1 SCIENTIFIC REPORTS | (2019) 9:16076 | https://doi.org/10.1038/s41598-019-52450-9 www.nature.com/scientificreports Demonstration of facilitation between microalgae to face environmental stress Emna Krichen 1,2,3 , Alain Rapaport 2 , Emilie Le Floc’h 1 & Eric Fouilland 1* 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 modeling approaches the direct facilitation between two algal strains during the colonization phase. Our results demonstrate that the frst 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 favoring the specialist species initially inhibited by free ammonia toxicity. This latter species ultimately dominated the algal community through 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 modify their growth conditions. We suggest that facilitation within the aquatic microbial communities is a widespread ecological interaction under a vast range of environmental stress. One of the major challenges in microbial ecology is to understand the dynamics of communities of interacting species. Understanding the biological interactions and the time scales over which they occur is necessary to inter- pret the results of the directional succession process of communities’ development in the natural environment and artifcial ecosystems. In aquatic systems, microalgae are present in natural waters such as oceans, lakes, rivers, and ponds and play a prominent role in the marine and fresh-water ecosystems where they drive major ecosys- tem processes. Strong similarities exist between marine and freshwater phytoplankton ecology 1 when they face similar changes in growth conditions leading to temporal species succession. Abiotic forcing and biotic interac- tions can both result in successional trends in phytoplankton. Te scientifc discussion around the phytoplankton growth periodicity and succession has been dominated by the role of the environmental drivers including global climatic change (e.g. light, temperature, wind) 2–6 local hydrological variations 7,8 biological disturbances such as species invasion 9 , and chemical efects such as toxic pollutants, nutrient enrichment, or change in pH (see refer- ences 6,7,10–12 ). On the other hand, the conditions governing phytoplankton growth over the seasonal change in plankton communities have mostly been discussed in the context of exploitative competition (e.g. Tilman, 1982) 13 or algae-grazer interactions (e.g. Porter, 1977) 14 . Te competition for limiting nutrients is an important factor explaining phytoplankton species temporal suc- cessions. In marine ecosystems, small-cell diatoms usually grow rapidly in the frst stage afer a strong nutrient enrichment because of their higher growth rates and are then followed by larger-cell diatoms and dinofagellates, which are more likely to occur when nutrients are depleted 15,16 . Similarly, the seasonal patterns of succession in freshwater ecosystems might be explained by the frst occurrence of invasive small-sized species 17 , which can be expected to continue to expand until they either run out of nutrient or light energy or are controlled by zoo- plankton grazing 17,18 . Tese pioneer invasive species can be replaced by other phytoplankton species more prone to grow under nutrient depletion because of mixotrophy ability or mobility allowing them to exploit patches of nutrients not available to other microalgae 17 . Te ability to colonize a specifc habitat usually explains the dominance and succession under changes in envi- ronmental conditions. For instance, changes in the algal assemblage in natural bioflm communities have been 1 MARBEC, Univ. Montpellier, CNRS, IFREMER, IRD, Sète, France. 2 MISTEA, Univ. Montpellier, INRA, SupAgro, Montpellier, France. 3 ADEME, Agence de l’environnement et de la Maîtrise de l’Energie, 20 avenue du Grésillé, BP 90406, 49004 Angers Cedex 01, France. *email: eric.fouilland@cnrs.fr OPEN Corrected: Author Correction