Encystment by the mixotrophic flagellate Ochromonas pinguis Conrad (Chrysophyceae) under manipulated laboratory conditions

Chrysophyte stomatocysts are well documented in lake sediments, but except for a few studies on sexually reproducing colonial chrysophytes, little is known regarding encystment cues in these algae. Ochromonas pinguis Conrad, a mixotrophic chrysophyte with an unknown sexual history was observed to produce stomatocysts, albeit at a low frequency. To determine if the low rate of encystment was typical or whether manipulating environmental conditions might enhance the rate of resting cyst production O. pinguis was subjected to various physical and chemical conditions in a batch culture. O. pinguis cultures were manipulated by culturing them in DY–V inorganic medium in variable light, pH, and temperature conditions, and at varying levels of N and P limitation. Except for growth in continuous darkness, all treatments resulted in an increase in O. pinguis concentration. Five treatments resulted in a corresponding increase in stomatocyst concentration. In the remaining treatments stomatocyst production was steady although sporadic. The encystment frequency however was minimal in all manipulations and ranged from 0.008 to 0.03% of the population undergoing cyst formation. In this study, O. pinguis stomatocyst production was not influenced by laboratory manipulations but rather a minute proportion of the population produced stomatocysts regardless of environmental conditions.