The goal of the work in this dissertation was to determine the nature of carbon metabolism that supports mixotrophic growth in the mixotrophic dinoflagellate, Karlodinium micrum. This remained a question following work previously done with K. micrum, which primarily focused on its distribution in nature and the environmental / cellular conditions that increased the propensity of autotrophic cultures to ingest prey, Storeatula major (cryptophyte). The research was approached in three experimental chapters that focused on (Chp. 2) autotrophic growth, (Chp. 3) mixotrophic growth, and (Chp 4) the relationship of predator / prey fatty acids. An experimental design was developed to allow separate examination of autotrophic and heterotrophic C metabolism during mixotrophic growth, without the need to mechanically separate predator from prey, a step that had previously proven cumbersome and inconsistent. Chp 5. includes observations and experiments conducted on natural dinoflagellate blooms, drawing upon techniques that were used in culture studies.

Strict autotrophic growth of K. micrum was typical for dinoflagellates, characterized by a relatively low growth rate that had its bases in low chl:C and growth efficiency. As a mixotroph, at growth rates similar to or greater than autotrophic growth rates, cellular photosynthetic performance was lower than in autotrophic cells and gross C gain was dominated by phagotrophy, especially at higher feeding rates. Further, the fraction of photosynthate allocated to protein synthesis was lower in mixotrophic cells than autotrophic cells, while protein was the main assimilation product of phagotrophy. The increased growth rate typical of mixotrophic cells was not accompanied by an increase in growth efficiency, and was a result of increased C gain largely attributable to phagocytosis. These observations suggest a strong heterotrophic capability during mixotrophic growth of K. micrum, yet feeding and growth do not take place in the absence of light in this obligately autotrophic dinoflagellate. I hypothesize that fatty acid biosynthesis is a plastid-specific, essential, metabolic activity that underlies obligate autotrophy in K. micrum. Observations of fatty acids in mixotrophic K. micrum cultures were not inconsistent with this hypothesis, although further testing is required to satisfactorily resolve the issue of obligate autotrophy in K. micrum. Use of laboratory derived relationships between cell-specific properties and physiological state were used to infer the physiological state of K. micrum drawn from natural populations. The measurements were consistent with a high light-acclimated / mixotrophic population with a high cellular growth rate.