Nutrient availability and grazing influence priority effects in aquatic microbial communities

When bacterial communities mix, immigration history can fundamentally affect the community composition as a result of priority effects. Priority effects arise when an early immigrant exhausts or alters existing resources and/or habitat conditions thereby influencing the establishment success of the late arriver. The strength of priority effects is context dependent and expected to be higher if environmental conditions favor the growth of the first arriver. In this study we conducted a two-factorial experiment testing the importance of nutrient availability and grazing on the strength of priority effects in complex microbial communities. We did so by mixing two dissimilar communities simultaneously and with a 38 hours' time-delay. Priority effects were measured as the invasion resistance of the first community to the invading second community. We found priority effects in treatments with high nutrient availability and absence of grazing, which had high initial cell counts prior to the introduction of the second communities. At population level, the results were complex, but priority effects may have been driven by bacteria belonging to for example the genera Rhodoferax and Herbaspirillum. Our study highlights the importance of arrival timing in complex bacterial communities especially in environmental conditions which favor rapid community growth.