Regulation of Phagotrophy by Prey, Low Nutrients, and Low Light in the Mixotrophic Haptophyte Isochrysis galbana.

Mixotrophy combines autotrophy and phagotrophy in the same cell. However, it is not known to what extent the phagotrophy influences metabolism, cell composition, and growth. In this work, we assess, on the one hand (first test), the role of phagotrophy on the elemental and biochemical composition, cell metabolism, and enzymes related to C, N, and S metabolism of Isochrysis galbana Parke, 1949. On the other hand, we study how a predicted increase of phagotrophy under environmental conditions of low nutrients (second test) and low light (third test) can affect its metabolism and growth. Our results for the first test revealed that bacterivory increased the phosphorous and iron content per cell, accelerating cell division and improving the cell fitness; in addition, the stimulation of some C and N enzymatic routes help to maintain, to some degree, compositional homeostasis. Under nutrient or light scarcity, I. galbana grew more slowly despite greater bacterial consumption, and the activities of key enzymes involved in C, N, and S metabolism changed according to a predominantly phototrophic strategy of nutrition in this alga. Contrary to recent studies, the stimulation of phagotrophy under low nutrient and low irradiance did not imply greater and more efficient C flux.