionus angularis was investigated. Fed rotifers exposed to the flagellate experienced a mortality rate indistinguishable from starvation. Unfed rotifers exposed to the flagellate experienced a higher mortality rate. The mortality rate appears to depend on the flagellate concentration. Higher doses of flagellates resulted in quicker rotifer death. These laboratory results are consistent with the hypothesis that the occurrence of B. angularis in the field may be negatively related to the presence of P. malhamensis and related flagellates.