It has been considered that physico-chemical conditions (e.g. pH, temperature, salinity) affect the growth of organisms and naturally selected a species, which could adapt to the changed conditions. Many organisms also alter their surrounding conditions via various processes (e.g. offset carbon emission by plants). This connection implies feedback between organisms and its surrounding physico-chemical conditions. However, the theoretical basis for such feedbacks is poorly understood.

Recently, we demonstrated that the feedback might promote the resistance of the physico-chemical condition and the coexistence of species (Seto and Akagi, 2005, 2007; Akagi, 2006). To examine how this immanent regulative property might be universal, we examined an interaction between pH and pH-affecting microorganisms (e.g. cyanobacteria) using a chemostat model, which has been a well-studied model for an open system with purely exploitative competition. We considered two cases where only 1 species or 2 species existed in the chemostat. pH resistance against acid loads arose for both the cases. This implies the existence of links between the organisms and the resistance of physico-chemical conditions in ecosystems.