Eutrophication leads to frequent outbreaks of cyanobacterial blooms, especially those of the genera Dolichospermum and Microcystis. The contribution of bacteria attached to algal cells to cyanobacterial blooms is still not clear and specific. To gain a deeper understanding of functional genes and their role in bacteria attached to different bloom-forming cyanobacteria, we carried out microbial experiments associated with Dolichospermum and Microcystis in four fish ponds.
The significantly positive relationships between Dolichospermum density and total nitrogen (TN) and between Microcystis density and particle nitrogen (PN) indicated the strong nitrogen (N) demand of these two species. The lack of functional genes mediating the nitrification process in bacteria attached to both Microcystis and Dolichospermum indicated that these two species preferred ammonium (NH 4 + -N). Dolichospermum could overcome N limitation through N 2 fixation expressed by high nitrogenase genes abundance. Compared to bacteria attached to Dolichospermum cells, bacteria attached to Microcystis cells showed a higher activity of leucine aminopeptidase (LAP) and a significantly higher abundance of functional genes (such as nrfA , nirB and aminopeptidase genes) mediating the dissimilatory nitrate reduction to ammonium (DNRA). The significantly higher abundance of functional genes (carbon degradation) and β-glucosidase (GLU) activity of bacteria attached to Microcystis than those of bacteria attached to Dolichospermum suggested the abundant organic carbon bound Microcystis cells, which was prerequisite for DNRA. Also, Microcystis had a great advantage in solving phosphorus (P) stress, including high levels of organic phosphorus hydrolysis associated with high levels of phosphatase genes of attached bacteria. The difference of functional community compositions of bacteria attached to Microcystis and Dolichospermum resulted in the functional differentiation.
Dolichospermum and Microcystis growth was susceptible to P and N (especially NH 4 + -N) limitation, respectively, the latter of which could be effectively solved by attached bacteria through DNRA and ammonification. The P acquisition disadvantage of Dolichospermum resulted in its frequent replacement by Microcystis , especially in conditions of P deficiency. Hence, the evaluation of nutrient limitation (N or P) type of algal growth should combine the nutrient concentration and ratio as well as the ability to solve nutrient deficiency.