Dynamics of an experimental microbial invasion

Abstract

The ecological dynamics underlying species invasions have been a major focus of research in macroorganisms for the last five decades. However, we still know little about the processes behind invasion by unicellular organisms. To expand our knowledge of microbial invasions, we studied the roles of propagule pressure, nutrient supply, and biotic resistance in the invasion success of a freshwater invasive alga, Prymnesium parvum, using microcosms containing natural freshwater microbial assemblages. Microcosms were subjected to a factorial design with two levels of nutrient-induced diversity and three levels of propagule pressure, and incubated for 7 d, during which P. parvum densities and microbial community composition were tracked. Successful invasion occurred in microcosms receiving high propagule pressure whereas nutrients or community diversity played no role in invasion success. Invaded communities experienced distinctive changes in composition compared with communities where the invasion was unsuccessful. Successfully invaded microbial communities had an increased abundance of fungi and ciliates, and decreased abundances of diatoms and cercozoans. Many of these changes mirrored the microbial community changes detected during a natural P. parvum bloom in the source system. This role of propagule pressure is particularly relevant for P. parvum in the reservoir-dominated southern United States because this species can form large, sustained blooms that can generate intense propagule pressures for downstream sites. Human impact and global climate change are currently causing widespread environmental changes in most southern US freshwater systems that may facilitate P. parvum establishment and, when coupled with strong propagule pressure, could put many more systems at risk for invasion.

Keywords: Prymnesium; diversity; invasion resistance; microbial ecology; propagule pressure.

Fig. S1.

Composition of (A) eukaryotic and (B) bacterial communities for microcosms at the time of Prymnesium parvum invasion (day 8) and at the end of the experiment (day 15), visualized using NMDS ordination. Shape of dots indicates nutrient treatment whereas colors indicate day, as indicated in the legend. NMDS stress values are 0.043 for bacteria and 0.119 for eukaryotes.

Fig. 1.

Population densities of P. parvum in microcosms during the experiment in the three invading propagule pressure treatments (high, 6,430 cells per mL, red stars; medium, 640 cells per mL, blue squares; and low, 64 cells per mL, gray circles). Points for days 11 and 15 are offset for clarity.

Fig. 2.

Community conditions of experimental microcosms at the end of the experiment (day 15) showing (A) taxonomic richness and (B) alpha diversity for eukaryotes (blue symbols) and bacteria (red symbols) for three propagule pressure treatments: low (circles), medium (triangles), and high (crosses). Richness (Chao1) and diversity (Simpson’s reciprocal) of microbial communities were calculated using 97%-similarity OTU abundances.

Fig. 3.

Composition of (A) eukaryotic and (B) bacterial communities in microcosms at the end of the experiment (day 15) visualized using NMDS ordination. Colors indicate low (blue), medium (green), or high (yellow) propagule pressure; shape indicates either low (triangles) or high (circle) nutrient treatments. Communities from control bottles appear as orange squares. High (High PP) and all remaining (Non-high PP) propagule pressure treatments are surrounded by 95% confidence dispersion ellipses. NMDS stress values are 0.145 for bacteria and 0.160 for eukaryotes.

Fig. S2.

Composition of (A) eukaryotic and (B) bacterial communities for high and non-high propagule pressure (PP) treatments, using 97%-similarity OTUs. Eukaryotes are shown at the level of rank 3 (previously phylum) and bacteria are shown at the level of class. OTUs that constituted less than 0.2% of each microcosm library have been omitted for clarity.