Résumé
Positive interactions such as facilitation play an important role during the biological colonization and species succession in harsh or changing environments. However, the direct evidence of such ecological interaction in microbial communities remains rare. Using common freshwater microalgae isolated from a High Rate Algal Pond (HRAP) treating wastewaters, we investigated with both experimental and modelling approaches the direct facilitation between two algal strains during the colonization phase. Our results demonstrate that the first colonization by microalgae under a severe chemical condition arose from the rapid growth of pioneer species such as Chlorella sorokiniana, which facilitated the subsequent colonization of low growth specialists such as Scenedesmus pectinatus. The pioneer species rapidly depleted the total available ammonia nitrogen favouring the specialist species initially inhibited by free ammonia toxicity. This latter species ultimately dominated the algal community in accordance with the competitive exclusion under low nutrient conditions. We show that microbial successions are not only regulated by climatic conditions but also by interactions between species based on the ability to indirectly modify their growth conditions. Furthermore, the theoretical study of algal resilience and succession times as proxies of the facilitation efficiency showed that control of algal production processes might be possible by modifying the initial populations' densities.