Are K-strategists yield-strategists in disguise? An example from autotrophic nitrogen removal.

The behavior of heterotrophic bacteria growing in systems with low or no external supply of chemical oxygen demand (COD) has become more relevant within the wastewater context. Growth strategies help to clarify how bacteria behave and adapt to different environmental conditions. In the case of substrate limited conditions, research has been mainly focused on the k-strategy, whereas another important strategy: the yield strategy has not been explored intensely. Some authors have, however, demonstrated the implications of bacteria pursuing the yield strategy when living in structured environments and facing low-substrate concentrations. This study uses a one-dimensional biofilm model to study the influence of the affinity constant, the maximum growth rate, and the growth yield on the heterotrophic formation of dinitrogen gas (N ) in a completely autotrophic partial nitritation anammox system. The effect of these parameters on the composition and the diversity of the heterotrophic community is also evaluated. In a first scenario, heterotrophic bacteria are allowed to grow only on the COD produced by biomass decay. In a second step, the competition with a second group of heterotrophs using external COD as electron donor is assessed. For both evaluated scenarios, the results suggest that the yield plays a crucial role in the heterotrophic biomass and dinitrogen gas formation. Moreover, in the case of the community diversity the yield seems to be the decisive parameter. Finally, we conceptually compared the K and the yield strategy and give some insight to the possibility of both either being closely related or even being the same strategy.