The origin of the Chromalveolata was thought to be a secondary endosymbiosis between a bikont ( cell with two flagella ) and a red alga . This union led to plastids containing chlorophyll c. In Cavalier-Smith 's classification , the Chromalveolata was one of the six major groups within the eukaryotes . Chromalveolata was proposed to represent the result of a single secondary endosymbiosis between a line descending from a bikont and a red alga that became the progenitor of chlorophyll c containing plastids .