Species of Chrysochromulina (Prymnesiophyceae) were screened for their ability to ingest inert material and live cells of a small green flagellate. The species C. brevifilum showed a marked preference for the small green flagellate over carmine particles and the rate of ingestion of the flagellate was proportional to its concentration and inversely proportional to light intensity. Ingestion was also higher by phosphate-starved Chrysochromulina, and a clear predator/prey relationship was demonstrated with C. brevifilum clearly benefiting, in terms of growth rate, from the ingestion of the small green flagellate. Electron micrographs confirmed that digestion occurs.
1. Mixotrophs are organisms which combine phototrophy and heterotrophy; such nutritional behaviour is widespread among protists. This ability to combine multiple modes of nutrition varies between species and is not related to their taxonomic grouping. A classification of mixotrophic protists, based on their behaviour, is proposed, dividing them into four groups.
2. Group A includes protists whose primary mode of nutrition is heterotrophy and where phototrophy is employed only when prey concentrations limit heterotrophic growth. In groups B, C and D phototrophy is the dominant mode of nutrition. In group B phagotrophy supplements growth when light is limiting, therefore ingestion of prey is inversely proportional to light intensity; in group C phagotrophy provides essential substances for growth and ingestion is proportional to light intensity; and group D includes those who have very low ingestion rates, ingesting prey only, for example, for cell maintenance during prolonged dark periods.
3. This classification is aimed towards predicting the impact of any particular mixotrophic protist on the aquatic food web, and how this impact may vary depending on the environmental conditions. A model representation of the four groups is discussed.
Summary
The seasonal variations of protist and rotifer populations were monitored over 1 year in a small artificial pond. Grazing rates on fluorescently labelled bacteria were also determined.
The data showed population dynamics similar to other small freshwater bodies; diatoms were numerous during the spring, chlorophytes dominated during the summer months, and mixotrophs, in particular Gymnodinium, dominated during the autumn and winter.
The mixotrophic dinoflagellates were responsible for a high chlorophyll concentration during the autumn and winter. Mixotrophs were important consumers of bacteria, particularly during the autumn when population densities of pure heterotrophs were low. Mixotrophs were an important component of the microbial food web in this pond.
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