Physiological Responses of Phytoflagellates to Dissolved Organic Substrate Additions. 1. Dominant Role of Heterotrophic Nutrition in Poterioochromonas malhamensis (Chrysophyceae)

Algal heterotrophy is a potentially important considerationin the flow of carbon through aquatic food webs. The physiologicalresponses to organic compound additions under various lightintensities were examined with Poterioochromonas malhamensis,a freshwater chrysophyte with an exceptionally high heterotrophiccapability. P. malhamensis demonstrated a much greater potentialfor heterotrophic growth than for photoautotrophic growth. Whenorganic substrates (glucose, glycerol, or ethanol) were addedto the culture medium, the growth rate of P. malhamensis significantlyincreased while the chlorophyll content cell ^–1 decreased,even at light intensities saturating for photoautotrophic growth.After an initial decline in chlorophyll production caused byorganic substrate uptake, chlorophyll cell¹ increased and theuptake rate of organic substrates decreased, despite the persistenceof a relatively high substrate concentration in the medium.The results are consistent with the production of substance(s)by P. malhamensis that conditioned the culture medium, leadingto a relief of the inhibitory effect of organic substrates onchlorophyll production. (Received October 29, 1990; Accepted May 8, 1991)