Ecological Impacts of Calcium Enrichment and Sediment Sterilization on Harmful Algal Bloom Dynamics

The present study evaluated the potential interactions between harmful algal blooms (HABs) and the microbial communities following in situ sediment modifications in the Three Gorges Reservoir (TGR), China. An in situ mesocosm experiment was conducted in Gaoyang Lake, Pengxi River, one of the most eutrophicated first-order tributaries in TGR, using four treatments: unaltered control, sterilized sediment, and calcium (Ca2+) enrichment at 100 mg L-1 and 300 mg L-1. High-throughput 16S and 18S rRNA gene sequencing revealed that Proteobacteria, Bacteroidota, and Firmicutes were dominant bacterial phyla, with the genera Exiguobacterium, Acinetobacter, and Chryseobacterium showing marked increases in relative abundance (RA) specifically under the Ca2+-enriched and sterilized sediment treatments. Among eukaryotes (18S), Cercozoa initially dominated but were replaced by fungi, particularly Rozella, which became relatively more abundant, up to 68% in sterilized and Ca2+-enriched sediments. Notably, Ca2+ enrichment significantly stimulated the free-living protist Agitata proliferation after an initial suppression. Microscopic phytoplankton analysis showed early Cryptophyta dominance shifting to the persistent blooms of Dinophyta, especially Ceratium hirundinella (RA > 90%) across all treatments. Dolichospermum circinalis (D. circinalis) declined under sterilization but persisted in Ca2+-enriched conditions. Functional redundancy within Cyanophyta and host-linked dynamics of Rozella were evident. The resource ratio based on the TN:TP molar ratio confirmed high TN:TP in Ca2+-enriched mesocosms, indicating possible Ca2+-induced P immobilization to the legacy store, resulting in a dramatic N increase relative to P. These findings demonstrate that sediment sterilization and Ca2+ enrichment distinctly shape microbial-alga interactions, offering novel insights for managing eutrophication and HABs in reservoirs.