Figure 1.
Conceptual scheme of the model.
The reduction of intracellular structures is a mean to reduce the cell size of the predatory flagellates. A consequence of this size reduction is an optimized predator-prey-size ratio for the smallest flagellate when preying upon the smallest bacteria, i.e. ultramicrobacteria, which largely escape predation by larger flagellates (Pernthaler 19). This optimization is thus an evolutionary driving force behind the differentiation of mixotrophic algae into obligate heterotrophic flagellates.
Table 1.
Values and description of the model parameters.
Figure 2.
Relative dominance of Mixotrophs vs. Heterotrophs.
Ratio of Mixotroph biomass (M) to Heterotroph biomass (H) at the equilibrium, as a function of carbon input (ci) and photosynthetic growth rate (rM×1000). Left panel (A), represents low light availability (KM = 0.5), while right panel (B) represents high light availability (KM = 2.5). The horizontal plane marks the 1∶1 ratio. Note the different scale in both panels. At a relatively high carrying capacity (B), the mixotrophs dominate the community for most of the range of carbon input (ci) and growth rate (rM). Only in a very carbon-poor environment and with a low photosynthetic ability can the heterotroph reach higher biomass. The mixotroph dominance is, again, particularly strong at the combination of low ci and high photosynthesis. However, as in the previous case, at high photosynthetic rates the dominance of the mixotroph decreases sharply when carbon input increases, thus allowing a coexistence with heterotrophs at equal densities.
Figure 3.
Relative dominance of large vs. small bacteria.
Ratio of Large bacteria biomass (L) to Small bacteria biomass (S) at the equilibrium, as a function of carbon input (ci) and photosynthetic growth rate (rM×1000). Values above 10 are not plotted. Left panel (A), represents low light availability (KM = 0.5), while right panel (B) represents high light availability (KM = 2.5). The horizontal plane marks the 1∶1 ratio. Values higher than 10 are not presented, which causes the “serrated-edge” effect in the figure. The large bacteria dominated over the small ones in richer environments (due to their higher growth rate at higher DOC concentrations), and also as the mixotroph photosynthetic growth rate increased, because the mixotroph grazing pressure (mostly directed to the large bacteria) declines as its photosynthetic ability increases.