Microcystis aeruginosa is one of the world’s most widespread causes of cyanobacterial harmful algal blooms in eutrophic lakes due to its ability to aggregate in large, mucilage-rich colonies. Under laboratory conditions, M. aeruginosa isolates lose this ability. Factors that contribute to aggregation include high inorganic nitrogen concentrations, association with specific heterotrophic bacteria, and concentrations of the toxin microcystin. These controlling factors have been studied individually yet their interactions remain largely unexplored.
Toxic and non-toxic strains of M. aeruginosa were co-cultured with Exiguobacterium indicum, which induces aggregate formation, or Exiguobacterium undae, a non-aggregate inducing bacterium. Nutrient treatments followed a full-factorial design, with two phosphorus concentrations typical of eutrophic and mesotrophic lakes and three N:P ratios: 78:1 (N-replete), 16:1 (Redfield ratio), and 4:1 (N-limited). M. aeruginosa was predicted to exhibit greater aggregate frequency and size in co-culture with E. indicum than with E. undae, and the effects of co-culture with a bacterium was predicted to be enhanced by high N:P ratios. Furthermore, it was hypothesized that toxic M. aeruginosa would produce more microcystin under P-limited conditions, resulting in more aggregation than in N-limited treatments.
Aggregates were two to eight times larger and two to twelve times more frequent in co-cultures with E. indicum than in negative controls. In the eutrophic N-replete treatment, E. undae had no effect relative to negative controls, and aggregates were two to three times more frequent in E. indicum co-cultures than in E. undae co-cultures. E. undae enhanced frequency of aggregation six- to eight-fold relative to negative controls in all other conditions, including the mesotrophic N-replete treatment, where the frequency of aggregates was roughly equal in E. undae and E. indicum co-cultures. Although nutrients alone did not induce aggregation, nutrient concentration and N:P ratio can enhance or diminish the effects of aggregate-inducing bacteria. Toxic and non-toxic strains did not differ in overall aggregation, but significant interactions of strain with nutrient level, N:P ratio, and co-culture were detected, indicating that toxic and non-toxic M. aeruginosa respond to the same conditions in different ways. Further tests will quantify microcystin in toxic cultures to assess how it varies with co-culture and N:P ratio. Results of this study will advance understanding of how the interplay between nutrient inputs and the microbial community contributes to occurrence, persistence, and toxicity of Microcystis blooms.