Fig. 1. The MOCHA model.

(A) Model schematic illustrating the cell of a constitutive mixotroph that can obtain carbon (C) and nitrogen (N) by investing in phototrophy and/or prey consumption and then use C and N to build three structures: plastids (green), feeding vacuoles (magenta), and growth machinery (gold). The optimal strategies vary with environmental conditions so that (B) phototrophy is optimal if prey is limited but light and external inorganic nitrogen (DIN) are replete, (C) phagotrophy is optimal if prey is abundant but light and DIN are limited, and (D) mixotrophy is optimal if light and prey are replete but DIN is limited. Ternary plots [(B) to (D)] show heatmaps of growth rate as a function of the three-structure investment strategy (dark gray = slow or negative growth; white = fast growth). Lines indicate strategies that produce equivalence of growth components: Solid lines represent C flux = N flux, dashed lines represent C flux = growth flux, and dotted lines represent N flux = growth flux. Note that when strict phagotrophy or strict phototrophy are optimal, only one of these equalities is true. The growth-maximizing strategy (red dot) is at the convergence point of the lines (or, in the cases of strict phagotrophy or phototrophy, where the nonoptimal metabolic investment is set to zero).