Dominance shifts between planktonic and attached algae associated with differential phosphorus utilization and microbial processes

Phosphorus (P) availability is closely associated with algal bloom types, yet filamentous attached algae frequently proliferate in clear waters or low-P systems. To examine associations between algal dominance and microbial P-cycling potential, sediment–periphyton assemblages from a low-P artificial water-diversion canal were incubated for 120 days under three water matrices representing oligotrophic, lake-like, and eutrophic conditions. Algal dominance was assessed by microscopy and metagenomic relative abundance profiling, together with sediment P fractions, bacterial communities, and P-cycling genes. Filamentous attached algae remained dominant under low-P treatments, accompanied by depletion of sediment P, especially redox-sensitive BD-P, and higher relative abundances of genes associated with P scavenging and organic P utilization, including aphA and glpQ. In contrast, eutrophic treatments were associated with planktonic algae accounting for more than 70% of the algal community at the bloom stage and with distinct bacterial assemblages and P-cycling gene profiles. Candidate phosphate-solubilizing taxa, including Ideonella, Runella, Sphingopyxis, and Gemmatimonas, were statistically associated with community variation. These results suggest that the level of P in the water, microbial P-cycling potential, and sediment P dynamics may jointly contribute to algal niche differentiation at the water–sediment interface, providing a framework for understanding filamentous algal proliferation in low-P aquatic systems.