Demonstration of a facilitation between microalgae to face ammonia toxicity in a High Rate Algal Pond

International audienceThe use of intensive microalgae cultivation for both wastewater treatment and valorization of the produced algal biomass has received increased attention in the literature. Maintaining the stability and economic viability of the algal production during the wastewater treatment process is of crucial importance. However, the presence of growth inhibitors in untreated sewage may damage the algae culture. Ammonia NH3 is considered as an inhibitor of photosynthesis at high pH that can cause failure of high rate oxidation ponds when supplemented with substantial concentrations of ammonium NH4+.Many studies highlighted the effects of high ammonium concentrations on growth, which vary significantly within classes of microalgae and within species. However, little is known about interactions between species under a potentially toxic environment and no studies have reported the effect of ammonia toxicity on algal species successions which impact the performance of the waste removal process. We hypothesize that an ecological facilitation can occur between two microalgae, when one species being more tolerant to ammonia can contribute to reduce the ammonium availability, making the growth of more sensitive microalgae possible. Such hypothesis has been suggested to explain the widely observed succession of Chlorella sp. and Scenedesmus sp. reported in High Rate Algal Ponds supplemented with urban wastewaters